Effect of Surface Mechanical Treatments on the Microstructure

materials

Review Eﬀect of Surface Mechanical Treatments on the Microstructure-Property-Performance of Engineering Alloys

Dharmesh Kumar 1,2 , Sridhar Idapalapati 1,* , Wei Wang 2 and Srikanth Narasimalu 3 1 School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore 2 Advanced Remanufacturing and Technology Centre, Agency for Science, Technology and Research (A*STAR), 3 CleanTech Loop, Singapore 637143, Singapore 3 Energy Research Institute, Nanyang Technological University, Singapore 639798, Singapore * Correspondence: [email protected]

Received: 13 June 2019; Accepted: 30 July 2019; Published: 7 August 2019

Abstract: Fatigue is a dominant failure mechanism of several engineering components. One technique for increasing the fatigue life is by inducing surface residual stress to inhibit crack initiation. In this review, a microstructural study under various bulk (such as severe plastic deformation) and surface mechanical treatments is detailed. The eﬀect of individual microstructural feature, residual stress, and strain hardening on mechanical properties and fatigue crack mechanisms are discussed in detail with a focus on nickel-based superalloys. Attention is given to the gradient microstructure and interface boundary behavior for the mechanical performance. It is recommended that hybrid processes, such as shot peening (SP) followed by deep cold rolling (DCR), could enhance fatigue life. The technical and scientiﬁc understanding of microstructural features delineated here could be useful for developing materials for fatigue performance.

Keywords: nickel-based superalloys; surface mechanical treatments; severe plastic deformation; microstructure; fatigue performance

1. Introduction The type of material and its physical, chemical, and mechanical properties determine the performance of a component during its period of operation. Every product is made from one type of material dictated by its performance properties. Examples of such materials include the following: nickel-based alloys used for aerospace applications due to their excellent combination of fatigue, creep, and corrosion/oxidation resistance [1]; biocompatible titanium alloys (Ti-6Al-4V), Co-Cr, and 316L stainless steel alloys used for biomedical hard-tissue replacement applications [2]; steels used for load-bearing structural applications due to their combination of strength, durability, and cost [3]; and magnesium alloys used for automotive applications owing to their high specific strength [4]. However, mechanical properties of engineering alloys may be dramatically compromised under severe operating conditions such as cyclic loading, vibrations [1], corrosive environments, and elevated temperatures [5]. These conditions may lead to premature failure of the components during the service life. Thus, there is a constant need to improve the fatigue performance of the materials as approximately 50% of engineering components failure is due to fatigue, which is probabilistic in nature [6]. Fatigue life can be divided into the number of cycles for crack initiation and its propagation, leading to the ultimate failure of the component. As a rule of thumb, a hard mirror-like surface finish through polishing, compressive surface residual stress, and elimination of stress concentration zones through design modifications improve the fatigue life.

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A variety of surface mechanical treatments (SMTs) such as shot peening (SP) [7–11], laser shock peening (LSP) [12–14], deep cold rolling (DCR) [15–19], and vibro peening (VP) [6,20] have been developed to mitigate the fatigue failure on a range of critical mechanical components in various industries [1]. The SP treatment is widely used in aerospace industry as it is generally believed that the compressive residual stress (CRS) induced by shot peening may impede the crack initiation and propagation at the subsurface region, hence contributing to the enhanced fatigue performance [21]. Laser shock peening achieves an appreciable CRS at a much deeper depth for certain fatigue critical components. The DCR treatment is another alternative process that could achieve both CRS and deep penetration depth [18]. The strengthening and deformation mechanisms in the nickel-based superalloy, Udimet720Li, during deep cold rolling process were proposed in our previous study [22]. The existing limitations of these processes, such as inaccessibility to treat complex geometries and rough surface finish, always motivate industries to develop new processes. The VP process is a newly developed process which produces a better surface finish and comparable CRS profile than that of shot peened parts, as reported in our previous study [23]. However, knowledge is still limited to residual stress and strain hardening, whereas, the microstructural changes under various surface mechanical treatments and their influence on final fatigue performance are not fully understood. Microstructural features play a vital role in the fatigue behavior of polycrystalline materials. When tensile stress is present, surface/subsurface material imperfections could become stress concentration points and lead to crack initiation. This can be impeded by the introduction of CRS and strain hardening in the material after surface treatments (as shown in Figure1). For the crack propagation, microstructural features and materials properties could have significant effects. For instance, grain boundaries impede the crack propagation by providing resistance to the crack tip, and provide strength to the material by impeding the motion of dislocations from one grain to another as it behaves as the pinning point. Therefore, a comprehensive understanding of fatigue damage due to microstructural features interacting is required to develop an optimized surface/subsurface material treatment process. This literature review focuses on the influence of residual stress and microstructural features on fatigue crack initiation and propagation. A variety of surface mechanical treatments are compared based on the process intensity, residual stress distribution, microstructure, mechanical property, and fatigue performance. Materials 2019, 12, x FOR PEER REVIEW 3 of 41

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Figure 1. Schematic showing various microstructural features aaffectingﬀecting fatiguefatigue mechanisms.mechanisms.

TheThe firstfirst twotwo sectionssections covercover thethe introductionintroduction andand variousvarious aspectsaspects ofof fatiguefatigue failure,failure, factorsfactors responsibleresponsible forfor thethe failure,failure, crackcrack initiation,initiation, andand propagationpropagation mechanismmechanism inin polycrystallinepolycrystalline engineeringengineering alloys.alloys. The secondsecond sectionsection provides provides a a report report on on the the fundamental fundamental microstructural microstructural features features responsible responsible for controllingfor controlling fatigue. fatigue. These These features features include include the e theffect effe of grainct of grain size (coarse size (coarse/fine/ultrafine/nanograin),/fine/ultrafine/nanograin), grain distributiongrain distribution (homogeneous /gradient),(homogeneous/gradi interface boundariesent), (highinterface/low angle /coherentboundaries/incoherent (high/low/twins), dislocationangle/coherent/incoherent/twins), density, slip planes, residual dislocation stress density, distribution, slip planes, and strain residual hardening stress ondistribution, fatigue life. and In thestrain last hardening two sections, on fatigue the microstructural life. In the last two features sections, generated the microstructural through various features process generated routes through such as severevarious plastic process deformation, routes such surface as severe mechanical plastic deformation, treatments, andsurface their mechanical effect on fatigue treatments, is investigated. and their Ultimately,effect on fatigue a matrix is investigated. is prepared Ultimately, for understanding a matrix the is prepared contribution for understanding of an individual the microstructural contribution of parameteran individual on mechanicalmicrostructural properties parameter and on the mechan fatigueical performance. properties Theand mainthe fatigue objective performance. of this review The ismain to provideobjective a of direction this review to optimize is to provide the fatigue a direction performance to optimize of nickel-basedthe fatigue performance superalloys of through nickel- microstructuralbased superalloys modifications through microstructural for aero-engine modifications components. for aero-engine components. 2. Fatigue Mechanism 2. Fatigue Mechanism Fatigue cracking is one of the critical failure mechanisms of structural components. Under cyclic Fatigue cracking is one of the critical failure mechanisms of structural components. Under cyclic loading conditions, a material often fails at a stress level below its nominal strength. The fatigue life of loading conditions, a material often fails at a stress level below its nominal strength. The fatigue life a component is presented as the number of loading cycles required to initiate a fatigue crack and to of a component is presented as the number of loading cycles required to initiate a fatigue crack and propagate the crack to a critical size. Therefore, fatigue failure occurs in two stages, crack initiation to propagate the crack to a critical size. Therefore, fatigue failure occurs in two stages, crack initiation and crack propagation [24]. Fatigue cracks initiate from surface defects or regions of high-stress and crack propagation [24]. Fatigue cracks initiate from surface defects or regions of high-stress concentration, surface roughening generated due to vibrations or high repetitive stress amplitudes, concentration, surface roughening generated due to vibrations or high repetitive stress amplitudes, and internal defects or material inhomogeneities. and internal defects or material inhomogeneities. Fatigue is a stochastic process influenced by microstructure, surface topography, geometry, stress Fatigue is a stochastic process influenced by microstructure, surface topography, geometry, amplitude, frequency, and mean stress ratio, etc. Figure2 shows the interaction of crack initiation and stress amplitude, frequency, and mean stress ratio, etc. Figure 2 shows the interaction of crack propagation with grain boundaries, dislocations, and other obstacles. These barriers, such as hard initiation and propagation with grain boundaries, dislocations, and other obstacles. These barriers, precipitates and grain boundaries, impede the propagation of a crack and improve the fatigue life [25]. such as hard precipitates and grain boundaries, impede the propagation of a crack and improve the It is important to understand crack initiation and propagation behavior and their interaction with microstructural features.

Materials 2019, 12, x FOR PEER REVIEW 4 of 41 fatigueMaterials life2019 ,[25].12, 2503 It is important to understand crack initiation and propagation behavior and their4 of 41 interaction with microstructural features.

FigureFigure 2. 2. SchematicSchematic diagram diagram showing showing interaction interaction betw betweeneen crack crack propagation, propagation, precipitates, precipitates, and and microstructuralmicrostructural features. features. 2.1. Crack Initiation Mechanism 2.1. Crack Initiation Mechanism Microstructural features, state of stress, materials properties, and surface topography are the Microstructural features, state of stress, materials properties, and surface topography are the critical factors in the crack initiation stage. Microstructural features such as dislocations, interface critical factors in the crack initiation stage. Microstructural features such as dislocations, interface boundaries (i.e., grain boundaries, slip planes/bands, twin boundaries, and defects) are the primary boundaries (i.e., grain boundaries, slip planes/bands, twin boundaries, and defects) are the primary sites for crack initiation [26]. It is observed that after a significant number of loading cycles, dislocations sites for crack initiation [26]. It is observed that after a significant number of loading cycles, pile up creating persistent slip bands (PSB), which are the areas that rise above (extrusion) or fall dislocations pile up creating persistent slip bands (PSB), which are the areas that rise above below (intrusion) the surface of the component due to the movement of material along slip planes [27]. (extrusion) or fall below (intrusion) the surface of the component due to the movement of material This leaves tiny steps on the surface that serve as stress risers where fatigue cracks are initiated. This is along slip planes [27]. This leaves tiny steps on the surface that serve as stress risers where fatigue the first aspect of pure fatigue failure, and cracks approach in the region of the maximum dislocation cracks are initiated. This is the first aspect of pure fatigue failure, and cracks approach in the region activity in the active slip planes/bands [26]. Crack initiation is more prevalent in coarse grains where of the maximum dislocation activity in the active slip planes/bands [26]. Crack initiation is more persistent slip bands (PSBs) are prone to generate. However, in small grains, the crack initiation is prevalent in coarse grains where persistent slip bands (PSBs) are prone to generate. However, in small difficult due to the increment in the stress intensity threshold for nucleation. Under cyclic loading, grains, the crack initiation is difficult due to the increment in the stress intensity threshold for dislocations pile up at the grain boundaries and induce stress concentration that initiates cracks to nucleation. Under cyclic loading, dislocations pile up at the grain boundaries and induce stress minimize the internal energy state. It means grain boundaries are a source of crack nucleation. concentration that initiates cracks to minimize the internal energy state. It means grain boundaries are2.2. a Cracksource Propagation of crack nucleation. Mechanism

2.2. CrackThe Propagation crack propagation Mechanism mechanism is affected by microstructural features, precipitates, and compressive residual stress. It also depends on the threshold intensity factor below which fatigue crack propagationThe crack is notpropagation possible. mechanism Once the value is affected of applied by stressmicrostructural intensity isfeatures, higher than precipitates, the threshold, and compressivecrack propagation residual starts stress. in the It grainsalso depends and it propagates on the threshold until a microstructuralintensity factor hindrancebelow which like fatigue a grain crackboundary, propagation twin boundary, is not possible. inclusions, Once or precipitates the value deceleratesof applied it.stress Therefore, intensity grain is refinementhigher than causes the threshold,strengthening crack of propagation the material bystarts the insertionin the grains of microstructural and it propagates barriers. until Surface a microstructural mechanical hindrance treatments likecontribute a grain to boundary, the increase twin in someboundary, microstructural inclusions, barriers or precipitates per unit lengthdecelerates due toit. the Therefore, misorientation grain refinementof grains. causes The di ffstrengtheningerent orientation of the of material grainsand by the grain insertion boundary of microstructural provides an obstruction barriers. Surface to the mechanicalpropagation treatments of the crack contribute and simultaneously to the increase diff usesin some the microstructural crack [28]. Thus, barriers these processes per unit increaselength due the tofatigue the misorientation life of the material of grains. significantly The different by generating orientation various of grains microstructural and grain boundary features, i.e.,provides interface an obstructionand dislocations. to the propagation of the crack and simultaneously diffuses the crack [28]. Thus, these processesIn summary, increase the fatigue microstructural life of the features material such significantly as grain by size, generating interface various boundaries, microstructural slip bands, features,and dislocations i.e., interface govern and the dislocations. crack initiation and propagation mechanism. Thus, further experimental investigationIn summary, to improve the microstructural the fundamental features understanding such as grain of size, microstructure-damage interface boundaries, mechanics slip bands, is andstrongly dislocations required. govern the crack initiation and propagation mechanism. Thus, further experimental investigation to improve the fundamental understanding of microstructure-damage mechanics is strongly3. Influencing required. Factors of Ni-Based Alloy Fatigue Behavior This section of the review covers the features that drive the fatigue crack initiation and propagation 3. Influencing Factors of Ni-Based Alloy Fatigue Behavior mechanism in the material.

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This section of the review covers the features that drive the fatigue crack initiation and propagation mechanism in the material. Materials 2019, 12, 2503 5 of 41 3.1. Grain Size and Distribution

3.1.The Grain effect Size andof grain Distribution size, i.e., coarse (grain size more than 20 µm), fine (1–20 µm), ultrafine (between 100–1000 nm), nanocrystalline grains (less than 100 nm) on material properties is µ µ summarizedThe effect here. of grain size, i.e., coarse (grain size more than 20 m), fine (1–20 m), ultrafine (between 100–1000 nm), nanocrystalline grains (less than 100 nm) on material properties is summarized here. 3.1.1. Coarse, Fine, Ultrafine, and Nanocrystalline Grains 3.1.1. Coarse, Fine, Ultrafine, and Nanocrystalline Grains Grain size is one of the most important factors to influence mechanical property and Grain size is one of the most important factors to influence mechanical property and performance. performance. The Hall–Petch relation predicts that as the grain size decreases the yield strength The Hall–Petch relation predicts that as the grain size decreases the yield strength increases and it has increases and it has been experimentally found to be an effective model for materials with grain sizes been experimentally found to be an effective model for materials with grain sizes ranging from 1 mm ranging from 1 mm to 1 micrometer [29]. For example, coarse grains (see stage I) exhibit low strength to 1 micrometer [29]. For example, coarse grains (see stage I) exhibit low strength as the grain size as the grain size is relatively large as shown in Figure 3. However, the ductility and toughness of is relatively large as shown in Figure3. However, the ductility and toughness of coarse grains are coarse grains are relatively higher than fine grain structures. For fine and ultrafine grains (stage II relatively higher than fine grain structures. For fine and ultrafine grains (stage II and III), the decreasing and III), the decreasing grain size increases the strength at different rates at the expense of ductility. grain size increases the strength at different rates at the expense of ductility. The mechanism involved The mechanism involved in the rate decrement is still ambiguous. In stage IV for nanograins, it offers in the rate decrement is still ambiguous. In stage IV for nanograins, it offers an ultra high-yield an ultra high-yield strength, fracture strength, lower elongation, toughness, and excellent wear strength, fracture strength, lower elongation, toughness, and excellent wear resistance. However, the resistance. However, the critical grain size and the depth of the nanocrystalline layer play a critical critical grain size and the depth of the nanocrystalline layer play a critical role in the enhancement role in the enhancement of these properties. In stage IV, below 10 nm (typical critical value) materials of these properties. In stage IV, below 10 nm (typical critical value) materials soften due to grain soften due to grain boundary sliding [30] and follow the inverse Hall–Petch relationship which boundary sliding [30] and follow the inverse Hall–Petch relationship which depicts that the saturation depicts that the saturation of grain refinement takes place up to a certain point, and then, further of grain refinement takes place up to a certain point, and then, further grain coarsening and dislocation grain coarsening and dislocation annihilation mechanism takes place. This softening can be from annihilation mechanism takes place. This softening can be from various factors such as incomplete various factors such as incomplete densification and atomic sliding at the grain boundaries [31]. densification and atomic sliding at the grain boundaries [31].

Figure 3. Schematic representation of the yield stress and grain size in microcrystalline, ultrafine grains Figureand nanocrystalline 3. Schematic metalsrepresentation and alloys of (redrawnthe yield basedstress onand [29 grain]). size in microcrystalline, ultrafine grains and nanocrystalline metals and alloys (redrawn based on [29]). The Hall–Petch relationship [32–35] is expressed as: The Hall–Petch relationship [32–35] is expressed as: k σ = σo + k (1) σ= σ + √ d (1) √𝑑 where, σ is yield stress, σo is lattice friction stress required to move individual dislocation, k is the where, 𝜎is yield stress, 𝜎 is lattice friction stress required to move individual dislocation, k is the stressstress intensity intensity for for plastic plastic yielding yielding across across polycrystalline polycrystalline grain grain boundaries boundaries (constant), (constant), and and dd isis the the averageaverage grain size. AsAs perper Hall–Petch, Hall–Petch, smaller smaller grain grain size size increases increases the the strength strength through through increased increased grain grainboundary boundary area forarea blocking for blocking the dislocations, the dislocations, and hence and there hence has there been greathas been interest great in developinginterest in nanocrystalline materials [36]. Strengthening depends on the average grain size, grain size distribution, and grain boundary structure [37]. Strengthening is due to dislocation pile-up at the grain boundary that provides the resistance to deformation from structural refinement. In addition, it is also believed Materials 2019, 12, 2503 6 of 41 that the interactions of high- and low-range shear bands, which are developed by plastic deformation, contribute to the generation of subgrains, local dynamic recovery, and the recrystallization process that yields the grain refinement together with highly misoriented grains [38]. However, the grain refinement usually leads to a reduction in the ductility of the material. In contrast, plastic deformation by the dislocation glides can result in crystal rotation and a twist of grain boundaries (GBs) to improve the ductility of the material. Nanocrystalline structures are also notable for their mechanically unstable structure, since under high stress and deformation they undergo grain coarsening [39]. Maintaining both strength as well as ductility is practically difficult, however, the authors suggest a few methods to preserve the strength without the expense of ductility in the Section 3.1.2. A material cannot develop high strength and ductility at the same time [40]. This is called the “paradox of strength and ductility” [41,42]. This loss of ductility at very small grain size comes from the low rate of strain hardening and strain sensitivity. During high strain, most of the dislocation boundaries induced by deformation develop into high angle grain boundaries and their frequency increases with the applied strain [43]. Thus, when the rate of strain hardening is high, dislocations accumulate at the grains which result in a reasonable level of ductility [44]. The formation of the ultrafine structure with non-equilibrium and high angle grain boundaries generates boundary sliding which maintains ductility in materials. Ultrafine crystalline structures with equiaxed grains and high angle grain boundaries impede the motion of dislocations, which enhances the strength of the material. Concurrently, these may facilitate another deformation mechanism, such as grain boundary sliding, and improve grain rotation which induces enhanced ductility [45]. It is accepted that the fatigue properties of materials are strongly dependent on their grain size [46]. Gayda et al. [47] reported on microcrystalline metals, where the grain size is typically greater than 1 µm, and found that an increase in grain size results in a reduction in the fatigue endurance limit. On the other hand, a coarser grain structure can lead to an increased fatigue threshold stress intensity factor, as well as a decrease in the rate of fatigue crack propagation. Conversely, grain size was found to govern the rate of crack growth under mechanical fatigue, with all other structural factors held constant. The microcrystalline Ni alloys measured a slower crack growth rate than that of nanocrystalline or ultrafine structures, as shown in Figure4. The relevance of this pattern to microcrystalline, ultrafine-crystalline metals, and nanocrystalline metals is still unclear. Such lack of understanding is primarily a consequence of the paucity of experimental data on the fatigue behavior of metals with very fine grains. Materials 20192019,, 1212,, 2503x FOR PEER REVIEW 77 of 4141

Figure 4. Comparison of (a) stress range and (b) crack growth in microcrystalline (MC), ultrafine grains (UFG) and nanocrystalline (NC) Ni at fixed stress intensity factor (adapted with permission from [48]). Figure 4. Comparison of (a) stress range and (b) crack growth in microcrystalline (MC), ultrafine Gaydagrains (UFG) et al. [and47] describednanocrystalline the temperature(NC) Ni at fixed/grain stress size intensity dependence factor of(adapted the crack with growth permission behavior. At a lowerfrom [48]). temperature (T < 0.2 Tm), reduction in grain size/crystallite size and irregularity of grain boundaries can improve the resistance towards short cracks propagation, but this resistance is lower for longGayda cracks et al. propagation. [47] described However, the temperature/grai at the middle-temperaturen size dependence range of (Tthe crack0.4 T growthm), smaller behavior. grain ∼ sizeAt a promptslower temperature intergranular (T fatigue< 0.2 T fracturem), reduction in the in presence grain size/crystallite of high grain boundaries size and irregularity density due of tograin the reductionboundaries in can ductility improve [49 ].the resistance towards short cracks propagation, but this resistance is lower for long cracks propagation. However, at the middle-temperature range (T ~ 0.4Tm), smaller grain 3.1.2.size prompts Gradient intergranular and Homogenous fatigue Structure fracture in the presence of high grain boundaries density due to the reductionGradient microstructuresin ductility [49]. (GS) with a grain size gradient have been recently introduced to optimize the mechanical properties of structural materials [50]. It is reported that multiscale grain-size structures can3.1.2. be Gradient achieved and by gradientHomogenous plastic Structure deformation processes such as surface impacting, grinding, and rolling.Gradient Gradient microstructures microstructures (GS) could with obtain a grain the trade-osize gradientff between have strength been recently and ductility; introduced both are to mutuallyoptimize the exclusive mechanical properties. properties It has of been structural proven materials that gradient [50]. grain-sizeIt is reported structures that multiscale yield superior grain- propertiessize structures to homogenous can be achieved structures by gradient [51]. According plastic deformation to reported processes results, the such superior as surface property impacting, is the significancegrinding, and of therolling. optimum Gradient thickness microstructures of the GS layer could (volume obtain %), the but trade-off it is still challengingbetween strength to quantify and theductility; optimum both thickness are mutually of the GSexclusive layer [52 properties]. The gradient. It has nanostructure been proven can th beat further gradient classified grain-size into thestructures following yield four superior categories: properties (a) grain to size homogenous gradient, (b) structures twin thickness [51]. According gradient, (c) to lamellar reported thickness results, gradient,the superior and property (d) columnar is thesize, significance as shown of in the Figure optimum5. thickness of the GS layer (volume %), but it is still challenging to quantify the optimum thickness of the GS layer [52]. The gradient nanostructure can be further classified into the following four categories: (a) grain size gradient, (b) twin thickness gradient, (c) lamellar thickness gradient, and (d) columnar size, as shown in Figure 5.

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Figure 5. Gradient nanostructures with (a) grain size gradient, (b) twin thickness gradient, (c) lamellar Figure 5. Gradient nanostructures with (a) grain size gradient, (b) twin thickness gradient, (c) lamellar Figurethickness 5. Gradient gradient, nanostructures and (d) columnar with size (a) gradientgrain size (adapted gradient, with (b) twin permission thickness from gradient, [52]). (c) lamellar thicknessthickness gradient, gradient, and ( d) columnar size gradient (adapted with permission from [[52]).52]). It should should be be noted noted that that the the concept concept of gradient of gradient nano nanostructure,structure, here, here, is very is verydifferent diﬀ erentfrom the from strain the It should be noted that the concept of gradient nanostructure, here, is very different from the strain straingradient gradient plasticity plasticity [52]. First, [52]. First,the range the range of the of thegrain grain size size gradient gradient should should be be in in the the order order of gradient plasticity [52]. First, the range of the grain size gradient should be in the order of approximately, or more than, hundred grains, which is far less thanthan otherother reportedreported results of strainstrain approximately, or more than, hundred grains, which is far less than other reported results of strain gradient plasticity mechanism [53]. [53]. Secondly, Secondly, the the applied applied strain strain throughout throughout the the depth depth is is the same, gradient plasticity mechanism [53]. Secondly, the applied strain throughout the depth is the same, unlike the strain variance throughout the depth in itsits counterpart. Lastly, Lastly, the grain size should vary unlike the strain variance throughout the depth in its counterpart. Lastly, the grain size should vary from the the surface surface to to the the depth depth of ofthe the material material (as (asshown shown in Figure in Figure 6), which6), which results results in varying in varying yield yieldstress from the surface to the depth of the material (as shown in Figure 6), which results in varying yield stress stressin the inmaterial, the material, and the and applied the applied strain is strain the same is the throughout same throughout the depth. the depth. in the material, and the applied strain is the same throughout the depth.

Figure 6. Representation of (a) ductility-strength synergy and (b) strain softening and hardening in Figure 6. Representation of (a) ductility-strength synergy and (b) strain softening and hardening in gradient nanograined structures (adapted with permission from [54]). Figuregradient 6. nanograinedRepresentation structures of (a) ductility-strength (adapted with permission synergy and from (b [54]).) strain softening and hardening in gradientLu [53] andnanograined Liu et al. structures [55] have (ada employedpted with apermission surface mechanical from [54]). grinding treatment (SMGT) for bulkLu copper [53] and nickelLiu et toal. produce[55] have strong, employed as well a surface as tough, mechanical microstructures grinding [56 treatment]. In this method,(SMGT) thefor Lu [53] and Liu et al. [55] have employed a surface mechanical grinding treatment (SMGT) for surfacebulk copper of the and material nickel is to under produce shear strong, force, as and well the as core tough, of the microstructures material is not [56]. subjected In this to method, signiﬁcant the bulk copper and nickel to produce strong, as well as tough, microstructures [56]. In this method, the loading.surface of Hence, the material surface is layers under produce shear force, nanograins, and the twins core/ twinof the boundaries, material is andnot subjected slip planes to/bands significant in the surface of the material is under shear force, and the core of the material is not subjected to significant presenceloading. Hence, of shear surface force without layers produce altering nanograins, the core microstructure. twins/twin boundaries, The topmost and layers, slip whichplanes/bands comprise in loading. Hence, surface layers produce nanograins, twins/twin boundaries, and slip planes/bands in the nanograins,presence of provide shear force strengthening without altering to the material the core due microstructure. to dislocation slips The andtopmost accumulation layers, which of the the presence of shear force without altering the core microstructure. The topmost layers, which coarsecomprise grains the innanograins, the core, providing provide ductility strengthening or toughness. to the Thismaterial means due the overallto dislocation material slips softening and comprise the nanograins, provide strengthening to the material due to dislocation slips and andaccumulation hardening of isthe taking coarse place grains simultaneously in the core, providing creating ductility strong or and toughness. tough material. This means Furthermore, the overall accumulationmaterial softening of the and coarse hardening grains in is thetaking core, place providing simultaneously ductility or creating toughness. strong This and means tough the material. overall material softening and hardening is taking place simultaneously creating strong and tough material.

Materials 2019, 12, 2503 9 of 41 the synergy achieved by the grain size gradient is due to the gradient yield stress and mechanical incompatibility caused by a mismatch of the Poisson’s ratio in the outer plastic and inner elastic core. For example, the nanograin structure throughout the material depth provides strengthening and the coarse grain structure alone provides toughness and ductility, as shown in Figure6. It is to be noted, as shown in Figure6, that a microstructure with coarse grain embedded in the nanosize grains due to the homogeneous plastic deformation provides both low strength and ductility. The generation of bimodal grain size is also reported to be a method for generating high strength and ductility at the same time [57]. The nanocrystalline structure provides the strength, and embedded large grains stabilize the tensile deformation of the material [58]. It is to be noted that the gradient nanocrystalline structure (GNG) improves the fatigue life, whereas, the surface layer enhances the fatigue crack initiation threshold and coarse grains deﬂect the propagation paths of fatigue cracks by grain boundaries, thus, introducing crack closure and decreasing the rate of crack growth [59]. Thus, this synergy concept between two mutually exclusive properties, such as strength and ductility, together with high fatigue life, can be achieved through a gradient microstructure and a bimodal structure, which can be a potential application in aerospace and structural component designs [51].

3.2. Interface Boundaries Interface boundaries play a major role in altering the material properties and failure mechanics. Factors that induce failure, such as crack nucleation and propagation, directly interact through the boundaries between grains [60]. Therefore, the behavior of the boundary aﬀects the resistance toward its movement into another grain [52]. The geometry of the grain boundary could facilitate either slip transfer (in low angle GBs or twin boundaries) or resist the slip movement.

3.2.1. Grain and Twin Boundaries Twinning usually occurs as a coordinated movement of a large number of atoms together in a crystal by shearing action. This narrow misoriented region is considered as twin and the boundary that separates this region is termed the twin boundary. The major influencing factors for twinning are the plastic deformation, stacking fault energy, and grain size [61]. The face centred cubic (FCC) materials with low stacking fault energy deform by twins when subjected to plastic deformation. Further twinning can be observed in the materials with high stacking fault energy when subjected to a high strain rate [62]. Meanwhile, both high and low stacking fault energies are beneficial for the grain refinement process, since high stacking fault energy materials have a faster recovery rate and low stacking fault energy materials have twin formation [63]. Ultimately, stacking fault energy is the deciding factor for the twin formation as it influences the probability of cross slip. Cross slip and dislocation climb mechanisms are responsible for dynamic recovery [64,65]. Stacking fault energy can be modified through alloying in FCC metals [66]. At a specific temperature, the chance of twin grain formation increases if the stacking fault energy is low since dislocation annihilation is difficult [67]. For example, FCC metals with low stacking fault energies, such as Ag and Ni, usually deform by twinning [62,68–71], whereas, coarse-grained FCC metals have high stacking fault energy, and thus normally deform by dislocation slip. Twinning is easier in the nanocrystalline materials as the grain size is in the 10 nm range, but it becomes difficult as the grain size further decreases [72–74]. Deformation twinning can be observed in the materials subjected to severe plastic deformation when the strain rate reaches a critical value [69]. Interestingly, in nanocrystalline material, thinner twins (<10 nm) play a major role in strengthening. In the nanometre range, twinning shows a strong resistance to the slip/slip bands. The persistent slip bands (PSB) twin boundary (TB) interaction (PSB–TB), can affect the fatigue response of the material. P Furthermore, nanoscale twin boundaries (CSL 3) provide better resistance to the movement of dislocations and slip planes due to a narrow region of atomic mismatch and by decreasing their mean free path [71]. This atomic mismatch provides extra strengthening to the material as it induces Materials 2019, 12, x FOR PEER REVIEW 10 of 41 Materials 2019, 12, 2503 10 of 41 dislocation pile-ups and hinders crack propagation, and thus exhibits high strength and thermal stability at elevated temperatures. Extreme segregation of dislocations on the twin/grain boundary dislocation pile-ups and hinders crack propagation, and thus exhibits high strength and thermal induces stress concentration. To maintain the mechanical equilibrium, the position of this dislocation stability at elevated temperatures. Extreme segregation of dislocations on the twin/grain boundary nucleation shifts to other grains or it can initiate cracks at the grain boundary. However, the resistance induces stress concentration. To maintain the mechanical equilibrium, the position of this dislocation of dislocation movement through grain boundaries is low. Therefore, the dislocation pile-ups are rare nucleation shifts to other grains or it can initiate cracks at the grain boundary. However, the resistance in grain boundaries, as shown in Figure 7a, and no significant strengthening is noted. Twin boundary of dislocation movement through grain boundaries is low. Therefore, the dislocation pile-ups are rare spacing, λ, is a critical feature that should be small for ultra-strong material properties, as shown in in grain boundaries, as shown in Figure7a, and no significant strengthening is noted. Twin boundary Figure 7b. The twin boundary increases the work hardening rate by acting as an obstacle for gliding spacing, λ, is a critical feature that should be small for ultra-strong material properties, as shown in dislocations [75]. Together, slip transfer at the boundary of nanoscale growth twins in FCC structures Figure7b. The twin boundary increases the work hardening rate by acting as an obstacle for gliding also provides the strengthening mechanism and ductility enhancement [76]. It has been dislocations [75]. Together, slip transfer at the boundary of nanoscale growth twins in FCC structures demonstrated that the twinning mechanism can simultaneously maintain high strength and ductility also provides the strengthening mechanism and ductility enhancement [76]. It has been demonstrated of the material. that the twinning mechanism can simultaneously maintain high strength and ductility of the material.

Figure 7. Schematic diagram of (a) grain boundaries and (b) twin boundaries (redrawn based on [70]). Figure 7. Schematic diagram of (a) grain boundaries and (b) twin boundaries (redrawn based on [70]). For the fracture mechanism, the twin boundaries play a vital role in crack initiation and propagation. CracksFor usually the fracture initiate mechanism, from the twin the boundariestwin boundaries due to play the concentrationa vital role in gradient crack initiation generated and by propagation.dislocation pile-ups Cracks andusually the twininitiate boundaries from the imparttwin boundaries higher resistance due to tothe crack concentration propagation gradient in the generatedgrains. The by wide dislocation atomic mismatchpile-ups betweenand the grainstwin boundaries deflects the impart crack tip higher movement resistance and makesto crack its propagation in di ffithecult. grains. However, The wide there atomic is always mismatch conflict between among grains researchers deflects with the crack respect tipto movement the exact andinteraction makes its mechanism propagation of PSB-dislocation difficult. However, and there twin is boundaries. always conflict among researchers with respect to the exact interaction mechanism of PSB-dislocation and twin boundaries. 3.2.2. High Angle and Low Angle Grain Boundaries 3.2.2.Interface High Angle boundaries and Low are Angle also categorizedGrain Boundaries into high angle and low angle grain boundaries based on theirInterface angle of misalignmentboundaries are from also neighboring categorized grains.into high If theangle atomic and low misorientation angle grain angle boundaries between based two grains is more than 15 (θ 15 ), it is considered to be a high angle grain boundary, otherwise it is low on their angle of misalignment◦ ≥ ◦ from neighboring grains. If the atomic misorientation angle between twoangle grains grain is boundary, more than as 15° shown (θ ≥ 15°), in Figure it is 8considered[ 36]. High to angle be a high grain angle boundaries grain boundary, are generated otherwise through it islarge low accumulated angle grain boundary, strains (>6–8) as shown in the materialin Figure and 8 [36]. help High to arrest angle the grain cracks boundaries because they are generated retard slip throughband formation large accumulated and dislocation strains movement. (>6–8) in Simultaneously, the material and they help induce to arrest more the grain cracks boundary because sliding they retardwhich yieldsslip band high formation grain rotation and anddislocation misorientation movement. which Simultaneously, ultimately results they in induce high ductility more grain and boundarytoughness sliding of the material. which yields high grain rotation and misorientation which ultimately results in high ductility and toughness of the material.

Materials 2019, 12, x FOR PEER REVIEW 11 of 41

Materials 2019, 12, 2503 11 of 41 Materials 2019, 12, x FOR PEER REVIEW 11 of 41

Figure 8. Representation of interface boundaries (redrawn based on [77]). Figure 8. Representation of interface boundaries (redrawn based on [77]). The formationFigure of ultrafine 8. Representation grains with of interfacehigh angl boundae nonequilibriumries (redrawn grain based boundaries on [77]). is very prone to sliding,The formation which is of not ultrafine possible grains in low with angle high grain angle boundaries nonequilibrium [57]. grainThis results boundaries in a high is very ductility prone towith sliding, highThe formation whichstrength, is notfatigue,of ultrafine possible and grains intoughness. low with angle high The grain anglconcept boundariese nonequilibrium behind [57this]. mechanism This grain results boundaries is in that a high theis very ductilityultrafine prone withgrainto sliding, high possesses strength, which higher is fatigue, not strength, possible and toughness.and in lowgrain angle boundary The grain concept boundariessliding behind due this to[57]. high mechanism This angle results nonequilibrium is in that a high the ultrafine ductility grain grainboundarieswith possesseshigh strength, contributes higher fatigue, strength, to the and higher and toughness. grainductility boundary The of theconcept material. sliding behind due tothis high mechanism angle nonequilibrium is that the ultrafine grain boundariesgrainLow possesses angle contributes highergrain boundaries tostrength, the higher and are ductility grain usually boundary of induc the material.ed sliding during due plastic to high deformation angle nonequilibrium which leads grain to strainboundariesLow hardening angle contributes grain of the boundaries surface. to the higher They are usually have ductility poor induced of grai then during material.boundary plastic sliding, deformation and thus which result leads in high to strainstrain hardeninghardeningLow ofanglebut the low grain surface. ductility boundaries They and havetoughness. are poor usually grainIn contrast,induc boundaryed aduring few sliding, low plastic angle and deformation grain thus resultboundaries which in high (i.e., leads strain low to hardeninganglestrain twist hardening but or lowtilt ofgrain ductility the surface.boundaries and They toughness. with have aligned poor In contrast, grai or nscrew boundary a few dislocations) low sliding, angle anddo grain not thus boundaries effectively result in high (i.e.,resist strain low the angledislocationhardening twist ormovement,but tilt low grain ductility hence, boundaries and they toughness. display with aligned lower In contrast, orstrengthening screw a dislocations)few low [78]. angle do grain not eboundariesffectively resist (i.e., thelow dislocationangle twist movement, or tilt grain hence, boundaries they display with aligned lower strengthening or screw dislocations) [78]. do not effectively resist the 3.2.3.dislocation Coherent movement, and Incoherent hence, theyGrain display Boundaries lower strengthening [78]. 3.2.3. Coherent and Incoherent Grain Boundaries Coherent grain boundaries with atom positions on either side that are in proper contact, as in 3.2.3. Coherent and Incoherent Grain Boundaries FigureCoherent 9, can be grain generated boundaries through with physical atom positionsand chemical on eitherprocesses side such that as are electrodeposition, in proper contact, sputter as in Figuredeposition,Coherent9, can plastic be generatedgrain deformation, boundaries through recrystallization,with physical atom and positions chemical and onphase processes either transformation. side such that as are electrodeposition, inThese proper boundaries contact, sputter playas in deposition,aFigure major 9, role can plastic in be the generated deformation,formation through of recrystallization,a dense physical network and chem andof hardical phase processesprecipitates. transformation. such For as exelectrodeposition, Theseample, boundaries the precipitates sputter play aγdeposition, major, γ’, roleand inplastic intermetallics the formation deformation, in of Ni a recrystallization, densealloys network are intended of and hard phasefor precipitates. strengthening transformation. For at example, elevated These theboundaries temperatures. precipitates play γSeverala, majorγ’, and authors role intermetallics in the[70,79,80] formation inhave Ni of reported alloysa dense are thatnetwork intended nanoscale of hard for coherent strengtheningprecipitates. boundaries For at ex elevatedareample, stable the temperatures. and precipitates generate Severalstrengtheningγ, γ’ authors, and intermetallicseffects [70,79 ,while80] have maintainingin reportedNi alloys thatthe are toughness nanoscale intended and coherentfor ductility.strengthening boundaries at areelevated stable andtemperatures. generate strengtheningSeveral authors eff ects[70,79,80] while have maintaining reported the that toughness nanoscale and coherent ductility. boundaries are stable and generate strengthening effects while maintaining the toughness and ductility.

Figure 9. Schematic representation of interface boundaries (a) incoherent, (b) semi-coherent, and (c) Figure 9. Schematic representation of interface boundaries (a) incoherent, (b) semi-coherent, and (c) coherent (redrawn based on [77]). coherent (redrawn based on [77]). Furthermore,Figure 9. Schematic Seita etrepresentation al. [81] conducted of interface a study boundaries on the e ﬀ(aect) incoherent, of hydrogen (b) semi-coherent, on fracture damage and (c) and foundcoherent that coherent (redrawn twin based boundaries on [77]). play a dual role in strengthening. Firstly, hydrogen embrittlement

Materials 2019, 12, 2503 12 of 41 weakens the grain boundary leading to intergranular fracture (IGF) in coherent twin boundaries (CTBs). Crack initiation results from the localization of dislocations along persistent slip bands. However, hydrogen is known to enhance dislocation density in the material as compared to another deformation process. Researchers have reported that high dislocation density results in high strength and hardness, therefore, high fatigue life is anticipated but results are contrasted. Secondly, the resistance to crack propagation is governed by a completely distinct physical mechanism as the crack interacts with dislocation; it does not provide the medium to proceed through the next grain, so crack arrest takes place at the dislocation site [81]. Incoherent/semi-coherent grain boundaries do not create close crystallographic registry between grains and atoms separated by interface boundaries, as shown in Figure9. A high fraction of incoherent grain boundaries obstructs the dislocation movement, thus, it signiﬁcantly increases the strength and hardness, but it decreases the ductility of the material. This makes the material diﬃcult to deform further as there is no scope for the accommodation of dislocations. In major structural applications, internal coherent boundaries with low excess energies was introduced as the main strengthening mechanism [70].

3.3. Dislocation Generation and Pile-Ups The generation of dislocations and their movements significantly contribute to the strengthening of a material. Dislocation movement is the first aspect of fatigue crack initiation. In addition, dislocation pile-up at grain boundaries usually promotes large cracks to propagate further in the material. During plastic deformation, the energy provided to the material is converted to heat that raises the temperature of the material. A small fraction of this energy stored in the material itself causes lattice defects in the material, i.e., mostly dislocations. Grain boundaries are considered as the dense arrangement of tangled dislocations, which contribute to stored strain energy. Under external loads, dislocation movement and its segregation results in gross plastic deformation. Rupture and reformation of the interatomic bonds are the fundamental mechanisms for dislocation motion [82–87]. The nature of dislocation movement depends on the dislocation type, i.e., edge/screw. Edge dislocation can be moved by slip and climb, while screw dislocation can be moved by slip and cross slip. During the plastic deformation, movement and interaction of existing dislocations take place, which are very complex phenomena as the number of dislocations are moving on some of the slip planes in various directions. The hindrance occurs during the movements due to vacancy, grain boundary, defects, and surface irregularity. A higher critical resolved stress for dislocations is required to move further into the grains. This additional stress which is needed for the movement provides the strengthening of the material [88]. The hardness of the material is related to the dislocation density by the following equation [89]: H = H∗ + αGb √ρ (2) where, H is microhardness, H∗, α, and G are the material constants, b is burger vector, and ρ is dislocation density. Alternatively, single weak grains are primary sites which activate the dislocation source and grain boundaries for dislocation pile-ups under stress, that induce stress concentration. However, piled up dislocations (only a few hundred) are not sufficient to form macroscopically observable extrusions and intrusions. The stress concentration in weak grains relaxes when the dislocation source moves to the adjacent grains and dislocation slips continuously form the nucleus of a fatigue crack of sufficient dimension. The mechanism for the formation of a fatigue crack source and the yield process are very similar to each other. According to the Hall–Petch relationship, the yield strength is dependent on grain size of the dislocation pile-up mechanism. Yielding is first initiated from a single grain once a dislocation source becomes active and emits a dislocation loop. This dislocation loop then moves to the grain boundary, which resists further movement and results in a pile-up of dislocations. If the number Materials 20192019,, 1212,, 2503x FOR PEER REVIEW 1313 of 4141

piled up at GBs is larger than geometrically required, then slip in grain 2 occurs immediately. The of dislocations piled up at GBs is larger than geometrically required, then slip in grain 2 occurs number of geometrically necessary dislocations before yielding is expressed as: immediately. The number of geometrically necessary dislocations before yielding is expressed as: 𝛼𝜋𝜏𝑑 𝑛=απτ d (3) n = 4 𝐺𝑏s (3) 4 Gb where, 𝛼 is a constant near unity, 𝜏 is the average resolved shear stress in the slip plane, G is the where,shear modulus,α is a constant d is the near grain unity, diameter,τs is the average and b is resolved the burger shear vector stress inlength. the slip This plane, accumulationG is the shear of modulus,dislocationsd is at the the grain grain diameter, boundary and resultsb is the in burger stress vector concentration length. This and accumulation if this stress ofconcentration dislocations atis theeffective, grain boundarya source in results neighboring in stress grains concentration will be acti andvated if this to stress emit concentration a dislocation isloop eﬀective, (Figure a source10). This in neighboringstress concentration grains will in other be activated grains tostarts emit plastic a dislocation deformation loop (Figure and this 10 phenomenon). This stress concentrationextends further in otherto the grains whole starts material plastic [90]. deformation and this phenomenon extends further to the whole material [90].

Figure 10.10. Pile-up formedformed inin graingrain 11 underunder an an applied applied resolved resolved shear shear stress stressτs. τs. S2 S2 is is a a source source in in grain grain 2. A2. dashedA dashed line line marks marks the the trace trace of theof the preferred preferred slip slip plane plane in eachin each grain grain (redrawn (redrawn based based on on [90 ]).[90]).

According toto thethe dislocationdislocation theorytheory [[91–93],91–93], asas thethe particle size decreases thethe valuevalue ofof the critical shearshear stressstress forfor dislocationdislocation nucleationnucleation increasesincreases suddenly.suddenly. The critical shear stress required forfor dislocation nucleationnucleation withwith thethe approximationapproximation thatthat sourcesource sizesize isis equalequal toto particleparticle sizesize isis asas follows:follows:

2𝑎𝐺𝑏 𝜏 = 2aGb f (4) τ = 𝐷 (4) f D and that required to nucleate the partial dislocation is: and that required to nucleate the partial dislocation is: 2𝑎𝐺𝑏 𝛾 𝜏 = + (5) 2aGb𝐷 f 𝑏γ τ = + (5) f D b where, bf and bp are the burgers vectors of the full and partialp dislocations, respectively, G is the shear modulus, and γ is the stacking fault energy. The factor a is taken as 0.5 for edge dislocation and 1.5 where, bf and bp are the burgers vectors of the full and partial dislocations, respectively, G is the shear for screw dislocations. modulus, and γ is the stacking fault energy. The factor a is taken as 0.5 for edge dislocation and 1.5 for Conversely, this dislocation pile-up theory is valid for pure metals, therefore, there is an screw dislocations. alternate theory of grain boundary source for understanding the mechanism. According to the model Conversely, this dislocation pile-up theory is valid for pure metals, therefore, there is an alternate of the grain boundary source, a grain boundary acts as the source of dislocation and the capacity to theory of grain boundary source for understanding the mechanism. According to the model of the emit the dislocation depends on the character of the grain boundary. This theory is still under grain boundary source, a grain boundary acts as the source of dislocation and the capacity to emit the investigation and further study is required for the enhanced understanding of the mechanism. dislocation depends on the character of the grain boundary. This theory is still under investigation and further study is required for the enhanced understanding of the mechanism. 3.4. Slip Band and Slip Planes 3.4. SlipThe Band slip andin crystal Slip Planes planes is the prominent mechanism of plastic deformation, which involves atomicThe sliding slip in on crystal different planes planes is the called prominent slip plan mechanismes, as shown of plastic in Figure deformation, 11. Slip occurs which when involves the atomicapplied sliding shear onstress diﬀ valueerent planesexceeds called the critical slip planes, limit asfor shown the atomic in Figure movement 11. Slip [94]. occurs Coherent when the slip applied band shearformation stress takes value place exceeds at grain the critical boundaries limit for because the atomic of a movementpile-up of [dislocations94]. Coherent at slipthe bandsite [36]. formation These takesobstacles place generate at grain microstress boundaries concentration because of a or pile-up discontinuity of dislocations on the surface, at the site encouraging [36]. These nucleation obstacles of fatigue cracks at the stress raised site. Furthermore, cyclic loading that causes dislocations to move back and forth results in weakening of slip bands. This repetitive process weakens the point where it

Materials 2019, 12, 2503 14 of 41 generate microstress concentration or discontinuity on the surface, encouraging nucleation of fatigue Materials 2019, 12, x FOR PEER REVIEW 14 of 41 cracks at the stress raised site. Furthermore, cyclic loading that causes dislocations to move back and forth results in weakening of slip bands. This repetitive process weakens the point where it is fractured, is fractured, i.e., shear decohesion which yields crack initiation [95]. Thus, slip plane characteristics, i.e., shear decohesion which yields crack initiation [95]. Thus, slip plane characteristics, pile-ups, leads pile-ups, leads to the slip band crack initiation and propagation. to the slip band crack initiation and propagation.

Figure 11. Differentiation between slip (a) and twin (b) interface (adapted from DoITPoMS/University Figureof Cambridge). 11. Differentiation Available between on: https: slip//www.doitpoms.ac.uk and twin interface (adapted/ldplib/shape_memory from DoITPoMS/University/background.php of , Cambridge).accessed date Available (6 August 2019)on: https://www.doitpoms [96]. .ac.uk/ldplib/shape_memory/background.php, accessed date (06 August 2019) [96]. 3.5. Strain Hardening Effects 3.5. Strain Hardening Effects Strain hardening is the strengthening of the material because of dislocation generation, multiplication,Strain hardening and their is movement the strengthening due to plastic of deformation.the material Afterbecause strain of hardening, dislocation the generation, saturation of multiplication,plastic deformation and their takes mo place,vement as it due cannot to plastic accommodate deformation. further After dislocation strain hardening, generation, the and saturation provides ofresistance plastic deformation to the deformation. takes place, This resistanceas it cannot to deformationaccommodate comes further from dislocation the obstacle generation, structure inand the providesmaterial, resistance which controls to the the deformation. movement of This the mobileresistance dislocations. to deformation Thus, it increasescomes from the yieldthe obstacle strength structure(τ), hardness, in the andmaterial, tensile which strength controls of the the material. moveme However,nt of the mobile dislocation dislocations. annihilation Thus, andit increases residual thestress yield relaxation strength takes (𝜏 place, hardness, due to atomicand tensile vibrations strength at the of higher the internalmaterial. energy However, generated dislocation due to a annihilationhigh temperature and residual condition stress [97 relaxation–101]. Hence, takes the place magnitude due to atomic and rate vibrations of stress relaxationat the higher depends internal on energythe degree generated of strain due hardening to a high developed temperature in thecondit materialion [97–101]. [102]. Hence, the magnitude and rate of stress Strainrelaxation hardening depends is on also the degree responsible of strain for hardening enhanced developed resistance in tothe crackmaterial initiation [102]. due to surfaceStrain/subsurface hardening strengthening. is also responsible On the other for hand,enhanced it causes resistance lower crackto crack growth initiation resistance due due to to surface/subsurfacematerial embrittlement strengthening. [103], i.e., lowerOn the ductility other hand, in aluminum. it causes Strainlower hardeningcrack growth or work resistance strengthening due to materialis the significance embrittlement of dislocation [103], i.e., arrest lower by grainductility boundary in aluminum. or any other Strain obstacle. hardening As reported or work by strengtheningGuechichi et al.is [the104 significance], the effect of of strain dislocation hardening arrest on by the grain fatigue boundary life improvement or any other of the obstacle. material As is reportedmore crucial by Guechichi than compressive et al. [104], residual the effect stress. of Almoststrain hardening half of the on residual the fatigue stress life is relaxed improvement during theof thefirst material few fatigue is more cycles crucial in torsion, than rotatorycompressive bending, residu andal thestress. tension-compression Almost half of the test. residual However, stress strain is relaxedhardening during does the not first change few fatigu appreciablye cycles andin torsion, is relatively rotatory more be stablending, evenand the at elevated tension-compression temperatures. test.Overall, However, there isstrain ambiguity hardening over does the dominance not change of appreciably compressive and residual is relatively stresses, more strain stable hardening even at or elevatedplastic deformation temperatures. on Overall, the fatigue there performance. is ambiguity It is over believed the dominance that the improvement of compressive is attributed residual to stresses,complex strain interactions hardening of all or factors plastic such deformation as material on properties, the fatigue microstructure, performance. It and is believed topography that [ 105the]. improvement is attributed to complex interactions of all factors such as material properties, microstructure, and topography [105].

3.6. Compressive Residual Stress Distribution

Materials 2019, 12, 2503 15 of 41

3.6. Compressive Residual Stress Distribution The generation of the compressive residual stress (CRS) fields in the surface or subsurface layers of the material are well known for fatigue life improvement. Strain hardening, CRS, surface Materials 2019, 12, x FOR PEER REVIEW 15 of 41 finish, and phase composition are the influencing factors for the fatigue failure mechanism [106]. The crackThe initiation generation could of the be compressive due to tensile residual stress stress present (CRS) on fields the surfacein the surface or subsurface, or subsurface and layers therefore the introductionof the material of are CRS well can known compensate for fatigue for life these im adverseprovement. eff ectsStrain as hardenin both areg, opposite CRS, surface in nature. finish, CRS resistsand the phase surface composition crack initiation are the influencing and could factors even force for the subsurface fatigue failure initiation, mechanism i.e., the[106]. tensile The crack region in the subsurfaceinitiation could [107 ].be It due has to been tensile reported stress thatpresen shallowt on the residual surface stressor subsurface, field is enoughand therefore to impede the the introduction of CRS can compensate for these adverse effects as both are opposite in nature. CRS crack initiation from the surface and higher fatigue endurance can be achieved if the crack source is resists the surface crack initiation and could even force subsurface initiation, i.e., the tensile region in under the strain hardening zone [103]. CRS also plays an important role in impeding the crack growth, the subsurface [107]. It has been reported that shallow residual stress field is enough to impede the as thecrack compressive initiation from stress the field surface closes and the higher crack fatigu tip toe endurance propagate can further be achieved [108]. However,if the crack the source individual is dominanceunder the of magnitudestrain hardening and depth zone of[103]. CRS CRS on fatiguealso plays crack an initiationimportant androle propagationin impeding isthe not crack yet fully understoodgrowth, andas the needs compressive further stress investigation. field closes the crack tip to propagate further [108]. However, the Atindividual elevated dominance temperatures of magnitude (T > 0.4 and Tm depth), the of stress CRS on relaxation fatigue crack is a initiation major concern and propagation that eliminates is the beneficialnot yet fully eff ectunderstood of compressive and needs stress further induced investigation. in the material due to material annealing. Overall, the beneficialAt elevated effect ontemperatures fatigue life (T is > pronounced0.4 Tm), the stress due relaxation to the complex is a major interactions concern that of eliminates strain hardening, the hardness,beneficial compressive effect of compressive residual stress, stress andinduced dense in dislocationthe material due structure to material [109 ].annealing. Overall, the beneficial effect on fatigue life is pronounced due to the complex interactions of strain hardening, 4. Severehardness, Plastic compressive Deformation residual Techniques stress, and fordense Bulk dislocation structure [109].

Microstructural4. Severe Plastic Deformation tailoring during Techniques bulk material for Bulk fabrication can provide superior properties [110] as these areMicrostructural nanostructural tailoring features during such bulk as material nanograins, fabrication nanotwins, can provide and supe nanoclustersrior properties [111 [110]]. In this section,as these different are nanostructural methods for generatingfeatures such tailored as nano microstructuresgrains, nanotwins, are and discussed. nanoclusters [111]. In this section, different methods for generating tailored microstructures are discussed. 4.1. Equal Channel Angular (ECA) Equal4.1. Equal channel Channel angularAngular (ECA) (ECA) pressing is an effective method (Figure 12a) for producing nanostructuredEqual materialchannel angular with a combination (ECA) pressing of excellent is an effective materials method and structural (Figure 12a) properties for producing [112]. A few researchersnanostructured have reported material onwith the a combination following varieties of excellent of interfacematerials and boundary: structural (1) properties low/high [112]. angle A GBs, (2) specialfew researchers and random have GBs, reported and (3)on equilibriumthe following/nonequilibrium varieties of interface GBs. boundary: Because nanograined(1) low/high angle materials are highlyGBs, (2) controlled special and by random interface GBs, structure, and (3) equi therefore,librium/nonequilibrium they display GBs. different Because material nanograined properties materials are highly controlled by interface structure, therefore, they display different material under various interfaces. Nanograins and nanotwins (Figure 12b) that are generated by a severe properties under various interfaces. Nanograins and nanotwins (Figure 12b) that are generated by a plastic deformation process improve the strength and ductility simultaneously [36]. The favorable severe plastic deformation process improve the strength and ductility simultaneously [36]. The conditionsfavorable for conditions generation for ofgeneration nanotwins of nanotwins are the following: are the following: (1) relatively (1) relatively low stackinglow stacking fault fault energy, (2) lowenergy, deformation (2) low deformation temperature, temperature, and (3) high and strain(3) high rate strain [73 rate]. [73].

FigureFigure 12. ( a12.) Schematic (a) Schematic diagram diagram of theof the equal equal channel channel angular angular (ECA) (ECA) process process andand ((bb)) BrightBright Field-TEMField- imageTEM of aimage grain of with a grain high with deformation high deformation twins tw (adaptedins (adapted with with permission permission from from [57 [57,110]),110]). .

Materials 2019, 12, 2503 16 of 41

Materials 2019, 12, x FOR PEER REVIEW 16 of 41 Ueno et al. [113] reported significant improvement in tensile strength (Figure 13a), high cycle fatigue (HCF)Ueno et (Figure al. [113] 13b), reported and fatigue significant endurance improvement limit in nanostructuredin tensile strength 316L (Figure stainless 13a), steel high without cycle compromisingfatigue (HCF) ductility. (Figure This 13b), improvement and fatigue wasendurance attributed limit to thein nanostructured high fraction of 316L nanotwins stainless and steel high stabilitywithout of twin compromising boundaries. ductility. Furthermore, This improvement Vinogadrov wa ets al. attributed [114] investigated to the high the fraction effect of of nanotwins strain path duringand thehigh ECA stability process of twin on grain boundaries. refinement Furthermore, and structural Vinogadrov features et foral. [114] Cu-Cr investigated alloys. The the grain effect shape of andstrain texture path eff ectduring on ductility the ECA andprocess strain on localizationgrain refinem wasent observed,and structural hence, features it could for influence Cu-Cr alloys. the cyclicThe loadinggrain behavior. shape and texture effect on ductility and strain localization was observed, hence, it could influence the cyclic loading behavior.

Figure 13. (a) Tensile properties and (b) Stress amplitude-number of cycles (S-N) curve for high cycle Figure 13. (a) Tensile properties and (b) Stress amplitude-number of cycles (S-N) curve for high cycle fatigue performance of 316L steel (adapted with permission from [113]). fatigue performance of 316L steel (adapted with permission from [113]). However, there are certain limitations of this process as it can be applied to only small/thin discs. However, there are certain limitations of this process as it can be applied to only small/thin discs. The components with relatively large and complex geometries are still challenging to process using The components with relatively large and complex geometries are still challenging to process using this method. this method. 4.2. High-Pressure Torsion (HPT) 4.2. High-Pressure Torsion (HPT) High-pressure torsion (HPT) (shown in Figure 14a) can generate an ultra-ﬁne crystallites High-pressure torsion (HPT) (shown in Figure 14a) can generate an ultra-fine crystallites (UFC)/nanocrystalline structure with excellent mechanical properties through combined compressive (UFC)/nanocrystalline structure with excellent mechanical properties through combined forcecompressive and torsional force straining and torsional [115]. Thisstraining is attributed [115]. This to ais large attributed fraction to ofa large high anglefraction grain of high boundaries angle withgrain high boundaries internal stress with inhigh Ti-6Al-4V internal stress [116]. in Zhilyaev Ti-6Al-4V et [116]. al. [117 Zhilyaev] reported et al. the [117] high reported fraction the of high high

Materials 2019, 12, 2503 17 of 41

Materials 2019, 12, x FOR PEER REVIEW 17 of 41 angleMaterials grain 2019 boundaries, 12, x FOR PEER (HAGBs) REVIEW (~68.1%) as compared with ECP (~60%) for nickel alloy17 of with 41 an fraction of high angle grain boundaries (HAGBs) (~68.1%) as compared with ECP (~60%) for nickel average grain size of ~0.27 µm. alloy with an average grain size of ~0.27 µm. However,fraction of high Zhilyaev angle etgrain al. boundaries [118] reported (HAGBs) signiﬁcant (~68.1%) grain as compared reﬁnement with andECP hardness(~60%) for incrementnickel alloyHowever, with an average Zhilyaev grain et al. size [118] of ~0.27reported µm. signific ant grain refinement and hardness increment in in pure nickel. A homogeneous microstructure with a large fraction of low angle grain boundaries pure However,nickel. A homogeneousZhilyaev et al. [118]micros reportedtructure signific with aant large grain fraction refinement of low and angle hardness grain incrementboundaries in (LAGBs), twins, special boundaries, and a small fraction of HAGBs was observed. Interestingly, grain (LAGBs),pure nickel. twins, A homogeneousspecial boundaries, micros andtructure a small with fraction a large of HAGBs fraction was of observ low angleed. Interestingly, grain boundaries grain size,size, interface(LAGBs), interface twins, boundaries, boundaries, special boundaries, and and material material and properties properties a small frac varied variedtion of from HAGBs from the the wasedge edge observ (Figure (Figureed. 14c) Interestingly, 14 toc) the to center the grain center of of the circularthesize, circular interface disc disc (Figure boundaries, (Figure 14b). 14b). Theand The material average average properties grain grain sizesize varied at the the centerfrom center the was was edge ~0.8µm ~0.8 (Figureµ andm and 14c)~1.2µm ~1.2to the atµm thecenter at edge the of edge as reportedasthe reported circular byXu bydisc Xu et (Figureal. et al. [119 [119] 14b).] for for The aluminum aluminum average grain (Al).(Al). Highsize High at dislocations dislocationsthe center was can can ~0.8µm be beobserved observed and ~1.2µm through through at the the TEM edge the TEM micrographsmicrographsas reported (Figure by (Figure Xu 14 etb,c) al.14b,c) [119] which which for are aluminum are not not widely widely (Al). separated High separated dislocations due due to to highcan high be stacking observedstacking fault faultthrough energy energy the andTEMand there there is easily cross slip. is easilymicrographs cross slip. (Figure 14b,c) which are not widely separated due to high stacking fault energy and there is easily cross slip.

FigureFigure 14. (14.a) Schematic(a) Schematic diagram diagram ofof thethe hi high-pressuregh-pressure torsion torsion (HPT) (HPT) proc process,ess, and TEM and micrograph TEM micrograph of of aluminumaluminumFigure 14. (Al) ((Al)a) Schematic disc at at 1.25 1.25 diagram GPa, GPa, ( bof) ( batthe) the at hi gh-pressure thecenter, center, and (torsion andc) at (thec )(HPT) atedge the proc(adapted edgeess, (adaptedand with TEM permission micrograph with permission from of from[57,117])aluminum [57,117]).. (Al) disc at 1.25 GPa, (b) at the center, and (c) at the edge (adapted with permission from [57,117]). ValievValiev et al. et [ 57al.] [57] used used HPT HPT to processto process pure pure Al Al (99.9%) (99.9%) disc disc of of 10–20 10–20 mm mm diameterdiameter and 0.2–0.5 mm thicknessmm thicknessValiev under et 6 under GPaal. [57] pressure. 6 used GPa HPTpressure. This to process produced This producedpure a Al nanocrystalline (99.9%) a nanocrystalline disc of structure10–20 structure mm withdiameter with deformation deformation and 0.2–0.5 twins, twins, nonequilibrium grain boundaries, dislocations, and solute segregation. Vickers hardness and nonequilibriummm thickness grain under boundaries, 6 GPa pressure. dislocations, This produced and solute a nanocrystalline segregation. structure Vickers with hardness deformation and tensile tensile strength of the material significantly increased. strengthtwins, of thenonequilibrium material signiﬁcantly grain boundaries, increased. dislocations, and solute segregation. Vickers hardness and tensileCommercially strength of thepure materi copperal significantly processed byincreased. HPT sh owed considerable improvement in fatigue Commercially pure copper processed by HPT showed considerable improvement in fatigue performanceCommercially of the purematerial copper as comparedprocessed toby thHPTe ECA showed process. considerable The nanograins, improvement nanotwins, in fatigue and performance of the material as compared to the ECA process. The nanograins, nanotwins, and dislocationperformance density of the havematerial more as fractioncompared of toth eth componentse ECA process. generated The nanograins, by HPT (Figurenanotwins, 15a) andas dislocationcompareddislocation density to densitythe have ECA have moreprocess more fraction (Figure fraction of 15b). the of However, components the components the generatedmicrostructure generated byHPT varied by (FigureHPT across (Figure 15thea) diameter as15a) compared as to theofcompared ECA the disk process toprocessed the (Figure ECA by process 15HPT.b). (FigureThe However, uniaxial 15b). the However,tensil microstructuree stress-strain the microstructure variedvariations across varied for the the across samples diameter the diameterbetween of the disk processedtheseof the two bydisk HPT.processes processed The show uniaxial by HPT.that HPT tensileThe showeduniaxial stress-strain a tensil remarkablee stress-strain variations improvement variations for the in samplesstrength for the samples(Figure between 16a)between these and two processesfatiguethese showtwo life processes(Figure that HPT 16b). show showed that HPT a remarkable showed a remarkable improvement improvement in strength in strength (Figure 16(Figurea) and 16a) fatigue and life (Figurefatigue 16b). life (Figure 16b).

Figure 15. TEM micrographs of (a) copper processed by HPT (5 turns at 6 GPa) and (b) processed by ECA (12 passes) (adapted with permission from [57]). Materials 2019, 12, x FOR PEER REVIEW 18 of 41

Figure 15. TEM micrographs of (a) copper processed by HPT (5 turns at 6 GPa) and (b) processed by MaterialsECA2019, (1212, 2503passes) (adapted with permission from [57]). 18 of 41

Figure 16. (a) Stress-strain curve and (b) S-N curve for HPT and ECP processes in copper (adapted withFigure permission 16. (a) Stress-strain from [120]). curve and (b) S-N curve for HPT and ECP processes in copper (adapted with permission from [120]). In conclusion, the microstructural architecture, such as bimodal grain structure/UFC structure with highIn conclusion, angle and the nonequilibrium microstructura grainl architecture, boundaries, suchcan as bimoda providel grain improved structure/UFC material structure properties suchwith as high strength, angle hardness, and nonequilibri ductility,um fatigue, grain boundaries, and wear resistance can provide [121 improved–123]. The material slip band properties formation, such as strength, hardness, ductility, fatigue, and wear resistance [121–123]. The slip band formation, substructure, and deformation phenomenon are also observed in copper during the ultrasonic vibration substructure, and deformation phenomenon are also observed in copper during the ultrasonic energy and the thermosonic energy bonding process which are being used in electronic applications. vibration energy and the thermosonic energy bonding process which are being used in electronic The wire bonding strength has been discussed based on the deformation mechanisms [124,125]. applications. The wire bonding strength has been discussed based on the deformation mechanisms These could be a topic of great interest to several researchers and industries for developing [124,125]. These could be a topic of great interest to several researchers and industries for developing next-generationnext-generation components. components. 5. Surface Modification Techniques 5. Surface Modification Techniques TheThe surface surface interacts interacts with with the the surrounding surrounding environmentenvironment and and loads. loads. Hence, Hence, it itis ismore more likely likely to to deterioratedeteriorate over over time, time, for for example, example, fretting,fretting, fatigue,fatigue, corrosion, wear, wear, and and creep creep [126]. [126 ].The The surface surface characteristicscharacteristics of of engineering engineering materials materials have have a significanta significant eff effectect on on the the serviceability serviceability and and component component life. Thus,life.it Thus, cannot itbe cannot neglected be neglected in design. in Surface design. modifications Surface modifications involve protective involve coating, protective cladding, coating, heat treatmentcladding, (e.g., heat nitriding treatment and (e.g., carburizing), nitriding and and carbur surfaceizing), mechanical and surface treatments. mechanical Surface treatments. coating Surface [127,128 ] andcoating deposition [127,128] methods and deposition are not eff ectivemethods for alteringare not effective the microstructure for altering driven the microstructure fatigue failure driven and are notfatigue discussed failure here and (readers are not discussed can refer here elsewhere (readers [129 can– 139refer] forelsewhere these processes). [129–139] for Surface these processes). mechanical treatmentsSurface mechanical are the focus treatments of this discourse. are the focus of this discourse. SurfaceSurface mechanical mechanical treatment treatment is ais method a method used used to alterto alter the the surface surface or subsurfaceor subsurface characteristics characteristics such assuch morphology, as morphology, microstructure, microstructu andre, materials and materials properties properties through through plastic plastic deformation. deformation. Modern Modern turbine bladesturbine of jetblades engines of arejet subjectedengines are to thesesubjected surface to treatmentsthese surface for improvedtreatments fatigue for improved performance fatigue [140 ]. Theperformance responsible [140]. mechanisms The responsibl for plastice mechanisms deformation for plastic are thedeformat slipping,ion are twinning, the slipping, and twinning, dislocation generation,and dislocation which generation, were described which in were the previousdescribed section.in the previous section. SurfaceSurface modifications modifications usually usually generate generate thethe high-density nanocrystalline nanocrystalline structure structure at atthe the surface surface thatthat attributes attributes to to the the strengthening. strengthening. ThisThis strengtheningstrengthening is is the the sum sum of of contributions contributions from from dislocation dislocation strengthening and boundary strengthening, apart from frictional stress, as given by [32]. strengthening and boundary strengthening, apart from frictional stress, as given by [32]. σ = σo + σρ + σb (6) σ = σo + σρ + σb (6)

where, σo is the friction stress, σρ is the forest hardening, and σb is the grain boundary hardening. where, Theσo is dislocation the friction in stress, the lowσρ isangle the forestboundaries hardening, and the and volumeσb is the between grain boundarythe boundaries hardening. causes forestThe hardening. dislocation Grain in the boundary low angle hardening boundaries is caus anded the by volume the high between angle boundaries, the boundaries which causes is taken forest hardening.to be inversely Grain proportional boundary hardening to the square is causedroot of byboundary the high spacing angle [141]. boundaries, which is taken to be inversely proportional to the square root of boundary spacing [141]. Microstructural evolution can be understood by the mathematical expression which depicts the blend effect of strain rate and deformation temperature during plastic deformation which is expressed as the Zener–Hollomon parameter (Z) [67]. It does signify that the low deformation temperature Materials 2019, 12, x FOR PEER REVIEW 19 of 41 Materials 2019, 12, 2503 19 of 41 Microstructural evolution can be understood by the mathematical expression which depicts the blend effect of strain rate and deformation temperature during plastic deformation which is and high strain rate can impede the dislocation annihilation mechanism, which will result in grain expressed as the Zener–Hollomon parameter (Z) [67]. It does signify that the low deformation refinement by the formation of new grain boundaries generated through dislocations. temperature and high strain rate can impede the dislocation annihilation mechanism, which will . result in grain refinement by the formationZ of= neεw graineQ/ boundariesRT generated through dislocations. (7) × 𝑍= 𝜀 𝑒⁄ (7) where, Q is the activation energy mechanism controlling the rate of deformation, T is the absolute temperature,where, Q andis theR activationis the universal energy mechanism gas constant. controlling the rate of deformation, T is the absolute temperature, and R is the universal gas constant. It is understood that strain rate, deformation temperature, plastic deformation, and strain gradient It is understood that strain rate, deformation temperature, plastic deformation, and strain are thegradient key factors are the for key the factors surface for modifications. the surface modifi Structuralcations. refinement Structural andrefinement reduction and ofreduction grain boundary of spacinggrain govern boundary the spacing dislocation govern multiplicationthe dislocation multiplication which provides which excellent provides strengthexcellent strength and mechanical and properties,mechanical although properties, further although grain refinement further grain can lead refinement to grain can coarsening lead to andgrain dislocation coarsening annihilation and duedislocation to grain boundary annihilation migration. due to gr Therefore,ain boundary the migration. high strain Therefor ratee, shear the high deformation strain rate can shear lead to higherdeformation dislocation can multiplication lead to higher withdislocation lower anglemultiplication grain boundaries with lower andangle lower grain grain boundaries size. This and shear deformationlower grain also size. generates This shear a deformation strain gradient also genera whichtes is a important strain gradient for thewhich creation is important of geometrically for the necessarycreation dislocations of geometrically [142, 143necessary]. dislocations [142,143]. SurfaceSurface mechanical mechanical treatment treatment (SMT) (SMT) processesprocesses incl includeude shot shot peening, peening, deep deep cold cold rolling, rolling, water water jet jet cavitation peening, laser peening, and vibro peening, as shown in Figure 17, and are discussed below. cavitation peening, laser peening, and vibro peening, as shown in Figure 17, and are discussed below. Several results are reported in the nascent microstructural condition of the sample, but in the post Severalsurface results treatment, are reported microstructural in the nascent evolution microstructural evidence is condition still limited. of the This sample, means but microstructural in the post surface treatment,evolution microstructural after surface treatment evolution and evidence governing is still mech limited.anism of This fatigue means crack microstructural damage requires evolution further after surfaceinvestigation. treatment and governing mechanism of fatigue crack damage requires further investigation.

FigureFigure 17. 17.Surface Surfacemechanical mechanical treatments: treatments: (a) (shota) shot peening peening (SP), (SP),(b) deep (b) cold deep rolling cold (DCR), rolling (c (DCR),) (c) ultrasonicultrasonic shot shot peening peening (USSP), (USSP), (d) laser (d) lasershock shockpeening peening (LSP), (e (LSP),) water jet (e) cavitation water jet peening cavitation (WJCP), peening (WJCP),and ( andf) vibro (f) vibropeening peening (VP) (a (VP)–e) adapted (a–e) adapted with permission with permission from [144,145]) from. [144,145]).

5.1. Shot5.1. Shot Peening Peening (SP) (SP) ShotShot peening peening (SP) (SP) is a surfaceis a surface mechanical mechanical treatment treatment method method in whichin which bombarding bombarding of of high-velocity high- shotvelocity media generates shot media plastic generates deformation plastic deformation and induces and compressiveinduces compressive residual residual stress andstress strain and strain hardening in thehardening material. in It the could material. improve It could the fatigueimprove life the significantly fatigue life significantly by impeding by theimpeding crack formationthe crack and formation and propagation in the surface/subsurface [144]. However, several microstructural propagation in the surface/subsurface [144]. However, several microstructural features also contribute features also contribute to the fatigue life improvement. to the fatigueSeveral life researchers improvement. [10,107,146–152] are working on its effect on material properties, residual stress,Several strain researchers hardening [ 10effect,,107 ,dislocation146–152] are density working, slip planes, on its and effect grain on boundary material properties,behavior. There residual stress,are strain few results hardening reported effect, for dislocationthe quantification density, of strain slip planes, hardening and effect, grain grain boundary size distribution, behavior. Thereand are fewinterface results reported boundaries. for Song the et quantification al. [153] reported of the strain fraction hardening of interface eff ect,bounda grainries sizeand its distribution, correlation and interfacewith boundaries.shot peening intensity Song et al.for [titanium153] reported alloys (T theC21). fraction In this of work, interface the EBSD boundaries technique and was its used correlation to withquantify shot peening the fraction intensity of low for angle titanium (θ < 15°) alloys and (TC21).high angle In grain this work,boundaries the EBSD(θ ≥ 15°) technique based on was the used to quantifymisorientation the fraction angle. ofThe low fraction angle of ( θlow< 15angle) and grain high boundaries angle grain was 0.59 boundaries and the fraction (θ 15 of) high based on ◦ ≥ ◦ the misorientationangle grain boundary angle. was The 0.38 fraction from a 100 of lowµm depth angle from grain the boundaries surface. Deep was in the 0.59 material and the (100–360 fraction of µm), the fraction of low angle grain boundary was 0.262, and the high angle grain boundary fraction high angle grain boundary was 0.38 from a 100 µm depth from the surface. Deep in the material was 0.72. The dislocation networks were composed of dislocation bands and slip bands in α phase of (100–360 µm), the fraction of low angle grain boundary was 0.262, and the high angle grain boundary the material. There were no twins and stacking faults observed in the surface layer [153]. Shekhar et fractional. [154] was studied 0.72. Thethe behavior dislocation of strain networks rate on werethe interface composed boundaries of dislocation and refinement bands and and quantified slip bands in α phase of the material. There were no twins and stacking faults observed in the surface layer [153].

Shekhar et al. [154] studied the behavior of strain rate on the interface boundaries and refinement and quantified the volume fraction of low angle and high angle grain boundaries. The advanced characterization, such as EBSD/TEM characterization, can distinguish the fraction of twin boundaries and atomic misorientation or kernel average misorientation [155]. Materials 2019, 12, x FOR PEER REVIEW 20 of 41 Materials 2019, 12, x FOR PEER REVIEW 20 of 41 the volume fraction of low angle and high angle grain boundaries. The advanced characterization, the volume fraction of low angle and high angle grain boundaries. The advanced characterization, Materialssuch as2019 EBSD/TEM, 12, 2503 characterization, can distinguish the fraction of twin boundaries and atomic20 of 41 suchmisorientation as EBSD/TEM or kernel characterization, average misorientation can distinguis [155].h the fraction of twin boundaries and atomic misorientation or kernel average misorientation [155]. Lainé et al. [156] studied the behavior of dislocation structure, deformation twinning, and crystallographicLainéLainé etet al. al. [156]effects [156] studied studiedon metallic the the shotbeha behavior peenedvior of of Ti-6Al-4V,dislocation as structur structure, showne, in deformationFigure 18, and twinning, twinning, reported and andthat crystallographiccrystallographicMSP generates longeffects effects wavy on on metallic metallictangled shot shotdislocation peened peened structuresTi-6Al-4V, Ti-6Al-4V, andas as shown shown shear inbands in Figure Figure in 18,the18, and andmaterial reported reported due that thatto a MSPMSPhigh-accumulated generates generates long long strain wavy wavy and tangled tangled strain dislocation dislocationrate (104 s −structures1 structures). The tangled and and shear sheardislocation bands bands structurein in the the material material was due due due to to tothe a a high-accumulated strain and strain rate (104 s−11). The tangled dislocation structure was due to the high-accumulatedlocalized work hardening strain and from strain multiple rate subsequent (10 s− ). The shot tangled impacts. dislocation The plastic structure deformation was duewas toin thethe localizedlocalizedform of deformation work work hardening hardening twins from (Figure from multiple multiple 18a,b) subsequent because subsequent it was shot shot very impacts. impacts. sensitive The The toplastic grain plastic deformation size, deformation which waswas was higherin the in formthefor formlarge of deformation ofgrain deformation size. twinsFurthermore, twins (Figure (Figure 18a,b) the depth18 becausea,b) of because stit rainwas ithardeningvery was sensitive very was sensitive to quantified grain to size, grain throughwhich size, was which the higher grain was forhigherorientation large for grain large spread size. grain (GOS) Furthermore, size. plot Furthermore, in EBSD, the depth as shown the of depth st inrain Figure of hardening strain 18c, hardening which was was quantified was 70 µm quantified from through the through top the surface. grain the orientationgrainA correlation orientation spread was spread(GOS)also reported plot (GOS) in EBSD, between plot in as EBSD, shown residual as in shownstressFigure depth 18c, in Figure which generated 18 wasc, which70 through µm wasfrom the 70the GOSµ topm fromand surface. hole the Atopdrilling correlation surface. method. Awas correlation Altenbergeralso reported was et alsobetween al. [157] reported residualalso betweeninvestigated stress residualdepth the generated effect stress of depth near-surface through generated the microstructureGOS through and hole the drillingGOSgenerated and method. holeby shot drilling Altenberger peening method. on et the al. Altenberger performance[157] also etinvestigated al.of [stainless157] also the steel investigated effect (AISI of near-surface304). the A e ffcomplexect ofmicrostructure near-surface subsurface generatedmicrostructuremicrostructure, by shot generated consisting peening by on shotof thenanocrystalline peening performance on the of performancelayers, stainless deformation steel of stainless (AISI bands,304). steel A (AISI andcomplex 304).strain subsurface A complexinduced microstructure,subsurfacemartensitic microstructure, twin consisting lamellae consisting withof nanocrystalline high of dislocation nanocrystalline layers, densities layers,deformation in deformation the austenitic bands, bands, matrixand and strain strainwas observedinduced induced martensiticmartensitic(Figure 19). twin twinNanocrystalline lamellae lamellae with with surface high high layersdislocation dislocation provided densities densities stability in in theagainst the austenitic austenitic cyclic loading,matrix matrix was waseven observed at high- (Figure(Figurestress amplitudes, 1919).). NanocrystallineNanocrystalline and impeded surface surface disl layers layersocation provided provided movement stability stability and against slip against band cyclic cyclic formation. loading, loading, even Interestingly, even at high-stress at high- the stressamplitudes,depth amplitudes, rather andthan impeded theand intensity impeded dislocation of disl stainocation movementhardening movement was and found slip and band slipbeneficial formation.band formation.for fatigue Interestingly, lifeInterestingly, improvement. the depth the depthratherLater, ratherZhan than the etthan al. intensity the[158] intensity investigated of stain of hardeningstain the hardening effect was of foundgradient was found beneficial microstructure beneficial for fatigue for generated fatigue life improvement. life by improvement. shot peening Later, Later,Zhanof steel etZhan (S30432). al. [158et al.] investigated [158]The microstrai investigated then, e ffmicrohardness, ectthe ofeffect gradient of gradient microstructureand domain microstructure size generated varied generated along by shot the by peeningdepth shot peeningand of steelwas of(S30432).observed steel (S30432). Thethrough microstrain, The X-Ray microstrai diffraction microhardness,n, microhardness, method. and domain and sizedomain varied size along varied the along depth the and depth was observedand was observedthrough X-raythrough diff X-Rayraction diffraction method. method.

Figure 18.18. TheThe electronelectron backscattered backscattered di ﬀdiffractionraction (EBSD) (EBSD) image image of treated of treated surface surface (a) inverse (a) inverse pole ﬁgure pole Figure 18. The electron backscattered diffraction (EBSD) image of treated surface (a) inverse pole image,figure image, (b) band (b) contrast, band contrast, and (c) and grain (c) orientation grain orientation spread spread (adapted (adapted from [156 from]). [156]). figure image, (b) band contrast, and (c) grain orientation spread (adapted from [156]).

Figure 19. Bright ﬁeld-TEM micrographs of shot peened area showing (a–c) dislocation structures (adapted from [156]). Materials 2019, 12, 2503 21 of 41

It is understood that induction of compressive residual stress, strain hardening, dislocation generation, and grain refinement are beneficial after shot peening. Unfortunately, the benefits of shot peening may be reduced or even completely eliminated at a high operating temperature (T > 0.4 Tm). Kim et al. [159] reported stress relaxation at an elevated temperature and isothermal fatigue reduced up to 50% at 650 ◦C to 725 ◦C for Udimat 720 Ni-based alloy. This relaxation occurs due to dislocation and diffusive movement of atoms that reduce the underlying misfit. At a short timescale at 350◦, the mechanism of stress relaxation was due to the accelerated kinetics at the high stored energy level. At longer times, this mechanism was due to creep-related phenomena as it significantly reduces the work hardened zone. Interestingly, the effect of thermal exposure was very strong on the residual stress rather than depth of cold work in the measured temperature range. On the surface, the percentage cold work decreased instantly as the temperature increased beyond 650 ◦C[160], which shows the temperature dependence of mechanical properties [161]. Shot peening has been demonstrated as a fatigue life improvement process as reported by several researchers for steel [8,162–166], aluminum [9,11,14,167–171], magnesium [7,172], nickel [97,147,173–175], and titanium alloys [176–188] and as summarized or quantified in Table1.

Table 1. Comparison of fatigue life enhancement by various microstructural features.

Material SP Parameters Microstructural Features Fatigue Life 10 percent surface pitting Intensity: 0.18–0.23 mm A, coverage: 40% increment in CRS for SP gear at fatigue life increased by AISI 9310 Steel [189] 200%, media: cast steel 070 depth of maximum shear stress 1.6 times than standard gear (unpeened) 6–7 times increment after 3.75 times higher RS ( 1530 MPa) up Bearing steel Media: 0.3 mm diameter, − treatment in bending to 90 µm, 1.18 times higher Vickers (JIS-SUJ2) [190] coverage:300% coverage fatigue, 0.3% in ultimate hardness (1019 HV) tensile strength The maximum measured stress of The decrease in 30% 51CrV4 high-grade Media: 0.8–0.9 mm diameter shots, 1100 MPa arises at a distance of sustainable stress spring steel alloy [56] 0.3% intensity, 100% coverage 210 mm measured from the amplitude due to high specimen’s middle point surface roughness Intensity: 15 A (CSP),100%,0.42 mm diameter 0.3–0.35 mm thick strain hardening 4% increment after SSP 7 C (SSP), 1500% coverage, 0.58 mm depth, 0.65 (CSP), 1.38 mm (SSP), Low alloy steel [191] and 10% after repeening diameter, (60–61 HRC) 580 MPa in both, observed NC − SSP on fatigue strength 10 N (RSSP),100%, 0.10 mm nanocrystalline layer using TEM diameter CRS: (1100 S, 1400 M) MPa, up to 200 µm depth Dwell fatigue relax the Stress relaxation: (400 S, 900 M) MPa, compressive residual Nickel-based superalloy Intensity:6–8 A, media: 110 H, up to 200 µm depth, peak shifted from stress and strain RR1000 [97] coverage: 200% 50 to 75 µm hardening in the material SH: strain hardening depth in the ﬁrst few cycles (100–125 µm), 21% relaxation in percentage hardening CRS: (826 S, 1094 M) MPa up to depth Nickel-based superalloy Intensity: 4–5 A, media: 110H, of 140 µm, 30% increment in Vickers - Udimet 720Li [23] coverage: 125% microhardness Intensity: range [0.0027 A (8 psi), 0.0063 A (13 psi), 0.0083 A (18 psi), 9–12% increment in AISI 4340 steel [162] CRS:1200 MPa up to 0.175 mm depth 0.0141 A (45 psi)], coverage: 200%, fatigue life media: S 230 (0.7 mm diameter)

In conclusion, there is still limited knowledge with respect to the correlation of fatigue performance and microstructural features which require further investigation to engineer the material with optimum microstructural features. This work could extend to explore other surface enhancement techniques for optimum microstructural features development. Materials 2019, 12, 2503 22 of 41

5.2. Vibro Peening (VP) Vibro peening (VP) [6] is a hybrid technique of shot peening and vibro polishing, which generates a peened and polished surface as compared to other available techniques such as shot peening or laser peening. The use of an unbalanced motor vibrates media bowl, allowing it to flow against the surface. The kinetic energy generated during the vibration of media deforms the surface through shear and compression. The major fraction of the kinetic energy of media transferred perpendicular to the surface (compression) induce the plastic deformation and the other fraction as shear action induce burnishing [6]. This process induces compressive residual stress deep into the material and maintains roughness on the surface. This advanced technology is cost effective and reduces overall process time because it combines both mechanical treatment and polishing. Feldmann et al. [6] reported that the low surface roughness value (Ra 0.25 µm) can be achieved ≤ by vibro peening, cold rolling, and vibro finishing, but shot peening generates the high roughness value (Ra 0.65 µm) which is beyond the acceptable range of aerospace components. However, the ≈ compressive residual stress is lower in magnitude and shallower in depth in vibro peening (maximum 800 MPa at 50 µm) as compared with shot peening (maximum 1050 MPa at 100µm), deep cold rolling (maximum 780 MPa at 150 µm), and the combination of shot peening followed by vibro finishing (maximum 1050 MPa at 45 µm). Corresponding to these values, the HCF life of components has been increased by 32% in deep cold rolling, 35% in vibro peening, 61% in shot peening, and 66% in a combination of shot peening and vibro finishing in the IN718 HPC blisk aerofoils [5]. Ardi et al. [20] investigated the effect on residual stress, cold work, and surface integrity on RR1000 and found that a desirable compressive stress with low cold work could be generated for optimizing the fatigue performance of the components. The quantification of the depth of strain hardening was done by grain orientation spread (GOS) using an electron backscattered diffraction (EBSD) technique. Kumar et al. [23] reported on a comparative microstructural and mechanical property study between shot peening and vibro peening of nickel-based superalloys when subjected to the same peening intensity and observed that vibro peening generates deeper compressive residual stress as compared to shot peening at the same peening intensity (4–5 A) with good thermal stability at an elevated temperature. The CRS, microhardness, full-width half maxima (FWHM), plastic strain, and GOS profiles are compared in shot peening and vibro peening, as shown in Figure 20. The elevated temperature fatigue, corrosion fatigue, and sulphidation experiments have not been explored. Therefore, further investigation on the microstructure of surface and high-temperature fatigue is required for a comprehensive understanding of the process. Materials 2019, 12, 2503 23 of 41 Materials 2019, 12, x FOR PEER REVIEW 23 of 41

Figure 20.20. A comparison of residual stress, microhardness,microhardness, full-widthfull-width half maximamaxima (FWHM), plastic strain, andand graingrain orientationorientation spreadspread (GOS).(GOS). ((aa)) SPSP andand ((bb)) VPVP (adapted(adapted withwith permissionpermission fromfrom [[23]).23]). 5.3. Deep Cold Rolling (DCR) 5.3. Deep Cold Rolling (DCR) In deep cold rolling (DCR), a hydrostatically controlled ball is pressed and rolled against the In deep cold rolling (DCR), a hydrostatically controlled ball is pressed and rolled against the surface in order to induce severe plastic deformation in the subsurface region [192]. The magnitude surface in order to induce severe plastic deformation in the subsurface region [192]. The magnitude of surface compressive residual stress is relatively low but deeper into the subsurface as compared of surface compressive residual stress is relatively low but deeper into the subsurface as compared to the SP process. This process is currently limited to treat mostly flat surfaces and is difficult to to the SP process. This process is currently limited to treat mostly flat surfaces and is difficult to implement on intricate geometries [17]. The combinations of high percentage cold work, higher depth implement on intricate geometries [17]. The combinations of high percentage cold work, higher depth of compressive residual stress, and lower roughness can yield a material with excellent fatigue life of compressive residual stress, and lower roughness can yield a material with excellent fatigue life which is approximately five times that of conventional treatment methods [193]. Deep cold rolling is which is approximately five times that of conventional treatment methods [193]. Deep cold rolling is highly beneficial if an elevated thermal and mechanical overload is not present. The high cold work highly beneficial if an elevated thermal and mechanical overload is not present. The high cold work generated by the deep cold rolling process can relax the compressive stress rapidly. generated by the deep cold rolling process can relax the compressive stress rapidly. Wong et al. [17] conducted DCR experiments on Ti-6Al-4V with three different designs of tools to Wong et al. [17] conducted DCR experiments on Ti-6Al-4V with three different designs of tools show the feasibility of peened complex features. The maximum compressive residual stress reported to show the feasibility of peened complex features. The maximum compressive residual stress was 1275 MPa magnitude within 1 mm. The process parameters, such as feed rate and overlapping, reported was 1275 MPa magnitude within 1 mm. The process parameters, such as feed rate and played a very insignificant role on the residual stress in the material as well the residual stress was overlapping, played a very insignificant role on the residual stress in the material as well the residual higher consistently in the lateral direction. stress was higher consistently in the lateral direction. Wagner et al. [194] studied the effect of DCR on the behavior of fatigue crack initiation Wagner et al. [194] studied the effect of DCR on the behavior of fatigue crack initiation and and propagation of the aluminum, titanium, and magnesium alloys and illustrated that the cold propagation of the aluminum, titanium, and magnesium alloys and illustrated that the cold work work contributed to the retardation of crack nucleation but accelerated the propagation. However, contributed to the retardation of crack nucleation but accelerated the propagation. However, compressive residual stress had little effect on the initiation but impeded the propagation of the cracks, compressive residual stress had little effect on the initiation but impeded the propagation of the whereas, surface roughness accelerated the crack initiation due to stress concentration. Nikitin et al. [195] cracks, whereas, surface roughness accelerated the crack initiation due to stress concentration. Nikitin studied the subsurface microstructure and cyclic deformation behavior of shot peened and deep rolled et al. [195] studied the subsurface microstructure and cyclic deformation behavior of shot peened and deep rolled austenitic stainless steel AISI 304 under varying pressure and intensity. The complex

Materials 2019, 12, 2503x FOR PEER REVIEW 2424 of of 41 microstructural features included deformation bands, nanocrystalline layer, and strain induced twins afteraustenitic both stainlesssurface steeltreatments. AISI 304 Thus, under these varying featur pressurees are and highly intensity. responsible The complex for the microstructural cyclic creep deformationfeatures included behavior. deformation bands, nanocrystalline layer, and strain induced twins after both surface treatments.Nalla et Thus, al. [16] these invest featuresigated are the highly effect responsibleof DCR and forlaser the peen cyclicing creep on the deformation fatigue of the behavior. Ti-6Al-4V at a temperatureNalla et al. [ 16range] investigated of ambient the to e450ffect °C. of The DCR resi anddual laser stress peening completely on the fatiguerelaxed of at the 450 Ti-6Al-4V °C thermal at exposure,a temperature but the range benefit of ambient of the surface to 450 treatment◦C. The residual was due stress to the completely generation relaxed of work at hardening 450 ◦C thermal layer andexposure, nanocrystalline but the benefit layer of formation the surface on treatment the subsur wasface due that to theimpeded generation the driving of work force hardening for fatigue layer failure.and nanocrystalline Recently, Kumar layer formation et al. [22] on and the subsurfaceNagarajan thatet al. impeded [196,197] the drivingpresented force the for residual fatigue failure.stress distribution,Recently, Kumar microstructural et al. [22] and evol Nagarajanution, and et proposed al. [196,197 deformation] presented and the residualstrengthening stress mechanisms distribution, inducedmicrostructural during evolution, deep cold and rolling proposed of deformationnickel-based and superalloy strengthening Udimet mechanisms 720Li under induced different during hydrostaticdeep cold rolling pressure of nickel-based levels (10–50 superalloy MPa). Deformation Udimet 720Li induced under defects, different such hydrostatic as dislocation pressure cells, levelsshear (10–50bands, MPa).and their Deformation interactions, induced were defects,observed such during as dislocation the DCR cells,process, shear as bands,presented and in their Figure interactions, 21. The wereplastic observed deformation during during the DCR DCR process, was predominant as presentedly in Figuredriven 21through. The plastic the slip deformation and dislocation during multiplicationDCR was predominantly mechanism. driven through the slip and dislocation multiplication mechanism.

Figure 21. Schematic of (a) proposed microstructural evolution and strengthening mechanism during Figure 21. Schematic of (a) proposed microstructural evolution and strengthening mechanism during the DCR process and (b) eﬀect of individual microstructural feature (adapted with permission from [22]). the DCR process and (b) effect of individual microstructural feature (adapted with permission from [22]).

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5.4. Laser Shock Peening (LSP) 5.4. Laser Shock Peening (LSP) Laser shock peening (Figure 22) induces a high magnitude residual stress deep in the material Laser shock peening (Figure 22) induces a high magnitude residual stress deep in the material by maintaining the low surface roughness [198] which is beneficial for surface-related failure such as by maintaining the low surface roughness [198] which is beneficial for surface-related failure such fretting wear, fatigue, and stress corrosion cracking. High energy pulsed laser shock waves have been as fretting wear, fatigue, and stress corrosion cracking. High energy pulsed laser shock waves have used for the generation of compressive residual stress. Nanosecond pulsed laser strikes on a been used for the generation of compressive residual stress. Nanosecond pulsed laser strikes on transparent overlay (water, oil) confine the pulse energy on the metal surface and generate the high- a transparent overlay (water, oil) confine the pulse energy on the metal surface and generate the pressure plasma. This plasma utilizes the mechanical effect of shock waves rather than thermal effects high-pressure plasma. This plasma utilizes the mechanical effect of shock waves rather than thermal to deform the target elastically and plastically. Additionally, an absorbent coating with low effects to deform the target elastically and plastically. Additionally, an absorbent coating with low impedance on the surface can further boost the intensity of shock waves by absorbing the pulse impedance on the surface can further boost the intensity of shock waves by absorbing the pulse energy energy and provides interaction with material surface directly after expanding. Elastic deformation, and provides interaction with material surface directly after expanding. Elastic deformation, which is which is under the Hugoniot elastic limit of deformation, regains its position and the rest plastic under the Hugoniot elastic limit of deformation, regains its position and the rest plastic deformation deformation causes the compressive residual stress and cold work in the material [199]. causes the compressive residual stress and cold work in the material [199].

FigureFigure 22. 22. SchematicSchematic diagram diagram of of LSP LSP (adapted (adapted with with permission permission from from [199]). [199]).

The LSP process parameters can be optimized to generate a higher volume of compressive stress The LSP process parameters can be optimized to generate a higher volume of compressive stress with constraints on the depth of their influence and on the magnitude of tensile stress [200,201]. As the with constraints on the depth of their influence and on the magnitude of tensile stress [200,201]. As shock wave propagates through the material, its magnitude decreases according to the attenuation the shock wave propagates through the material, its magnitude decreases according to the rate, which yields more compressive stress at the surface and a relatively decreased value towards the attenuation rate, which yields more compressive stress at the surface and a relatively decreased value depth. The effects of laser peening parameters, i.e., fluence, coverage, repetition rate, etc. were studied towards the depth. The effects of laser peening parameters, i.e., fluence, coverage, repetition rate, etc. by several researchers [21,202–205]. Smith et al. [206] studied the effect of power density and repetition were studied by several researchers [21,202–205]. Smith et al. [206] studied the effect of power density rate on Ti-6Al-4V. The main conclusion was that power density has no effect on the residual stress and and repetition rate on Ti-6Al-4V. The main conclusion was that power density has no effect on the cold work on the given conditions, but the number of pulses per spot has an effect on both residual residual stress and cold work on the given conditions, but the number of pulses per spot has an effect stress and cold work. The residual stress magnitude is high enough ( 650 MPa) at the surface and the on both residual stress and cold work. The residual stress magnitude− is high enough (−650 MPa) at subsurface. The surface stresses (most likely the result of machining stresses) decayed to a minimum the surface and the subsurface. The surface stresses (most likely the result of machining stresses) value (0 to 250 MPa) at the nominal mid-thickness of the section (i.e., 0.40 mm). Compressive decayed to a− minimum value (0 to −250 MPa) at the nominal mid-thickness∼ of the section (i.e., ∼0.40 residual stresses (CRS) and percentage cold work are both analogs to each other. mm). Compressive residual stresses (CRS) and percentage cold work are both analogs to each other. LSP is widely used for fatigue life improvement by inducing residual stress deep into the LSP is widely used for fatigue life improvement by inducing residual stress deep into the material and optimum microstructural features as reported by several researchers [21,89,207,208]. material and optimum microstructural features as reported by several researchers [21,89,207,208]. Prevey et al. [209] revealed the effect of the percentage cold work on the stress relaxation behavior of Prevey et al. [209] revealed the effect of the percentage cold work on the stress relaxation behavior of thethe IN718 IN718 material. material. The The main main conclusion conclusion was was that that laser laser peening peening and and ball ball burnishing burnishing offer offer minimal minimal percentagepercentage coldcold work,work, thus thus providing providing better better resistance resistance to thermal to relaxationthermal atrelaxation elevated temperatures.at elevated temperatures.Nikitin et al. [ 195Nikitin] reported et al. that [195] the cycle,reported stress that amplitude the cycle, and stress temperature-dependent amplitude and temperature- relaxation of dependentcompressive relaxation residual of stress compressive is more pronouncedresidual stress than is more the decrease pronounced of near-surface than the decrease work hardening of near- surfaceafter thermal work exposurehardening in after austenitic thermal stainless exposure steel in AISI austenitic 304. However, stainless gradual steel AISI stress 304. relaxation However, was gradualobserved stress for steelsrelaxation above was 450 observ◦C[210ed]. for steels above 450 °C [210].

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Peyre et al. [[211]211] studied the high cycle fatiguefatigue performance of laser peened aluminum alloys. alloys. The major eeffectffect observedobserved fromfrom other other technology technology was was that that the the depth depth of of penetration penetration of of residual residual stress stress is isapproximately approximately 1 mm,1 mm, by by far far above above any any other other process. process. The Theshortcoming shortcoming ofof thethe LSPLSP process, on other hand, was the hardening region that was 10% increm incrementent from the unpeened material and almost half of the conventionally peened material material.. Yang Yang et al. [212] [212] illustrated th thee effect effect of laser peening on the aluminum alloys fatigue performance with a fastener hole, multiple crack stop holes, and single-edge notch. The The process was performed in a confined confined ablation mode using a Neodymium (Nd):glass laser 2 at 5 GW GW cm cm−−2 powerpower density. density. The The compressive compressive residual residual stress stress generated generated due due to shock to shock waves waves was was385 MPa385 MPa magnitude magnitude in the in thesubsurface subsurface leading leading to togood good fatigue fatigue life life with with improved improved surface surface roughness. roughness. Tenaglia etet al.al. [[213]213] claimedclaimed thatthat the the LSP LSP produced produced a a number number of of beneficial beneficial eff effectsects for for Ti-6Al-4V Ti-6Al-4V such such as ashigh high fatigue fatigue life, life, and and higher higher resistance resistance towards towards fretting fretting wear wear and and stress stress corrosion corrosion cracking cracking than than the theshot shot peening peening process process due to due the relativelyto the relatively high magnitude high magnitude of compressive of compressive residual stressresidual generated stress generatedin LSP. in LSP. LainéLainé etet al. al. [156] [156] investigated investigated the the microstructural microstructural features features of of laser laser peened peened Ti-6Al-4V Ti-6Al-4V material and found complex microstructuresmicrostructures such such as as numerous numerous shears shears bands, bands, low low angle angle subgrain subgrain boundaries, boundaries, and and a afew few nanotwins nanotwins (Figure (Figure 23 23).). The The directional directional arrays arrays ofof planarplanar dislocationsdislocations withwith cellularcellular structure and small subgrains with low angle grain boundaries werewere noticed. Several othe otherr researchers have also reported the same microstructural features features after after la laserser peening peening on on different different materials materials [207,214–216]. [207,214–216]. Dislocation density density was was increased increased significantly significantly in la inser laser peened peened aluminum aluminum alloys alloys such suchas welded as welded 5086- H32,5086-H32, 6061-T6 6061-T6 [217], [217 2024-T62], 2024-T62 [218], [218 and], and low low carbon carbon steel steel [219], [219 ],but but no no quantitative quantitative analysis analysis was conducted. Strain hardening in the material was comparatively less and caused low-stress relaxation at an elevated temperature. Lack of fundamental understanding of the laser peening literature has been noted for the interaction of the atomic or molecular structure with laser-induced shock waves and the resulting changes in the microstructure.

Figure 23. ((aa)) BF-TEM BF-TEM micrograph micrograph of subgrains subgrains in the LSP sample, ( b) TKD EBSD image, and ( c) TKD EBSD inverse pole figurefigure image showing low angleangle subgrainsubgrain boundariesboundaries (adapted(adapted fromfrom [[156])156]).. 5.5. Water Jet Cavitation Peening (WJCP) 5.5. Water Jet Cavitation Peening (WJCP) Water jet cavitation peening (WJCP) uses a high-velocity water jet to impact the surface of a Water jet cavitation peening (WJCP) uses a high-velocity water jet to impact the surface of a material for multiple processes such as surface cleaning, paint removal, and cutting [220–222]. Water jet material for multiple processes such as surface cleaning, paint removal, and cutting [220–222]. Water cavitation peening (WJCP) or water peening (WP) is similar to SP except that it uses high-pressure jet cavitation peening (WJCP) or water peening (WP) is similar to SP except that it uses high-pressure droplets that disintegrate in the water jet flow field instead of solid shots. There are other similar droplets that disintegrate in the water jet flow field instead of solid shots. There are other similar concepts of peening like water droplet peening, water jet cavitation peening in water or air, and oil jet concepts of peening like water droplet peening, water jet cavitation peening in water or air, and oil peening [223]. The water peening (WP) process is a physically complex technique. jet peening [223]. The water peening (WP) process is a physically complex technique. A comparative study of shot peening and cavitation shot peening (CSP) on carburized steel was A comparative study of shot peening and cavitation shot peening (CSP) on carburized steel was done by Odhiambo et al. [224]. Surprisingly, the residual stress was 1189 MPa in SP with a 7% increase done by Odhiambo et al. [224]. Surprisingly, the residual stress was 1189 MPa in SP with a 7% increase in fatigue life from pristine sample, whereas, CSP produced only 560 MPa compressive residual stress in fatigue life from pristine sample, whereas, CSP produced only 560 MPa compressive residual stress (CRS), but with an improved fatigue life of 11%. Furthermore, Taguchi optimization of the process (CRS), but with an improved fatigue life of 11%. Furthermore, Taguchi optimization of the process identified the influencing factors responsible for higher fatigue life. It was observed that cavitation identified the influencing factors responsible for higher fatigue life. It was observed that cavitation number played a significant role in residual stress and surface roughness while nozzle size had a larger number played a significant role in residual stress and surface roughness while nozzle size had a larger impact on surface roughness. A higher cavitation number yielded better results for improving the fatigue strength of chrome-molybdenum steel.

Materials 2019, 12, 2503 27 of 41 impact on surface roughness. A higher cavitation number yielded better results for improving the fatigue strength of chrome-molybdenum steel. Grinspan et al. [223] peened AISI 1040 medium carbon steel with oil jet, which led to 390 MPa compressive residual stress and 332 MPa distribution up to 50 µm with 20% improvement in fatigue life. The eﬀects of peening by various oils on a range of engineering alloys was carried out by Pai et al. [225]. It was observed that vegetable oil exhibits the best medium for erosion resistance for all metals due to its high viscosity index and materials with high hardness had less cavitation damage for all lubricants [225]. Soyama et al. [226] reported a 50% improvement in fatigue life for aluminum alloys when subjected to cavitation shotless peening using a 30 MPa plunger pump. The pits resulting from cavitation shotless peening were of various sizes from 20 µm to 100 µm, giving a compressive residual stress of 200 MPa with a surface roughness of 0.22 µm. Han et al. [227] conducted water cavitation with aeration on SAE 1050 steel, and measured 215 MPa residual stress which gradually reduced to zero at a depth of 110 µm. The samples were quenched leading to an increase in residual stress up to 535 MPa to a depth of 140 µm. Hence, post processing of WJCP samples further improved the fatigue life. Several other researchers [152,228–231] reported a fatigue life improvement of 40–60% with variations of process parameters and medium due to compressive residual stress. In another study, Ju et al. [145] investigated the microstructural features after WJCP treatment of pure titanium and reported that the twinning generated by the treatment played a vital role in plastic deformation and residual stress of hexagonal structure. The deformation twinning, twinning interaction, and high dislocation density in the strain hardening region were observed which could be responsible for the stability of residual stress [232].

5.6. Ultrasonic Shot Peening (USSP) The principle of ultrasonic shot peening (USSP) is based on the vibration of spherical shots using a high-power ultrasound at high frequency. The vibrations shot peen the surface by repeated impacts over a short period. The main parameters of the USSP process are the vibration frequency of the chamber driven by an ultrasonic generator, shot diameter, and processing time [232]. Liu et al. [232] used USSP to treat 316L stainless steel and studied the effect of processing duration 30 s to 810 s on the microstructure. In all cases, a grain refinement in the surface layer and nanocrystalline layer of 30 µm thickness was observed. Similarly, the same deformation behavior and thickness were obtained in various types of steels [233] and aluminum alloys [234]. Villegas et al. [235] and [236], with WC/Co balls of 7.9 mm diameter for USSP, reported that nanocrystalline layers obtained on Ni-base C-2000 alloy increased with the processing time. The grain refinement occured due to deformation twin and was complemented by the dislocation activity, which justified the generation of nanocrystalline. The grain refinement induced, and the work-hardening associated with the USSP process itself led to a surface hardening. The hardness increment occurred in 16MnCr5 steel when displacement amplitude increased and the distance between nozzle and sample decreased [237]. Hou et al. [238] also reported high surface hardness when the grain size was very small (approximately 20 nm) on a magnesium alloy surface and 200 nm in bulk material, due to the generation of the nanocrystalline layer. Xing et al. [239] investigated the effect of the USSP process on the fatigue life of steel. A compressive residual stress (309 MPa) to a depth of 300 µm was developed in the subsurface and improved the total strength, stiffness, and fatigue life of the material. The equiaxed nanocrystals with a grain size of a few nanometers (i.e., 10 nm) were observed in the subsurface layers. The grain refinement mechanism could be related to the activity of the high density of dislocations and the formation of small shear bands. A similar trend of results has been published by several other researchers on iron [240,241] and stainless steel [232,242]. However, as Dai et al. [243] reported, the surface roughness generated by the indentation process of the bombarding balls may induce stress concentration at specific sites, which may result in crack initiation and fatigue failure. To avoid these cracks, protective coating must be applied during the peening process. These coatings can also absorb the energy and induce a greater concentration of Materials 2019, 12, 2503 28 of 41

Materials 2019, 12, x FOR PEER REVIEW 28 of 41 energyMaterials on 2019 the, 12 surface, x FOR PEER or they REVIEW can also impede the impact eﬀect on the surface. Various coatings28 provideof 41 a diﬀerent role in surface topography and properties. Overall, a comparative study was conducted for residual stress and fatigue performance based Overall,Overall, a a comparative comparative study study was was conducted conducted for for residual residual stressstress andand fatiguefatigue performanceperformance based on on the experimental data published for various materials and processing parameters. For example, theon experimental the experimental data data published published for variousfor various materials materials and and processing processing parameters. parameters. For For example, example, deep deep cold rolling (DCR) or deep rolling (DR) generated a relatively high magnitude of residual stress colddeep rolling cold rolling (DCR) or(DCR) deep or rolling deep rolling (DR) generated (DR) generate a relativelyd a relatively high magnitudehigh magnitude of residual of residual stress stress deeper deeper in the subsurface than LSP and, hence, showed a higher cycle to failure (as shown in Figure deeper in the subsurface than LSP and, hence, showed a higher cycle to failure (as shown in Figure in the24) at subsurface various temperatures than LSP and, for hence,given input showed parame a higherters [16]. cycle In toLSP, failure the power (as shown density in used Figure was 24 7) at 24) at various temperatures for given input parameters [16]. In LSP, the power density used was 7 2 various temperatures−2 for given input parameters [16]. In LSP, the power density used was 7 GWcm− GWcm−2 with 200% coverage and for DCR, 500 N hydrostatic force with 1000% coverage. withGWcm 200% with coverage 200% andcoverage for DCR, and for 500 DCR, N hydrostatic 500 N hydrostatic force with force 1000% withcoverage. 1000% coverage.

FigureFigure 24. 24.Enhancement Enhancement in in fatigue fatigue lifetimes lifetimes followingfollowing d deepeep rolling rolling and and laser laser shock shock peening peening for for test test Figure 24. Enhancement in fatigue lifetimes following deep rolling and laser shock peening for test temperatures of 25, 250, and 450 °C and stress amplitudes of 670, 550, and 460 MPa, respectively, for temperaturestemperatures of of 25, 25, 250, 250, andand 450450 ◦°CC and stress am amplitudesplitudes of of 670, 670, 550, 550, and and 460 460 MPa, MPa, respectively, respectively, for for Ti-6Al-4V (adapted with permission from [16]). Ti-6Al-4VTi-6Al-4V (adapted (adapted with with permission permission fromfrom [[16]).16]). ResidualResidual stress stress depth depth of influence inﬂuence is is higher higher in inLSP LSP followed followed by DCR, by DCR, USSP, USSP, SP, in SP,both in the both axial the Residual stress depth of influence is higher in LSP followed by DCR, USSP, SP, in both the axial and the tangential direction (as shown in Figure 25a,b) at the given input process conditions. axialand andthe thetangential tangential direction direction (as (asshown shown in Figure in Figure 25a,b) 25a,b) at the at thegiven given input input process process conditions. conditions. However, the residual stress magnitude is higher in DCR followed by LSP, SP, and USSP axial However,However, the the residual residual stress stress magnitude magnitude is higheris higher in DCR in DCR followed followed by LSP, by SP,LSP, and SP, USSP and axialUSSP direction. axial direction. In the tangential direction, SP residual stress magnitude is higher followed by LSP, USSP, Indirection. the tangential In the direction,tangential SPdirection, residual SP stress residual magnitude stress magnitude is higher is followed higher followed by LSP, by USSP, LSP, and USSP, DCR. and DCR. Furthermore, full-width half maxima (FWHM) is higher in SP followed by LSP, USSP, Furthermore,and DCR. Furthermore, full-width half full-width maxima half (FWHM) maxima is higher(FWHM) in SPis higher followed in SP by followed LSP, USSP, by DCR, LSP, asUSSP, shown DCR, as shown in Figure 25c. SP at Almen intensity 0.012” A with 200% coverage was used for inDCR, Figure as 25shownc. SP in at Figure Almen 25c. intensity SP at Almen 0.012” intensity A with 0.012” 200% coverageA with 200% was coverage used for was generating used for the generating the deformation, and 30 MPa hydrostatic pressure with 530 rpm and 0.1 mm/rev was used deformation,generating the and deformation, 30 MPa hydrostatic and 30 MPa pressure hydrostati withc pressure 530 rpm with and 530 0.1 rpm mm and/rev 0.1 was mm/rev used forwas the used DCR for the DCR process. In USSP, a peening intensity of F20.12 A with 80 µm amplitude was applied for process.for the DCR In USSP, process. a peening In USSP, intensity a peening of intensity F20.12 Aof withF20.12 80 Aµ withm amplitude 80 µm amplitude was applied was applied for 120 for s to 120 s to maintain more than 125% coverage. In LSP, a high energy laser with power density 10 maintain120 s to moremaintain than more 125% than coverage. 125% Incoverage. LSP, a high In LSP, energy a high laser energy with powerlaser with density power 10 GWcmdensity 210and GWcm−2 and 8 ns pulse length with 100% coverage was applied. − 8 nsGWcm pulse−2 and length 8 ns with pulse 100% length coverage with 100% was coverage applied. was applied.

Figure 25. Cont.

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Figure 25. Comparison of Ti-6Al-4V for SP, USSP, DCR, LSP (a) axial, (b) tangential residual stress Figure 25. Comparison of Ti-6Al-4V for SP, USSP, DCR, LSP (a) axial, (b) tangential residual stress profiles, (c) FWHM, and (d) maximum stress for failure (adapted with permission from [144]). profiles, (c) FWHM, and (d) maximum stress for failure (adapted with permission from [144]). A comparison study of different surface treatments revealed that LSP generates a relatively A comparison study of different surface treatments revealed that LSP generates a relatively higherhigher fatigue fatigue lifelife (HCF)(HCF) followedfollowed by DCR, SP, and USSP for for Ti-6Al-4V Ti-6Al-4V for for given given input input parameters, parameters, as shownas shown in Figurein Figure 25d. 25d. The The values values are are presented presented based based on on the the experimental experimental evidence. evidence. TheThe fatigue life incrementlife increment was calculatedwas calculated based based on the on life the extension life extension after surfaceafter surface treatment treatment with reference with reference to respective to untreatedrespectivecondition. untreated Incondition. another In study another by Maawad study by [244 Maawad], it was [244], reported it was that reported HCF performance that HCF of Ti-2.5Cuperformance was superiorof Ti-2.5Cu after was ball superior burnishing after asball compared burnishing to laseras compared peening to without laser peening coating without or the shot peeningcoating or process. the shot Therefore, peening process. a comprehensive Therefore, a study comprehensive on different study materials on different with amaterials variety with of surface a treatmentsvariety of andsurface different treatments processing and parametersdifferent proc is requiredessing parameters to confirm theis required effectiveness to confirm of each surfacethe treatmenteffectiveness process. of each surface treatment process.

6.6. MajorMajor ConclusionsConclusions and and Scope Scope for for Future Future Research Research Work Work FromFrom the the above above discourse, discourse, the the eﬀects effects of microstructure of microstructure on the on mechanical the mechanical properties properties of engineering of alloysengineering are listed alloys inTable are listed2. The in following Table 2. The broad following conclusions broad can conclusions be made from can be this made literature from review:this literature review: 1. To achieve a good combination of strength and ductility simultaneously, gradient microstructure,

1. i.e.,To nanocrystallineachieve a good on combination the surface andof coarsestrength in coreand canductility be produced simultaneously, in the material gradient using microstructure, i.e., nanocrystalline on the surface and coarse in core can be produced in the surface mechanical treatments. material using surface mechanical treatments. 2. Residual compressive stress and strain hardening are beneficial for improving fatigue crack 2. Residual compressive stress and strain hardening are beneficial for improving fatigue crack resistance. However, strain hardening is adversely affected at elevated temperatures due to resistance. However, strain hardening is adversely affected at elevated temperatures due to dominancedominance of creep. creep. 3.3. ForFor elevatedelevated temperature applications, applications, stress stress rela relaxationxation behavior behavior is a is recurrent a recurrent phenomenon phenomenon becausebecause ofof thethe dislocation dislocation annihilation annihilation mechanism, mechanism, but but it it can can be be controlled controlled by by resisting resisting the the atomic motionsatomic motions on slip on planes slip planes by introducing by introducing twin graintwin grain boundaries. boundaries. 4.4. SurfaceSurface mechanicalmechanical treatmentstreatments withwith varyingvarying strainstrain rates rates could could generate generate grain grain size, size, residual residual stress, strainstress, hardening,strain hardening, and other and other defects defects with varyingwith varying distribution distribution throughout throughout the the material material depth. Additionally,depth. Additionally, optimization optimization is essential is foressentia developingl for materialdeveloping with material excellent with performance. excellent 5. Forperformance. surface treatments, LSP shows relatively high fatigue performance and deeper residual stress 5. asFor compared surface treatments, with SP, USSP, LSP WJCP,shows andrelatively DCR forhigh the fatigue studied performance process parameters. and deeper However, residual the samestress inputas compared energy, coverage,with SP, andUSSP, material WJCP, isand required DCR forfor athe better studied comparison process ofparameters. the processes However, the same input energy, coverage, and material is required for a better comparison of and performance. Hybrid processes like shot peening followed by deep cold rolling could be the processes and performance. Hybrid processes like shot peening followed by deep cold beneficial for the fatigue life, as a high magnitude of compressive residual stress deeper in the rolling could be beneficial for the fatigue life, as a high magnitude of compressive residual stress material can be achieved. deeper in the material can be achieved.

Materials 2019, 12, 2503 30 of 41

Table 2. Eﬀect of microstructural features on properties and fatigue crack mechanism.

Crack Crack Features Specifications Properties Material Specifications Initiation Propagation Brass [245], Ti [246], stainless steel [247] Ultrafine or Strength ( ) Retards, or Electrodeposited ↑ Grain size nanocrystalline Hardness ( ) Retards Accelerate both nanocrystalline pure Ni ↑ ( ) Ductility ( ) reported and a cryomilled ↑ ↓ ultrafine-crystalline Al–Mg alloy, Ni films [248] Cu [50,53], steel [249] Gradient (torsion to cylindrical nanostructure Grain Strength ( ) twinning-induced or gradient ↑ Retards Retards distribution Ductility ( ) plasticity steel to generate nanotwined ↑ gradient nanotwinned structure ( ) ↑ structure) Nanotwinned Strength ( ) or coherent ↑ Twin GBs Ductility ( ) - Retards Cu [250] (CLSP) nanotwin ↑ Toughness ( )[250] boundaries ( ) ↑ ↑ Misorientation Ductility ( ) Low angle GBs ↓ - - All polycrystalline metals angle < 15 ( ) Hardness ( ) ◦ ↑ ↑ High angle Misorientation Ductility ( ) HAGB with UFC and ↑ -- GBs angle 15 ( ) Hardness ( ) nonequilibrium GBs [57] ≥ ◦ ↑ ↓ Strength ( ) Generation and ↑ Nanocrystalline/ultrafine Dislocations Hardening ( ) -- pile-ups () ↑ structure [57] Ductility ( ) ↓ Magnitude and Retards and Strain Hardness ( ) distribution ↑ Retards accelerate Polycrystalline materials hardening Ductility ( ) depth ( ) ↓ (both reported) ↑ Compressive Compressive stresses or Hardness ( ) (little Retards residual ↑ Retards Polycrystalline materials distribution influence) (little effect) stresses depth ( ) ↑

The role of individual factors is explained here:

1. Grain Size: Decreasing grain size increases strain hardening as per the Hall–Petch relationship, thus, resistance to crack initiation increases. Additionally, decreasing grain size increases the frequency of crack encounters boundaries, which provides more resistance to the crack growth. 2. Grain Distribution: Surface nanograin structure strengthens the material while interior coarse grains maintains the ductility of the material. The hard-and-deformed nanograined surface structure suppresses the crack initiation while the soft coarse-grain interior structure is effective in arresting the cracks [53]. Strain delocalization in gradient nanostructure is responsible for enhancing fatigue resistance in cyclic loading/unloading. 3. Twin GBs: Twin boundaries toughen the material by the dislocation-twin interaction that provides a dislocation nucleation site. When dislocations strike with twin boundaries, stress concentration occurs at GBs. To eliminate stress localization, dislocation nucleation takes place on another side of the grain boundary. This dislocation nucleation and pile-ups make twin boundary a source of dislocation generation, thus, improving the toughness of the material. 4. High Angle GBs: High angle GBs with ultrafine grain and nonequilibrium structure encourage grain boundary sliding, and thus, improve the ductility of the material. Simultaneously, ultrafine grain strengthens the material. Thus, a combination of high strength and ductility can be achieved through gradient microstructure. 5. Low Angle GBs: Low angle GBs are generally poor in grain boundary sliding which leads to lower ductility and improves the hardness of the material significantly. Materials 2019, 12, 2503 31 of 41

6. Dislocations: Resistance to dislocation movement provides the strengthening of the material. However, dislocation pile-ups at the grain boundaries or precipitates generate the stress concentration, and therefore dislocation either moves to another grain to reduce the stress intensity or initiates the cracks at the grain boundaries. 7. Strain Hardening: Strain hardening effect, generated due to dislocation nucleation and pile-up, increases the hardness but reduces the ductility of the material. The influence of strain hardening is still ambiguous as both beneficial and adverse effects on fatigue life have been reported by several researchers. The higher effective depth of the strain-hardened region rather than its high magnitude is anticipated for higher mechanical performance. 8. Compressive residual stress: Compressive residual stress compensates the tensile stress generated due to applied load that reduces the chances of crack initiation on the surface. Compressive residual stress and stress gradient throughout the depth are beneficial for fatigue life as they can provide high resistance towards both crack initiation and propagation mechanism. There are still arguments whether the high depth of influence or high magnitude of compressive residual stress is beneficial for high fatigue performance.

Author Contributions: D.K. conceptualized the paper content and structure under the supervision of S.I. and W.W. D.K. prepared the original paper draft, S.I. reviewed and edited the paper and response to reviewers thoroughly. W.W. and S.N. reviewed the paper and provided the scientific inputs in the paper. Funding: D. Kumar thanks the Nanyang Technological University of Singapore for the financial support in the form of a graduate research assistantship and Rolls-Royce for providing the resources. S. Idapalapati thanks the Advanced Remanufacturing Technology Centre (ARTC), Singapore for the additional financial support through the project agreement titled Role of Surface Modification on Subsurface of Nickel-Based Superalloys. S. Narasimalu wish to acknowledge the research grant LTA-UM004-0014 for supporting this research. Conflicts of Interest: The authors declare that they have no conflict of interest.

References

1. Pollock, T.M.; Tin, S. Nickel-based superalloys for advanced turbine engines: chemistry, microstructure and properties. J. Propuls. Power 2006, 22, 361–374. [CrossRef] 2. Liu, X.; Chu, P.K.; Ding, C. Surface modification of titanium, titanium alloys, and related materials for biomedical applications. Mater. Sci. Eng. R Rep. 2005, 47, 49–121. [CrossRef] 3. Cashell, K.A.; Baddoo, N.R. Ferritic stainless steels in structural applications. Thin Walled Struct. 2014, 83, 169–181. [CrossRef] 4. Luo, A.A. Recent magnesium alloy development for elevated temperature applications. Int. Mater. Rev. 2004, 49, 13–30. [CrossRef] 5. Feldmann, G.G.; Haubold, T. Mechanical surface treatment technologies for improving HCF strength and surface roughness of blisk-rotors. Mater. Sci. Forum 2013, 768–769, 510–518. [CrossRef] 6. Feldmann, G.; Wong, C.C.; Wei, W.; Haubold, T. Application of vibropeening on aero-engine component. Procedia CIRP 2014, 13, 423–428. [CrossRef] 7. Zhang, P.; Lindemann, J. Influence of shot peening on high cycle fatigue properties of the high-strength wrought magnesium alloy AZ80. Scr. Mater. 2005, 52, 485–490. [CrossRef] 8. Wang, Z.; Jiang, C.; Gan, X.; Chen, Y.; Ji, V. Influence of shot peening on the fatigue life of laser hardened 17-4PH steel. Int. J. Fatigue 2011, 33, 549–556. [CrossRef] 9. Gao, Y.K.; Wu, X.R. Experimental investigation and fatigue life prediction for 7475-T7351 aluminum alloy with and without shot peening-induced residual stresses. Acta Mater. 2011, 59, 3737–3747. [CrossRef] 10. Gujba, A.K.; Medraj, M. Laser peening process and its impact on materials properties in comparison with shot peening and ultrasonic impact peening. Materials 2014, 7, 7925–7974. [CrossRef][PubMed] 11. Luong, H.; Hill, M.R. The effects of laser peening and shot peening on high cycle fatigue in 7050-T7451 aluminum alloy. Mater. Sci. Eng. A 2010, 527, 699–707. [CrossRef] 12. Hatamleh, O. A comprehensive investigation on the effects of laser and shot peening on fatigue crack growth in friction stir welded AA 2195 joints. Int. J. Fatigue 2009, 31, 974–988. [CrossRef] Materials 2019, 12, 2503 32 of 41

13. Ganesh, P.; Sundar, R.; Kumar, H.; Kaul, R.; Ranganathan, K.; Hedaoo, P.; Tiwari, P.; Kukreja, L.M.; Oak, S.M.; Dasari, S.; et al. Studies on laser peening of spring steel for automotive applications. Opt. Lasers Eng. 2012, 50, 678–686. [CrossRef] 14. Hatamleh, O.; Lyons, J.; Forman, R. Laser and shot peening effects on fatigue crack growth in friction stir welded 7075-T7351 aluminum alloy joints. Int. J. Fatigue 2007, 29, 421–434. [CrossRef] 15. Tsuji, N.; Tanaka, S.; Takasugi, T. Effect of combined plasma-carburizing and deep-rolling on notch fatigue property of Ti-6Al-4V alloy. Mater. Sci. Eng. A 2009, 499, 482–488. [CrossRef] 16. Nalla, R.K.; Altenberger, I.; Noster, U.; Liu, G.Y.; Scholtes, B.; Ritchie, R.O. On the influence of mechanical surface treatments-deep rolling and laser shock peening-on the fatigue behavior of Ti-6Al-4V at ambient and elevated temperatures. Mater. Sci. Eng. A 2003, 355, 216–230. [CrossRef] 17. Wong, C.C.; Hartawan, A.; Teo, W.K. Deep cold rolling of features on aero-engine components. Procedia CIRP 2014, 13, 350–354. [CrossRef] 18. Altenberger, I.; Nalla, R.K.; Sano, Y.; Wagner, L.; Ritchie, R.O. On the effect of deep-rolling and laser-peening on the stress-controlled low- and high-cycle fatigue behavior of Ti-6Al-4V at elevated temperatures up to

550 ◦C. Int. J. Fatigue 2012, 44, 292–302. [CrossRef] 19. Kloos, K.H.; Fuchsbauer, B.; Adelmann, J. Fatigue properties of specimens similar to components deep rolled under optimized conditions. Int. J. Fatigue 1987, 9, 35–42. [CrossRef] 20. Ardi, D.T.; Wei, W.; Parr, I.; Feldmann, G.; Aramcharoen, A.; Wong, C.C. Investigations of the residual stresses and surface integrity generated by a novel mechanical surface strengthening. Mater. Res. Proc. 2017, 2, 311–316. 21. Montross, C.S.; Wei, T.; Ye, L.; Clark, G.; Mai, Y.W. Laser shock processing and its effects on microstructure and properties of metal alloys: A review. Int. J. Fatigue 2002, 24, 1021–1036. [CrossRef] 22. Kumar, D.; Idapalapati, S.; Wang, W.; Bhowmik, A. Microstructural characteristics and strengthening mechanisms in a polycrystalline Ni-based superalloy under deep cold rolling. Mater. Sci. Eng. A 2019, 753, 285–299. [CrossRef] 23. Kumar, D.; Idapalapati, S.; Wang, W.; Child, D.J.; Haubold, T.; Wong, C.C. Microstructure-mechanical property correlation in shot peened and vibro-peened Ni-based superalloy. J. Mater. Process. Technol. 2019, 267, 215–229. [CrossRef] 24. Briottet, L.; Moro, I.; Escot, M.; Furtado, J.; Bortot, P.; Tamponi, G.M.; Solin, J.; Odemer, G.; Blanc, C.; Andrieu, E. Fatigue crack initiation and growth in a CrMo steel under hydrogen pressure. Int. J. Hydrog. Energy 2015, 40, 17021–17030. [CrossRef] 25. Pang, H.T.; Reed, P.A.S. Fatigue crack initiation and short crack growth in nickel-base turbine disc alloys—The effects of microstructure and operating parameters. Int. J. Fatigue 2003, 25, 1089–1099. [CrossRef] 26. Boyd-Lee, A.D. Fatigue crack growth resistant microstructures in polycrystalline Ni-base superalloys for aeroengines. Int. J. Fatigue 1999, 21, 393–405. [CrossRef] 27. Tanaka, K.; Mura, T. A Dislocation Model for Fatigue Crack Initiation. J. Appl. Mech. 1981, 48, 97. [CrossRef] 28. Hussain, K. Short fatigue crack behaviour and analytical models: A review. Eng. Fract. Mech. 1997, 58, 327–354. [CrossRef] 29. Kumar, K.S.; Van Swygenhoven, H.; Suresh, S. Mechanical behavior of nanocrystalline metals and alloys. Acta Mater. 2003, 51, 5743–5774. [CrossRef] 30. Lu, L.; Chen, X.; Huang, X.; Lu, K. Revealing the maximum strength in nanotwinned copper. Science 2009, 323, 607–610. [CrossRef] 31. Zhang, X.; Misra, A.; Wang, H.; Nastasi, M.; Embury, J.D.; Mitchell, T.E.; Hoagland, R.G.; Hirth, J.P. Nanoscale-twinning-induced strengthening in austenitic stainless steel thin films. Appl. Phys. Lett. 2004, 84, 1096. [CrossRef] 32. Hansen, N. Hall-petch relation and boundary strengthening. Scr. Mater. 2004, 51, 801–806. [CrossRef] 33. Zhao, M.; Li, J.C.; Jiang, Q. Hall-Petch relationship in nanometer size range. J. Alloys Compd. 2003, 361, 160–164. [CrossRef] 34. Yuan, W.; Panigrahi, S.K.; Su, J.Q.; Mishra, R.S. Influence of grain size and texture on Hall-Petch relationship for a magnesium alloy. Scr. Mater. 2011, 65, 994–997. [CrossRef] 35. Nieh, T.G.; Wadsworth, J. Hall-petch relation in nanocrystalline solids. Scr. Metall. Mater. 1991, 25, 955–958. [CrossRef] Materials 2019, 12, 2503 33 of 41

36. Gao, N.; Wang, C.T.; Wood, R.J.K.; Langdon, T.G. Tribological properties of ultrafine-grained materials processed by severe plastic deformation. J. Mater. Sci. 2012, 47, 4779–4797. [CrossRef] 37. Phaniraj, M.P.; Prasad, M.J.N.V.; Chokshi, A.H. Grain-size distribution effects in plastic flow and failure. Mater. Sci. Eng. A 2007, 463, 231–237. [CrossRef] 38. Zrnik, J.; Dobatkin, S.V.; Mamuziˇc,I. Processing of metals by severe plastic deformation (SPD)—Structure and mechanical properties respond. Metalurgija 2008, 47, 211–216. 39. Kulkarni, Y.; Asaro, R.J. Are some nanotwinned fcc metals optimal for strength, ductility and grain stability? Acta Mater. 2009, 57, 4835–4844. [CrossRef] 40. Valiev, R. Nanomaterial advantage. Nature 2002, 419, 887–889. [CrossRef] 41. Kumar, P.; Kawasaki, M.; Langdon, T.G. Review: Overcoming the paradox of strength and ductility in ultrafine-grained materials at low temperatures. J. Mater. Sci. 2016, 51, 7–18. [CrossRef] 42. Mungole, T.; Kumar, P.; Kawasaki, M.; Langdon, T.G. A critical examination of the paradox of strength and ductility in ultrafine-grained metals. J. Mater. Res. 2014, 29, 2534–2546. [CrossRef] 43. Megumi Kawasaki, T.G.L. Review: achieving superplastic properties in ultrafine-grained materials at high temperatures. J. Mater. Sci. 2015, 19–32. 44. Lu, K.; Hansen, N. Structural refinement and deformation mechanisms in nanostructured metals. Scr. Mater. 2009, 60, 1033–1038. [CrossRef] 45. Valiev, R.Z.; Alexandrov, I.V.; Zhu, Y.T.; Lowe, T.C. Paradox of strength and ductility in metals processed by severe plastic deformation. J. Mater. Res. 2002, 17, 5–8. [CrossRef] 46. Reifsnider, K.; Rabbi, F.; Vadlamudi, V.; Raihan, R.; Brinkman, K. Critical path-driven property and performance transitions in heterogeneous microstructures. J. Mater. Sci. 2017, 52, 4796–4809. [CrossRef]

47. Gayda, J.; Miner, R.V. Fatigue crack initiation and propagation in several nickel-base superalloys at 650 ◦C. Int. J. Fatigue 1983, 5, 135–143. [CrossRef] 48. Hanlon, T.; Kwon, Y.-N.; Suresh, S. Grain size eﬀects on the fatigue response of nanocrystalline metals. Scr. Mater. 2003, 49, 675–680. [CrossRef] 49. Burke, J.J.; Weiss, V. Fatigue, Environment and Temperature Eﬀects; Plenum Press: New York, NY, USA, 1983; ISBN 0306411016. 50. Fang, T.H.; Li, W.L.; Tao, N.R.; Lu, K. Revealing extraordinary intrinsic tensile plasticity in gradient nano-grained copper. Science 2011, 331, 1587–1590. [CrossRef] 51. Wu, X.L.; Jiang, P.; Chen, L.; Zhang, J.F.; Yuan, F.P.; Zhu, Y.T. Synergetic strengthening by gradient structure. Mater. Res. Lett. 2014, 2, 185–191. [CrossRef] 52. Lu, K.; Faester, S.; Hansen, N.; Juul, J.D.; Ralph, B.; Sun, J. New frontiers of nanometals. In Proceedings of the 35th Risø International Symposium on Materials Science, Roskilde, Denmark, 1–5 September 2014. 53. Lu, K. Making strong nanomaterials ductile with gradients. Science 2014, 345, 1455–1456. [CrossRef] [PubMed] 54. Lu, K.; Tao, N.R.; Lu, K.; Kurzydłowski, K.J.; Pippan, R. Stabilizing nanostructures in metals using grain and twin boundary architectures. Nat. Rev. Mater. 2016, 1, 16019. [CrossRef] 55. Liu, X.C.; Zhang, H.W.; Lu, K. Strain-induced ultrahard and ultrastable nanolaminated structure in nickel. Science 2013, 342, 337–340. [CrossRef][PubMed] 56. Fragoudakis, R.; Saigal, A.; Savaidis, G.; Malikoutsakis, M.; Bazios, I.; Savaidis, A.; Pappas, G.; Karditsas, S. Fatigue assessment and failure analysis of shot-peened leaf springs. Fatigue Fract. Eng. Mater. Struct. 2013, 36, 92–101. [CrossRef] 57. Valiev, R. Nanostructuring of metals by severe plastic deformation for advanced properties. Nat. Mater. 2004, 3, 511–516. [CrossRef][PubMed] 58. Wang, Y.; Chen, M.; Zhou, F.; Ma, E. High tensile ductility in a nanostructured metal. Nature 2002, 419, 912–915. [CrossRef][PubMed] 59. Lu, K. Gradient nanostructured materials. Acta Met. 2015, 51, 1–10. 60. Masoumi, M.; Ariza, E.A.; Sinatora, A.; Goldenstein, H. Role of crystallographic orientation and grain boundaries in fatigue crack propagation in used pearlitic rail steel. Mater. Sci. Eng. A 2018, 722, 147–155. [CrossRef] 61. Zhu, Y.T.; Wu, X.L.; Liao, X.Z.; Narayan, J.; Mathaudhu, S.N.; Kecskés, L.J. Twinning partial multiplication at grain boundary in nanocrystalline fcc metals. Appl. Phys. Lett. 2009, 95, 031909. [CrossRef] Materials 2019, 12, 2503 34 of 41

62. Frøseth, A.G.; Derlet, P.M.; Van Swygenhoven, H. Twinning in Nanocrystalline fcc Metals. Adv. Eng. Mater. 2005, 7, 16–20. [CrossRef] 63. Wu, X.X.; San, X.Y.; Liang, X.G.; Gong, Y.L.; Zhu, X.K. Effect of stacking fault energy on mechanical behavior of cold-forging Cu and Cu alloys. Mater. Des. 2013, 47, 372–376. [CrossRef] 64. Sarma, V.S.; Wang, J.; Jian, W.W.; Kauffmann, A.; Conrad, H.; Freudenberger, J.; Zhu, Y.T. Role of stacking fault energy in strengthening due to cryo-deformation of FCC metals. Mater. Sci. Eng. A 2010, 527, 7624–7630. [CrossRef] 65. El-Danaf, E.; Kalidindi, S.R.; Doherty, R.D. Influence of grain size and stacking-fault energy on deformation twinning in fcc metals. Metall. Mater. Trans. A 1999, 30, 1223–1233. [CrossRef] 66. Gutierrez-Urrutia, I.; Zaefferer, S.; Raabe, D. The effect of grain size and grain orientation on deformation twinning in a Fe-22wt.% Mn-0.6wt.% C TWIP steel. Mater. Sci. Eng. A 2010, 527, 3552–3560. [CrossRef] 67. Farbaniec, L.; Abdul-Latif, A.; Gubicza, J.; Dirras, G. High purity ultrafine-grained nickel processed by dynamic plastic deformation: microstructure and mechanical properties. Adv. Eng. Mater. 2012, 14, 1027–1033. [CrossRef] 68. Liu, L.; Wang, J.; Gong, S.K.; Mao, S.X. High resolution transmission electron microscope observation of zero-strain deformation twinning mechanisms in Ag. Phys. Rev. Lett. 2011, 106, 175504. [CrossRef][PubMed] 69. Wu, X.L.; Ma, E. Deformation twinning mechanisms in nanocrystalline Ni. Appl. Phys. Lett. 2006, 88, 061905. [CrossRef] 70. Lu, K.; Lu, L.; Suresh, S. Strengthening materials by engineering coherent internal boundaries at the nanoscale. Science 2009, 324, 349–352. [CrossRef] 71. Bouaziz, O.; Allain, S.; Scott, C.P.; Cugy, P.; Barbier, D. High manganese austenitic twinning induced plasticity steels: A review of the microstructure properties relationships. Curr. Opin. Solid State Mater. Sci. 2011, 15, 141–168. [CrossRef] 72. Christian, J.W.; Mahajan, S. Deformation twinning. Prog. Mater. Sci. 1995, 39, 1–157. [CrossRef] 73. Valiev, R.Z.; Estrin, Y.; Horita, Z.; Langdon, T.G.; Zehetbauer, M.J.; Zhu, Y.T. Fundamentals of superior properties in bulk nanoSPD materials. Mater. Res. Lett 2016, 4, 1–21. [CrossRef] 74. Ghaderi, A.; Barnett, M.R. Sensitivity of deformation twinning to grain size in titanium and magnesium. Acta Mater. 2011, 59, 7824–7839. [CrossRef] 75. Bouaziz, O.; Allain, S.; Scott, C. Effect of grain and twin boundaries on the hardening mechanisms of twinning-induced plasticity steels. Scr. Mater. 2008, 58, 484–487. [CrossRef] 76. Gu, P.; Dao, M.; Zhu, Y. Strengthening at nanoscaled coherent twin boundary in f.c.c. metals. Philos. Mag. 2014, 94, 1249–1262. [CrossRef] 77. Callister, W.D. Materials Science and Engineering: An Introduction, 5th ed.; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 1999. 78. Barmak, K.; Darbal, A.; Ganesh, K.J.; Ferreira, P.J.; Rickman, J.M.; Sun, T.; Yao, B.; Warren, A.P.; Coffey, K.R. Surface and grain boundary scattering in nanometric Cu thin films: A quantitative analysis including twin boundaries. J. Vac. Sci. Technol. A 2014, 32, 061503. [CrossRef] 79. Lu, L.; Schwaiger, R.; Shan, Z.W.; Dao, M.; Lu, K.; Suresh, S. Nano-sized twins induce high rate sensitivity of flow stress in pure copper. Acta Mater. 2005, 53, 2169–2179. [CrossRef] 80. Ge, Z.-H.; Zhao, L.-D.; Wu, D.; Liu, X.; Zhang, B.-P.; Li, J.-F.; He, J. Low-cost, abundant binary sulfides as promising thermoelectric materials. Mater. Today 2016, 19, 227–239. [CrossRef] 81. Seita, M.; Hanson, J.P.; Gradecak, S.; Demkowicz, M.J. The dual role of coherent twin boundaries in hydrogen embrittlement. Nat. Commun. 2015, 6, 6164. [CrossRef] 82. Cai, W.; Bulatov, V.V.; Yip, S.; Argon, A.S. Kinetic Monte Carlo modeling of dislocation motion in BCC metals. Mater. Sci. Eng. A 2001, 309–310, 270–273. [CrossRef] 83. Kang, K.; Bulatov, V.V.; Cai, W. Singular orientations and faceted motion of dislocations in body-centered cubic crystals. Proc. Natl. Acad. Sci. USA 2012, 109, 15174–15178. [CrossRef] 84. Cai, W.; Bulatov, V.V.; Chang, J.; Li, J.; Yip, S. Chapter 64 Dislocation core effects on mobility. Dislocat. Solids 2005, 12, 1–80. 85. Peierls, R. The size of a dislocation. Proc. Phys. Soc. 1940, 52, 34–37. [CrossRef] 86. Eshelby, J.D. The equation of motion of a dislocation. Phys. Rev. 1953, 90, 248–255. [CrossRef] 87. Penn, R.L. Imperfect oriented attachment: dislocation generation in defect-free nanocrystals. Science 1998, 281, 969–971. [CrossRef] Materials 2019, 12, 2503 35 of 41

88. Callister, W.D. Materials Science and Engineering: An Introduction, 7th ed.; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2007; ISBN 13: 9780471736967. 89. Lin, D.; Ye, C.; Liao, Y.; Suslov, S.; Liu, R.; Cheng, G.J. Mechanism of fatigue performance enhancement in a laser sintered superhard nanoparticles reinforced nanocomposite followed by laser shock peening. J. Appl. Phys. 2013, 113, 133509. [CrossRef] 90. Russell, A.M.; Lee, K.L. Structure-property relations in nonferrous metals; Wiley-Interscience: Hoboken, NJ, USA, 2005; ISBN 9780471649526. 91. Chen, M.; Ma, E.; Hemker, K.J.; Sheng, H.; Wang, Y.; Cheng, X. Deformation twinning in nanocrystalline aluminum. Science 2003, 300, 1275–1277. [CrossRef] 92. Zhu, Y.T.; Liao, X.Z.; Wu, X.L. Deformation twinning in nanocrystalline materials. Prog. Mater. Sci. 2012, 57(1), 1–62. [CrossRef] 93. Liu, Y.; Van der Giessen, E.; Needleman, A. An analysis of dislocation nucleation near a free surface. Int. J. Solids Struct. 2007, 44, 1719–1732. [CrossRef] 94. Bayerschen, E.; McBride, A.T.; Reddy, B.D.; Böhlke, T. Review on slip transmission criteria in experiments and crystal plasticity models. J. Mater. Sci. 2016, 51, 2243–2258. [CrossRef] 95. Yang, Y.; Liu, C.T. Size effect on stability of shear-band propagation in bulk metallic glasses: An overview. J. Mater. Sci. 2012, 47, 55–67. [CrossRef] 96. DoITPoMS/University of Cambridge. Available online: https://www.doitpoms.ac.uk/ldplib/shape_memory/ background.php (accessed on 6 August 2019). 97. Foss, B.J.; Gray, S.; Hardy, M.C.; Stekovic, S.; McPhail, D.S.; Shollock, B.A. Analysis of shot-peening and residual stress relaxation in the nickel-based superalloy RR1000. Acta Mater. 2013, 61, 2548–2559. [CrossRef] 98. Zaroog, O.S.; Ali, A.; Sahari, B.; Zahari, R. Modelling of residual stress relaxation: A review. Pertanika J. Sci. Technol. 2009, 17, 211–218. 99. Ortiz, A.L.; Tian, J.W.; Villegas, J.C.; Shaw, L.L.; Liaw, P.K. Interrogation of the microstructure and residual stress of a nickel-base alloy subjected to surface severe plastic deformation. Acta Mater. 2008, 56, 413–426. [CrossRef] 100. Buchanan, D.J.; John, R. Relaxation of shot-peened residual stresses under creep loading. Scr. Mater. 2008, 59, 286–289. [CrossRef] 101. Buchanan, D.J.; John, R.; Brockman, R.A.; Rosenberger, A.H. A coupled creep plasticity model for residual stress relaxation of a shot peened nickel-base superalloy. JOM 2010, 62, 75–79. [CrossRef] 102. Prevey, P.S.; Hornbach, D.J.; Mason, P.W. Thermal residual stress relaxation and distortion in surface enhanced gas turbine engine components. In 17th Heat Treating Society Conference and Exposition and the 1st International Induction Heat Treating Symposium; ASM: Materials Park, OH, USA, 1998; pp. 3–12. 103. Benedetti, M.; Fontanari, V.; Scardi, P.; Ricardo, C.L.A.; Bandini, M. Reverse bending fatigue of shot peened 7075-T651 aluminium alloy: The role of residual stress relaxation. Int. J. Fatigue 2009, 31, 1225–1236. [CrossRef] 104. Guechichi, H.; Castex, L. Fatigue limits prediction of shot peened materials. Fatigue Fract. Steels 2006, 172, 221–228. 105. Soady, K.A. Life assessment methodologies incorporating shot peening process effects: Mechanistic consideration of residual stresses and strain hardening Part 1—Effect of shot peening on fatigue resistance. Mater. Sci. Technol. 2013, 29, 637–651. [CrossRef] 106. Sasahara, H. The effect on fatigue life of residual stress and surface hardness resulting from different cutting conditions of 0.45%C steel. Int. J. Mach. Tools Manuf. 2005, 45, 131–136. [CrossRef] 107. Klotz, T.; Delbergue, D.; Bocher, P.; Levesque, M.; Brochu, M. Surface characteristics and fatigue behavior of shot peened Inconel 718. Int. J. Fatigue 2018, 110, 10–21. [CrossRef] 108. Wang, S.; Li, Y.; Yao, M.; Wang, R. Fatigue limits of shot-peened metals. J. Mater. Process. Technol. 1998, 73, 57–63. [CrossRef] 109. Soady, K.A.; Mellor, B.G.; Reed, P.A.S. Life assessment methodologies incorporating shot peening process effects: Mechanistic consideration of residual stresses and strain hardening Part 2—Approaches to fatigue lifing after shot peening. Mater. Sci. Technol. 2013, 29, 652–664. [CrossRef] 110. Zhao, Y.-H.; Bingert, J.F.; Liao, X.-Z.; Cui, B.-Z.; Han, K.; Sergueeva, A.V.; Mukherjee, A.K.; Valiev, R.Z.; Langdon, T.G.; Zhu, Y.T. Simultaneously increasing the ductility and strength of ultra-fine-grained pure copper. Adv. Mater. 2006, 18, 2949–2953. [CrossRef] Materials 2019, 12, 2503 36 of 41

111. Zhu, Y.T.; Lowe, T.C.; Langdon, T.G. Performance and applications of nanostructured materials produced by severe plastic deformation. Scr. Mater. 2004, 51, 825–830. [CrossRef] 112. Langdon, T.G.; Furukawa, M.; Nemoto, M.; Horita, Z. Using equal-channel angular pressing for refining grain size. JOM 2000, 52, 30–33. [CrossRef] 113. Ueno, H.; Kakihata, K.; Kaneko, Y.; Hashimoto, S.; Vinogradov, A. Enhanced fatigue properties of nanostructured austenitic SUS 316L stainless steel. Acta Mater. 2011, 59, 7060–7069. [CrossRef] 114. Vinogradov, A.; Ishida, T.; Kitagawa, K.; Kopylov, V.I. Effect of strain path on structure and mechanical behavior of ultrafine grain Cu–Cr alloy produced by equal-channel angular pressing. Acta Mater. 2005, 53, 2181–2192. [CrossRef] 115. Kawasaki, M. Different models of hardness evolution in ultrafine-grained materials processed by high-pressure torsion. J. Mater. Sci. 2014, 49, 18–34. [CrossRef] 116. Valiev, R.R.; Smirnov, I.V. Effect of high pressure torsion technical parameters on grain refinement in Ti-6Al-4V alloy. In Proceedings of the 6th International Conference on Nanomaterials by Severe Plastic Deformation. IOP Conf. Ser. Mater. Sci. Eng. 2014, 63, 012073. [CrossRef] 117. Zhilyaev, A.P.; Langdon, T.G. Using high-pressure torsion for metal processing: Fundamentals and applications. Prog. Mater. Sci. 2008, 53, 893–979. [CrossRef] 118. Zhilyaev, A.P.; Nurislamova, G.V.; Kim, B.K.; Baró, M.D.; Szpunar, J.A.; Langdon, T.G. Experimental parameters influencing grain refinement and microstructural evolution during high-pressure torsion. Acta Mater. 2003, 51, 753–765. [CrossRef] 119. Xu, C.; Xia, K.; Langdon, T.G. The role of back pressure in the processing of pure aluminum by equal-channel angular pressing. Acta Mater. 2007, 55, 2351–2360. [CrossRef] 120. Khatibi, G.; Horky, J.; Weiss, B.; Zehetbauer, M.J. High cycle fatigue behaviour of copper deformed by high pressure torsion. Int. J. Fatigue 2010, 32, 269–278. [CrossRef] 121. Zhao, Y.H.; Bingert, J.F.; Zhu, Y.T.; Liao, X.Z.; Valiev, R.Z.; Horita, Z.; Langdon, T.G.; Zhou, Y.Z.; Lavernia, E.J. Tougher ultrafine grain Cu via high-angle grain boundaries and low dislocation density. Appl. Phys. Lett. 2008, 92, 081903. [CrossRef] 122. Sauvage, X.; Wilde, G.; Divinski, S.V.; Horita, Z.; Valiev, R.Z. Grain boundaries in ultrafine grained materials processed by severe plastic deformation and related phenomena. Mater. Sci. Eng. A 2012, 540, 1–12. [CrossRef] 123. Pumphrey, P.H.; Gleiter, H. On the structure of non-equilibrium high-angle grain boundaries. Philos. Mag. 1975, 32, 881–885. [CrossRef] 124. Murali, S.; Srikanth, N.; Vath, C.J. Grains, deformation substructures, and slip bands observed in thermosonic copper ball bonding. Mater. Charact. 2003, 50, 39–50. [CrossRef] 125. Murali, S.; Srikanth, N.; Vath, C.J. Part II: Grains, deformation substructures, and slip bands observed in thermosonic copper ball bonding of 6-mil-diameter wire. Mater. Charact. 2005, 54, 93–95. [CrossRef] 126. Kassner, M.E.; Smith, K.K.; Campbell, C.S. Low-temperature creep in pure metals and alloys. J. Mater. Sci. 2015, 50, 6539–6551. [CrossRef] 127. Vilardell, A.M.; Cinca, N.; Concustell, A.; Dosta, S.; Cano, I.G.; Guilemany, J.M. Cold spray as an emerging technology for biocompatible and antibacterial coatings: State of art. J. Mater. Sci. 2015, 50, 4441–4462. [CrossRef] 128. Stankiewicz, A.; Szczygieł, I.; Szczygieł, B. Self-healing coatings in anti-corrosion applications. J. Mater. Sci. 2013, 48, 8041–8051. [CrossRef] 129. Menthe, E.; Bulak, A.; Olfe, J.; Zimmermann, A.; Rie, K.T. Improvement of the mechanical properties of austenitic stainless steel after plasma nitriding. Surf. Coat. Technol. 2000, 133–134, 259–263. [CrossRef] 130. Stinville, J.C.; Villechaise, P.; Templier, C.; Riviere, J.P.; Drouet, M. Plasma nitriding of 316L austenitic stainless steel: Experimental investigation of fatigue life and surface evolution. Surf. Coat. Technol. 2010, 204, 1947–1951. [CrossRef] 131. Sirin, S.Y.; Sirin, K.; Kaluc, E. Effect of the ion nitriding surface hardening process on fatigue behavior of AISI 4340 steel. Mater. Charact. 2008, 59, 351–358. [CrossRef] 132. Hussain, K.; Tauqir, A.; Haq, A.; Khan, A.Q. Influence of gas nitriding on fatigue resistance of maraging steel. Int. J. Fatigue 1999, 21, 163–168. [CrossRef] 133. Xi, Y.T.; Liu, D.X.; Han, D. Improvement of corrosion and wear resistances of AISI 420 martensitic stainless steel using plasma nitriding at low temperature. Surf. Coat. Technol. 2008, 202, 2577–2583. [CrossRef] Materials 2019, 12, 2503 37 of 41

134. Genel, K.; Demirkol, M.; Çapa, M. Effect of ion nitriding on fatigue behaviour of AISI 4140 steel. Mater. Sci. Eng. A 2000, 279, 207–216. [CrossRef] 135. Alsaran, A.; Kaymaz, I.; Çelik, A.; Yetim, F.; Karakan, M. A repair process for fatigue damage using plasma nitriding. Surf. Coat. Technol. 2004, 186, 333–338. [CrossRef] 136. Mittemeijer, E.J. Fundamentals of nitriding and nitrocarburizing. ASM Handb. Steel Heat Treat. Fundam. Process. 2013, 4A, 619–646. 137. Czerwiec, T.; Michel, H.; Bergmann, E. Low-pressure, high-density plasma nitriding: Mechanisms, technology and results. Surf. Coat. Technol. 1998, 108–109, 182–190. [CrossRef] 138. Shibata, H.; Tokaji, K.; Ogawa, T.; Hori, C. The effect of gas nitriding on fatigue behaviour in titanium alloys. Int. J. Fatigue 1994, 16, 370–376. [CrossRef] 139. Basu, A.; Majumdar, J.D.; Alphonsa, J.; Mukherjee, S.; Manna, I. Corrosion resistance improvement of high carbon low alloy steel by plasma nitriding. Mater. Lett. 2008, 62, 3117–3120. [CrossRef] 140. Blodgett, M.P.; Nagy, P.B. Eddy current assessment of near-surface residual stress in shot-peened nickel-base superalloys. J. Nondestruct. Eval. 2004, 23, 107–123. [CrossRef] 141. Huang, X. Tailoring dislocation structures and mechanical properties of nanostructured metals produced by plastic deformation. Scr. Mater. 2009, 60, 1078–1082. [CrossRef] 142. Hughes, D.A.; Hansen, N.; Bammann, D.J. Geometrically necessary boundaries, incidental dislocation boundaries and geometrically necessary dislocations. Scr. Mater. 2003, 48, 147–153. [CrossRef] 143. Fleck, N.A.; Ashby, M.F.; Hutchinson, J.W. The role of geometrically necessary dislocations in giving material strengthening. Scr. Mater. 2003, 48, 179–183. [CrossRef] 144. Sonntag, R.; Reinders, J.; Gibmeier, J.; Kretzer, J.P. Fatigue performance of medical Ti6Al4V alloy after mechanical surface treatments. PLoS ONE 2015, 10, e0121963. [CrossRef] 145. Ju, D.Y.; Han, B. Investigation of water cavitation peening-induced microstructures in the near-surface layer of pure titanium. J. Mater. Process. Technol. 2009, 209, 4789–4794. [CrossRef] 146. Bagheri, S.; Guagliano, M. Review of shot peening processes to obtain nanocrystalline surfaces in metal alloys. Surf. Eng. 2009, 25, 3–14. [CrossRef] 147. Messé, O.M.D.M.; Stekovic, S.; Hardy, M.C.; Rae, C.M.F. Characterization of plastic deformation induced by shot-peening in a Ni-base superalloy. JOM 2014, 66, 2502–2515. [CrossRef] 148. Nouguier-Lehon, C.; Zarwel, M.; Diviani, C.; Hertz, D.; Zahouani, H.; Hoc, T. Surface impact analysis in shot peening process. Wear 2013, 302, 1058–1063. [CrossRef] 149. Unal, O.; Varol, R. Almen intensity effect on microstructure and mechanical properties of low carbon steel subjected to severe shot peening. Appl. Surf. Sci. 2014, 290, 40–47. [CrossRef] 150. Hong, T.; Ooi, J.Y.; Shaw, B. A numerical simulation to relate the shot peening parameters to the induced residual stresses. Eng. Fail. Anal. 2008, 15, 1097–1110. [CrossRef] 151. Bagherifard, S.; Fernandez-Pariente, I.; Ghelichi, R.; Guagliano, M. Effect of severe shot peening on microstructure and fatigue strength of cast iron. Int. J. Fatigue 2014, 65, 64–70. [CrossRef] 152. Kumar, H.; Singh, S.; Kumar, P. Modified shot peening processes—A review. Int. J. Eng. Sci. Emerg. Technol. 2013, 5, 12–19. 153. Song, Y.; Zhao, Z.; Lu, F. Experimental study of the influence of shot peening on the microstructure and properties of surface layer of a TC21 titanium alloy. Atlas J. Mater. Sci. 2014, 1, 17–23. [CrossRef] 154. Shekhar, S.; Cai, J.; Basu, S.; Abolghasem, S.; Shankar, M.R. Effect of strain rate in severe plastic deformation on microstructure refinement and stored energies. J. Mater. Res. 2011, 26, 395–406. [CrossRef] 155. Dingreville, R.; Karnesky, R.A.; Puel, G.; Schmitt, J.-H. Review of the synergies between computational modeling and experimental characterization of materials across length scales. J. Mater. Sci. 2016, 51, 1178–1203. [CrossRef] 156. Lainé, S.J.; Knowles, K.M.; Doorbar, P.J.; Cutts, R.D.; Rugg, D. Microstructural characterisation of metallic shot peened and laser shock peened Ti–6Al–4V. Acta Mater. 2017, 123, 350–361. [CrossRef] 157. Altenberger, I.; Scholtes, B.; Martin, U.; Oettel, H. Cyclic deformation and near surface microstructures of shot peened or deep rolled austenitic stainless steel AISI 304. Mater. Sci. Eng. A 1999, 264, 1–16. [CrossRef] 158. Zhan, K.; Fang, W.Q.; Zhao, B.; Yan, Y.; Feng, Q.; Jiang, C.H. Investigation of surface gradient microstructure of shot peened S30432 steel by X-ray line profile analysis method. Surf. Rev. Lett. 2016, 24, 1750078. [CrossRef] Materials 2019, 12, 2503 38 of 41

159. Kim, S.-B.; Shackleton, J.; Preuss, M.; Withers, P.J.; Evans, A.; Bruno, G. Stress relaxation of shot-peened Udimet 720Li under solely elevated-temperature exposure and under isothermal fatigue. Metall. Mater. Trans. A 2005, 36, 3041–3053. [CrossRef] 160. Evans, A.; Kim, S.B.; Shackleton, J.; Bruno, G.; Preuss, M.; Withers, P.J. Relaxation of residual stress in shot peened Udimet 720Li under high temperature isothermal fatigue. Int. J. Fatigue 2005, 27, 1530–1534. [CrossRef] 161. De Faoite, D.; Browne, D.J.; Chang-Díaz, F.R.; Stanton, K.T. A review of the processing, composition, and temperature-dependent mechanical and thermal properties of dielectric technical ceramics. J. Mater. Sci. 2012, 47, 4211–4235. [CrossRef] 162. Torres, M.A.S.; Voorwald, H.J.C. An evaluation of shot peening, residual stress and stress relaxation on the fatigue life of AISI 4340 steel. Int. J. Fatigue 2002, 24, 877–886. [CrossRef] 163. Trško, L.; Bok ˚uvka,O.; Nový, F.; Guagliano, M. Effect of severe shot peening on ultra-high-cycle fatigue of a low-alloy steel. Mater. Des. 2014, 57, 103–113. [CrossRef] 164. Aggarwal, M.L.; Agrawal, V.P.; Khan, R.A. A stress approach model for predictions of fatigue life by shot peening of EN45A spring steel. Int. J. Fatigue 2006, 28, 1845–1853. [CrossRef] 165. Widmark, M. Effect of material, heat treatment, grinding and shot peening on contact fatigue life of carburised steels. Int. J. Fatigue 1999, 21, 309–327. [CrossRef] 166. Sanjurjo, P.; Rodríguez, C.; Pariente, I.F.; Belzunce, F.J.; Canteli, A.F. The influence of shot peening on the fatigue behaviour of duplex stainless steels. Procedia Eng. 2010, 2, 1539–1546. [CrossRef] 167. Cernˇ ý, I. Growth and retardation of physically short fatigue cracks in an aircraft Al-alloy after shot peening. Procedia Eng. 2011, 10, 3411–3416. [CrossRef] 168. Majzoobi, G.H.; Ahmadkhani, A.R. The effects of multiple re-shot peening on fretting fatigue behavior of Al7075-T6. Surf. Coat. Technol. 2010, 205, 102–109. [CrossRef] 169. Asquith, D.T.; Yerokhin, A.L.; Yates, J.R.; Matthews, A. Effect of combined shot-peening and PEO treatment on fatigue life of 2024 Al alloy. Thin Solid Films 2006, 515, 1187–1191. [CrossRef] 170. Ali, A.; An, X.; Rodopoulos, C.A.; Brown, M.W.; O’Hara, P.; Levers, A.; Gardiner, S. The effect of controlled shot peening on the fatigue behaviour of 2024-T3 aluminium friction stir welds. Int. J. Fatigue 2007, 29, 1531–1545. [CrossRef] 171. Rodopoulos, C.A.; Romero, J.S.; Curtis, S.A.; de los Rios, E.R.; Peyre, D.P. Effect of controlled shot peening and laser shock peening on the fatigue performance of 2024-T351 aluminum alloy. J. Mater. Eng. Perform. 2003, 12, 414–419. [CrossRef] 172. Barry, N.; Hainsworth, S.V.; Fitzpatrick, M.E. Effect of shot peening on the fatigue behaviour of cast magnesium A8. Mater. Sci. Eng. A 2009, 507, 50–57. [CrossRef] 173. Cruchley, S.; Taylor, M.P.; Evans, H.E.; Bowen, P.; Hardy, M.C.; Stekovic, S. Microstructural characterisation of high temperature oxidation of nickel base superalloy RR1000 and the effect of shot-peening. In Proceedings of the Superalloys 2012: 12th International Symposium on Superalloys, Champion, PA, USA, 9–13 September 2012; pp. 751–758. 174. Zhang, G.; Yuan, H.; Li, F. Analysis of creep-fatigue life prediction models for nickel-based super alloys. Comput. Mater. Sci. 2012, 57, 80–88. [CrossRef] 175. Gibson, G.J.; Perkins, K.M.; Gray, S.; Leggett, A.J. Influence of shot peening on high-temperature corrosion and corrosion-fatigue of nickel based superalloy 720Li. Mater. High Temp. 2016, 33, 225–233. [CrossRef] 176. Mall, S.; Ng, J.L.; Madhi, E. Fretting fatigue behavior of shot-peened Ti-6Al-4V and IN100. J. ASTM Int. 2008, 5, 1–12. [CrossRef] 177. Sridhar, B.R.; Ramachandra, K.; Padmanabhan, K.A. Effect of shot peening on the fatigue and fracture behavior of two titanium alloys. J. Mater. Sci. 1996, 31, 5953–5960. [CrossRef] 178. Chen, G.Q.; Jiao, Y.; Tian, T.Y.; Zhang, X.H.; Li, Z.Q.; Zhou, W.L. Effect of wet shot peening on Ti-6Al-4V alloy treated by ceramic beads. Trans. Nonferrous Met. Soc. China 2014, 24, 690–696. [CrossRef] 179. Liu, K.K.; Hill, M.R. The effects of laser peening and shot peening on fretting fatigue in Ti-6Al-4V coupons. Tribol. Int. 2009, 42, 1250–1262. [CrossRef] 180. Mall, S.; Jain, V.K.; Fadag, H.A. Effects of shot-peening on fretting fatigue crack growth behavior in Ti-6Al-4V. Strain 2011, 47, e305–e318. [CrossRef] 181. Fridrici, V.; Fouvry, S.; Kapsa, P. Effect of shot peening on the fretting wear of Ti–6Al–4V. Wear 2001, 250, 642–649. [CrossRef] Materials 2019, 12, 2503 39 of 41

182. Lee, H.; Mall, S. Stress relaxation behavior of shot-peened Ti-6Al-4V under fretting fatigue at elevated temperature. Mater. Sci. Eng. A 2004, 366, 412–420. [CrossRef] 183. Lee, H.; Mall, S.; Sathish, S. Investigation into effects of re-shot-peening on fretting fatigue behavior of Ti-6Al-4V. Mater. Sci. Eng. A 2005, 390, 227–232. [CrossRef] 184. Li, K.; Fu, X.S.; Li, R.D.; Gai, P.T.; Li, Z.Q.; Zhou, W.L.; Chen, G.Q. Fretting fatigue characteristic of Ti-6Al-4V strengthened by wet peening. Int. J. Fatigue 2016, 85, 65–69. [CrossRef] 185. Murthy, H.; Mseis, G.; Farris, T.N. Life estimation of Ti-6Al-4V specimens subjected to fretting fatigue and effect of surface treatments. Tribol. Int. 2009, 42, 1304–1315. [CrossRef] 186. Namjoshi, S.A.; Jain, V.K.; Mall, S. Effects of shot-peening on fretting-fatigue behavior of Ti-6Al-4V. J. Eng. Mater. Technol. 2002, 124, 222. [CrossRef] 187. Sabelkin, V.; Martinez, S.A.; Mall, S.; Sathish, S.; Blodgett, M.P. Effects of shot-peening intensity on fretting fatigue crack-initiation behaviour of Ti-6Al-4V. Fatigue Fract. Eng. Mater. Struct. 2005, 28, 321–332. [CrossRef] 188. Jiang, X.P.; Man, C.S.; Shepard, M.J.; Zhai, T. Effects of shot-peening and re-shot-peening on four-point bend fatigue behavior of Ti-6Al-4V. Mater. Sci. Eng. A 2007, 468–470, 137–143. [CrossRef] 189. Townsend, D.P.; Zaretsky, E.V. Gearing. Scientific and Technical lnformation Branch; Lewis Research Centre, NASA: Cleveland, OH, USA, 1985. 190. Yoon, S.-J.; Park, J.-H.; Choi, N.-S. Fatigue life analysis of shot-peened bearing steel. J. Mech. Sci. Technol. 2012, 26, 1747–1752. [CrossRef] 191. Bagherifard, S.; Guagliano, M. Fatigue behavior of a low-alloy steel with nanostructured surface obtained by severe shot peening. Eng. Fract. Mech. 2012, 81, 56–68. [CrossRef] 192. Kumar, D.; Idapalapati, S.; Wei, W. Microstructural response and strain hardening in deep cold rolled nickel-based superalloy for aerospace application. Procedia CIRP 2018, 71, 374–379. [CrossRef] 193. Bozdana, A.T.; Gindy, N.N.Z.; Li, H. Deep cold rolling with ultrasonic vibrations—A new mechanical surface enhancement technique. Int. J. Mach. Tools Manuf. 2005, 45, 713–718. [CrossRef] 194. Wagner, L. Mechanical surface treatments on titanium, aluminum and magnesium alloys. Mater. Sci. Eng. A 1999, 263, 210–216. [CrossRef] 195. Nikitin, I.; Scholtes, B.; Maier, H.; Altenberger, I. High temperature fatigue behavior and residual stress stability of laser-shock peened and deep rolled austenitic steel AISI 304. Scr. Mater. 2004, 50, 1345–1350. [CrossRef] 196. Nagarajan, B.; Kumar, D.; Fan, Z.; Castagne, S. Effect of deep cold rolling on mechanical properties and microstructure of nickel-based superalloys. Mater. Sci. Eng. A 2018, 728, 196–207. [CrossRef] 197. Nagarajan, B.; Castagne, S. Microstructure study of nickel-based superalloys after deep cold rolling. Mater. Sci. Forum 2016, 879, 169–174. [CrossRef] 198. Zhang, X.C.; Zhang, Y.K.; Lu, J.Z.; Xuan, F.Z.; Wang, Z.D.; Tu, S.T. Improvement of fatigue life of Ti-6Al-4V alloy by laser shock peening. Mater. Sci. Eng. A 2010, 527, 3411–3415. [CrossRef] 199. Kumar, D.; Nadeem Akhtar, S.; Kumar Patel, A.; Ramkumar, J.; Balani, K. Tribological performance of laser peened Ti-6Al-4V. Wear 2015, 322–323, 203–217. [CrossRef] 200. Fabbro, R.; Peyre, P.; Berthe, L.; Sollier, A.; Bartnicki, E. Physics and applications of laser shock processing of materials. Proc. SPIE Int. Soc. Opt. Eng. 2000, 3888, 155–164. 201. Wang, Y.; Kysar, J.W.; Yao, Y.L. Analytical solution of anisotropic plastic deformation induced by micro-scale laser shock peening. Mech. Mater. 2008, 40, 100–114. [CrossRef] 202. Cellard, C.; Retraint, D.; François, M.; Rouhaud, E.; Le Saunier, D. Laser shock peening of Ti-17 titanium alloy: Influence of process parameters. Mater. Sci. Eng. A 2012, 532, 362–372. [CrossRef] 203. Tani, G.; Orazi, L.; Fortunato, A.; Ascari, A.; Campana, G. Warm laser shock peening: New developments and process optimization. CIRP Ann. Manuf. Technol. 2011, 60, 219–222. [CrossRef] 204. Ganesh, P.; Sundar, R.; Kumar, H.; Kaul, R.; Ranganathan, K.; Hedaoo, P.; Raghavendra, G.; Anand Kumar, S.; Tiwari, P.; Nagpure, D.C.; et al. Studies on fatigue life enhancement of pre-fatigued spring steel specimens using laser shock peening. Mater. Des. 2014, 54, 734–741. [CrossRef] 205. Wu, X.; Huang, C.; Wang, X.; Song, H. A new effective method to estimate the effect of laser shock peening. Int. J. Impact Eng. 2011, 38, 322–329. [CrossRef] 206. Smith, P.R.; Shepard, M.J.; Prevéy, P.S., III; Clauer, A.H. Effect of power density and pulse repetition on laser shock peening of Ti-6AI-4V. J. Mater. Eng. Perform. JMEPEG 2000, 9, 33–37. [CrossRef] Materials 2019, 12, 2503 40 of 41

207. Chahardehi, A.; Brennan, F.P.; Steuwer, A. The effect of residual stresses arising from laser shock peening on fatigue crack growth. Eng. Fract. Mech. 2010, 77, 2033–2039. [CrossRef] 208. Hu, Y.; Gong, C.; Yao, Z.; Hu, J. Investigation on the non-homogeneity of residual stress field induced by laser shock peening. Surf. Coat. Technol. 2009, 203, 3503–3508. [CrossRef] 209. Prevey, P.S. The effect of cold work on the thermal stability of residual compression in surface enhanced IN718. In Proceedings of the 20th ASM Materials Solutions Conference & Exposition, St. Louis, MO, USA, 10–12 October 2000. 210. Nikitin, I.; Altenberger, I. Comparison of the fatigue behavior and residual stress stability of laser-shock

peened and deep rolled austenitic stainless steel AISI 304 in the temperature range 25–600 ◦C. Mater. Sci. Eng. A 2007, 465, 176–182. [CrossRef] 211. Peyre, P.; Fabbro, R.; Merrien, P.; Lieurade, H.P. Laser shock processing of aluminum alloys application to high cycle fatigue behaviour. Mater. Sci. Eng. A 1996, 210, 102–113. [CrossRef] 212. Yang, J.-M.; Her, Y.C.; Han, N.; Clauer, A. Laser shock peening on fatigue behavior of 2024-T3 Al alloy with fastener holes and stopholes. Mater. Sci. Eng. A 2001, 298, 296–299. [CrossRef] 213. Tenaglia, R.D.; Lahrman, D.F. Shock tactics, Surface Treatment. Industry Perspective Technology Focus. Nat. Photonics 2009, 3, 267–269. [CrossRef] 214. Zhou, L.; He, W.; Luo, S.; Long, C.; Wang, C.; Nie, X.; He, G.; Shen, X.; Li, Y. Laser shock peening induced surface nanocrystallization and martensite transformation in austenitic stainless steel. J. Alloys Compd. 2016, 655, 66–70. [CrossRef] 215. Vukelić,S.; Kysar, J.W.; Lawrence Yao, Y. Grain boundary response of aluminum bicrystal under micro scale laser shock peening. Int. J. Solids Struct. 2009, 46, 3323–3335. [CrossRef] 216. Trdan, U.; Skarba, M.; Grum, J. Laser shock peening effect on the dislocation transitions and grain refinement of Al-Mg-Si alloy. Mater. Charact. 2014, 97, 57–68. [CrossRef] 217. Clauer, A.H.; Fairand, B.P.; Wilcox, B.A. Laser shock hardening of weld zones in aluminum alloys. Metall. Trans. A 1977, 8, 1871–1876. [CrossRef] 218. Hong, Z.; Chengye, Y. Laser shock processing of 2024-T62 aluminum alloy. Mater. Sci. Eng. A 1998, 257, 322–327. [CrossRef] 219. Bana´s, G.; Elsayed-Ali, H.E.; Lawrence, F.V.; Rigsbee, J.M. Laser shock-induced mechanical and microstructural modification of welded maraging steel. J. Appl. Phys. 1990, 67, 2380. [CrossRef] 220. Sun, Z.; Kang, X.Q.; Wang, X.H. Experimental system of cavitation erosion with water-jet. Mater. Des. 2005, 26, 59–63. [CrossRef] 221. Han, B.; Wang, Y.H.; Xu, C.L. Numerical simulation of residual stress field induced by water-jet cavitation peening. Appl. Mech. Mater. 2013, 345, 312–315. [CrossRef] 222. Gao, Y.; Wu, B.; Liu, Z.; Zhou, Y.; Shen, N.; Ding, H. Ultrasonic cavitation peening of stainless steel and nickel alloy. Trans. ASME J. Manuf. Sci. Eng. 2013, 136, 14502. [CrossRef] 223. Grinspan, A.S.; Gnanamoorthy, R. Effect of oil jet peening duration on surface modification and fatigue behavior of medium carbon steel, AISI 1040. Mater. Sci. Eng. A 2007, 456, 210–217. [CrossRef] 224. Odhiambo, D.; Soyama, H. Cavitation shotless peening for improvement of fatigue strength of carbonized steel. Int. J. Fatigue 2003, 25, 1217–1222. [CrossRef] 225. Pai, R.; Hargreaves, D.J. Performance of environment-friendly hydraulic fluids and material wear in cavitating conditions. Wear 2002, 252, 970–978. [CrossRef] 226. Soyama, H.; Saito, K.; Saka, M. Improvement of fatigue strength of aluminum alloy by cavitation shotless peening. J. Eng. Mater. Technol. 2002, 124, 135–139. [CrossRef] 227. Han, B.; Ju, D.Y.; Jia, W.P. Influence of water cavitation peening with aeration on fatigue behaviour of SAE1045 steel. Appl. Surf. Sci. 2007, 253, 9342–9346. [CrossRef] 228. Soyama, H. The use of cavitation peening to increase the fatigue strength of duralumin plates containing fastener holes. Mater. Sci. Appl. 2014, 5, 430–440. [CrossRef] 229. Soyama, H.; Macodiyo, D.O. Fatigue strength improvement of gears using cavitation shotless peening. Tribol. Lett. 2005, 18, 181–184. [CrossRef] 230. Soyama, H.; Nishizawa, K.; Mikami, M. Enhancement of cavitation aggressivity around a cavitating jet by injecting low-speed water jet for cavitation peening. In Proceedings of the 7th International Symposium on Cavitation (CAV2009), Ann Arbor, MI, USA, 16–20 August 2009. Materials 2019, 12, 2503 41 of 41

231. Soyama, H. Improvement of fatigue strength by using cavitating jets in air and water. J. Mater. Sci. 2007, 42, 6638–6641. [CrossRef] 232. Liu, G.; Lu, J.; Lu, K. Surface nanocrystallization of 316L stainless steel induced by ultrasonic shot peening. Mater. Sci. Eng. A 2000, 286, 91–95. [CrossRef] 233. Todaka, Y.; Umemoto, M.; Tsuchiya, K. Comparison of nanocrystalline surface layer in steels formed by air blast and ultrasonic shot peening. Mater. Trans. 2004, 45, 376–379. [CrossRef] 234. Wu, X.; Tao, N.; Hong, Y.; Xu, B.; Lu, J.; Lu, K. Microstructure and evolution of mechanically-induced ultrafine grain in surface layer of AL-alloy subjected to USSP. Acta Mater. 2002, 50, 2075–2084. [CrossRef] 235. Villegas, J.C.; Dai, K.; Shaw, L.L.; Liaw, P.K. Nanocrystallization of a nickel alloy subjected to surface severe plastic deformation. Mater. Sci. Eng. A 2005, 410–411, 257–260. [CrossRef] 236. Villegas, J.C.; Shaw, L.L. Nanocrystallization process and mechanism in a nickel alloy subjected to surface severe plastic deformation. Acta Mater. 2009, 57, 5782–5795. [CrossRef] 237. Abramov, V.O.; Abramov, O.V.; Sommer, F.; Gradov, O.M.; Smirnov, O.M. Surface hardening of metals by ultrasonically accelerated small metal balls. Ultrasonics 1998, 36, 1013–1019. [CrossRef] 238. Hou, L.F.; Wei, Y.H.; Liu, B.S.; Xu, B.S. Microstructure evolution of AZ91D induced by high energy shot peening. Trans. Nonferrous Met. Soc. China 2008, 18, 1053–1057. [CrossRef] 239. Xing, Y.M.; Lu, J. An experimental study of residual stress induced by ultrasonic shot peening. J. Mater. Process. Technol. 2004, 152, 56–61. [CrossRef] 240. Tao, N.R.; Sui, M.L.; Lu, J.; Lua, K. Surface nanocrystallization of iron induced by ultrasonic shot peening. Nanostruct. Mater. 1999, 11, 433–440. [CrossRef] 241. Guo, F.A.; Trannoy, N.; Lu, J. Microstructural analysis by scanning thermal microscopy of a nanocrystalline Fe surface induced by ultrasonic shot peening. Superlattices Microstruct. 2004, 35, 445–453. [CrossRef] 242. Marteau, J.; Bigerelle, M.; Mazeran, P.E.; Bouvier, S. Relation between roughness and processing conditions of AISI 316L stainless steel treated by ultrasonic shot peening. Tribol. Int. 2015, 82, 319–329. [CrossRef] 243. Dai, K.; Villegas, J.; Stone, Z.; Shaw, L. Finite element modeling of the surface roughness of 5052 Al alloy subjected to a surface severe plastic deformation process. Acta Mater. 2004, 52, 5771–5782. [CrossRef] 244. Maawad, E. Residual Stress Analysis and Fatigue Behavior of Mechanically Surface Treated Titanium Alloys; Helmholtz-Zentrum Geesthacht: Geesthacht, Germany, 2013. 245. Thompson, A.W.; Backofen, W.A. The effect of grain size on fatigue. Acta Metall. 1971, 19, 597–606. [CrossRef] 246. Ono, Y.; Yuri, T.; Sumiyoshi, H.; Matsuoka, S.; Ogata, T. Effect of grain size on high-cycle fatigue properties in alpha-type titanium alloy at cryogenic temperatures. Cryogenics 2003, 43, 483–489. [CrossRef] 247. Hong, Y.; Qiao, Y.; Liu, N.; Zheng, X. Effect of grain size on collective damage of short cracks and fatigue life estimation for a stainless steel. Fatigue Fract. Eng. Mater. Struct. 1998, 21, 1317–1325. [CrossRef] 248. Tanaka, K.; Sakakibara, M.; Kimachi, H. Grain-size effect on fatigue properties of nanocrystalline nickel thin films made by electrodeposition. Procedia Eng. 2011, 10, 542–547. [CrossRef] 249. Wei, Y.; Li, Y.; Zhu, L.; Liu, Y.; Lei, X.; Wang, G.; Wu, Y.; Mi, Z.; Liu, J.; Wang, H.; et al. Evading the strength-ductility trade-off dilemma in steel through gradient hierarchical nanotwins. Nat. Commun. 2014, 5, 3580. [CrossRef] 250. Ye, C.; Suslov, S.; Lin, D.; Liao, Y.; Cheng, G.J. Cryogenic ultrahigh strain rate deformation induced hybrid nanotwinned microstructure for high strength and high ductility. J. Appl. Phys. 2014, 115, 213519. [CrossRef]

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