metals

Review Recent Development of Superplasticity in Aluminum Alloys: A Review

Laxman Bhatta 1,2,3 , Alexander Pesin 4 , Alexander P. Zhilyaev 4, Puneet Tandon 5, Charlie Kong 6 and Hailiang Yu 1,2,3,* 1 State Key Laboratory of High Performance Complex Manufacturing, Central South University, Changsha 410083, China; [email protected] 2 College of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China 3 Light Alloys Research Institute, Central South University, Changsha 410083, China 4 Laboratory of Mechanics of Gradient Nanomaterials, Nosov Magnitogorsk State Technical University, Magnitogorsk 455000, Russia; [email protected] (A.P.); [email protected] (A.P.Z.) 5 PDPM Indian Institute of Information Technology, Design and Manufacturing, Jabalpur 482005, India; [email protected] 6 Electron Microscope Unit, University of New South Wales, Sydney, NSW 2052, Australia; [email protected] * Correspondence: [email protected]; Tel.: +86-731-8887-9351



Received: 10 December 2019; Accepted: 30 December 2019; Published: 2 January 2020


Abstract: Aluminum alloys can be used in the fabrication of intricate geometry and curved parts for a wide range of uses in aerospace and automotive sectors, where high stiffness and low weight are necessitated. This paper outlines a review of various research investigations on the superplastic behavior of aluminum alloys that have taken place mainly over the past two decades. The influencing factors on aluminum alloys superplasticity, such as initial grain size, deformation temperature, strain rate, microstructure refinement techniques, and addition of trace elements in aluminum alloys, are analyzed here. Since grain boundary sliding is one of the dominant features of aluminum alloys superplasticity, its deformation mechanism and the corresponding value of activation energy are included as a part of discussion. Dislocation motion, diffusion in grains, and near-grain boundary regions being major features of superplasticity, are discussed as important issues. Moreover, the paper also discusses the corresponding values of grain size exponent, stress exponent, solute drag creep and power law creep. Constitutive equations, which are essential for commercial applications and play a vital role in predicting and analyzing the superplastic behavior, are also reviewed here.

Keywords: aluminum alloys; superplasticity; ultrafine-grained materials; severe plastic deformation; grain boundary sliding

1. Introduction Superplasticity is a process in which polycrystalline materials undergo several hundred to several thousand percent of tensile elongation at an appropriate temperature and strain rate. It is greatly effective in reducing the weight and production cost by minimizing the processing steps of machining and joining. Furthermore, superplasticity can also be defined as the capacity of materials to be plastically deformed up to and exceeding 1000% without breaking [1]. Generally, in order to achieve the superplastic behavior of materials, temperature should be more than homologous temperature (T/Tm > 0.5; T: deformation temperature; Tm: melting point), where deformation mechanism deviates from dislocation activity to thermally activated mechanisms [2–4]. Likewise, to achieve better superplasticity, grain size should be less than 10 µm and strain rate interval in the tensile test should be within 1 10 5 s 1 to 1 10 1 s 1 [5]. Under superplastic conditions, deformation resistance is decreased × − − × − −

Metals 2020, 10, 77; doi:10.3390/met10010077 www.mdpi.com/journal/metals Metals 2020, 10, 77 2 of 26 while plasticity of materials is increased, resulting in a 50–80% decrease in energy compared to the conventional process, 50–90% cut down equipment expenses, and 20–50% reduction in raw materials consumption [6]. Moreover, compared to the conventional fabrication method, number of parts and joints decreases, along with their weight and cost, and intricate geometry can be fabricated. As grains in the polycrystalline matrix grow easily at an elevated temperature, therefore it is difficult to achieve smaller grain size at homologous temperature. Furthermore, the physics underlying this process is extremely complicated [7,8]. These restrictions resulted in increased attention in the research and the development of severe plastic deformation (SPD). Over the last two decades, alternative approaches are available for grain refinement through the application of SPD techniques [8,9]. SPD is an innovative and effective metal forming process where ultra-high plastic strain is introduced on a bulk metal in order to make ultra-fine grained metals [10–13]. SPD causes the formation of grain size in submicrometer to nanometer range, which results in the enhanced mechanical performance of the metals [11,12]. SPD techniques such as equal channel angular pressing (ECAP) [14–20], friction stir process (FSP) [21,22], high-pressure torsion (HPT) [23–25], accumulative roll bonding (ARB), and cryorolling, as well as asymmetric rolling are used for producing ultrafine grained materials. Aluminum alloys can be used in aero engines to manufacture compressor casing, cone, and inlet ring while aluminum composites are used in fan blades, fan casing, and cowls [26]. Superplasticity of aluminum alloys reduces manufacturing cost and has the ability to form parts of intricate geometry which would be difficult or even impossible to fabricate by other means. Xing et al. [27] pointed out that aluminum alloys can be used in the fabrication of complex shapes and curved parts for a wide range of uses in aerospace and automotive sectors, architectural design and small scale structural elements, where high stiffness and low weight are necessitated. Furthermore, superplasticity of aluminum alloys can be used for facade elements, automobile parts, uncritical parts, and mass-produced sheets [28]. With the development of several SPD techniques, exceptional grain refinement has led to enhanced superplastic property at low temperatures and high strain rates, which is impossible to attain using conventional processing techniques. Recently, Avtokratova et al. [29] achieved an exceptional superplastic elongation of 4100% at high strain rate in an Al-Mg-Sc-Zr alloy after processing by ECAP. Moreover, the recent development of nanoidentation technique, which requires limited sample volume is effective to measure the mechanical property [30]. In addition, quick superplastic forming technology, which was developed recently, improves the low production efficiency of traditional superplasticity by combining traditional superplastic forming and hot drawing technology. It not only reduces the production cycle but also decreases the manufacturing cost [31]. Likewise, García-Infanta et al. [32] used a small punch testing device, which requires a small amount of sample, to analyze the superplastic behavior of HPT processed Al alloy. The purpose of this review is to introduce the recent developments in Al alloys superplasticity, their mechanisms and the factors influencing the superplastic characteristics.

2. Influencing Factors on Al Alloys Superplasticity Studies on superplasticity of metals have a long history since Pearson [33] reported a noteworthy tensile elongation of 1950% in Pb-Sn eutectic alloy. Since then, multitudinous research has been reported in superplasticity of metals like aluminum, magnesium, iron, titanium and nickel-based alloys [34]. Generally, superplasticity can be experienced both at a lower and higher temperature. Presnyakov [35] achieved a maximum elongation of 650% in Al-Zn (eutectoid composition) at 250 ◦C. Lee [36] achieved maximum elongation to failure 2100% in the Mg-Al eutectic alloy at 400 ◦C. Ishikawa et al. [37] observed a maximum tensile strain of 2900% in Zn-22% Al eutectoid at a testing temperature of 200 ◦C and grain size of 2.5 µm, which was the highest strain reported in a superplastic material at that time. Likewise, single phase AA2004 aluminum alloy exhibited excellent superplastic behavior. Subsequently, reports on the superplasticity of alloys stronger than AA2004, particularly, high strength aerospace alloys of the 7000 series were published. The high cost of fuel in the late 1970s and enhanced mechanical properties of these alloys appropriate to aerospace applications attracted Metals 2020, 10, 77 3 of 26 much more attention in the aluminum alloys superplasticity. As a result, superplastic research on aluminum alloys like AA7075 and AA7475 was undertaken in many laboratories around the world. Researchers were investigating the alloy composition that can replace the airframe alloys and have low density and high stiffness than existing airframe alloys. Excellent superplastic properties of these aluminum alloys not only attracted the researchers but also encouraged companies like TI Group, Rockwell Company to manufacture and sell engineering components from aluminum sheet [38]. As a result, Horiuchi et al. [39] reported a tensile elongation of more than 600% in Al-Cu-Mg alloy and Wadsworth et al. [40] achieved the best elongation to failure of 1035% in Al-3Li-0.5Zr at 370 ◦C. Highashi et al. [41] experienced exceptional superplastic elongation of 5500% in aluminum bronze at 800 C and 6.3 10 3 s 1 strain rate. Exceptional grain refinement by SPD process was initiated ◦ × − − 30 years ago. Valiev et al. [9] utilized high-pressure torsion HPT to process Al-Cu-Zr alloy and obtained elongation of 250% at a low temperature of 220 ◦C while Wang et al. [42] took advantage of ECAP to process Al-3Mg alloy and achieved the grain size of 0.2 µm. In addition to these two SPD processes, several other processes have been developed since then but the two procedures receiving most interest are ECAP and HPT [34]. Table1 lists some experiments on the superplasticity of Al alloys performed by different researchers over the past 20 years. It can be inferred from the Table1 that generally superplastic elongation increases with decrease in grain size.

Table 1. Development of superplasticity in Al alloys in recent years.

Temp., Strain Grain Elongation Al alloys (wt.%) 1 Process Refs. Rate (◦C), (s− ) size (µm) (%) Al-4Mg-0.6Mn-0.3Sc 550, 7.5 10 4 8–14 Hot and cold rolling 1400 [1] × − Al-5%Mg 0.2% Sc 0.15%Zr 510, 1 10 2 5 FSP 1500 [5] × − Al–1.9Li–1.0Mg–1.7Cu–0.03Sc–0.08Zr 450, 1.4 10 2 1–3 Hot rolling 415 [43] × − Al-2.35Mg-0.16Cr-0.03Mn 425, 3.78 10 3 33 Cold Rolling 181 [44] × − Al–5%Mg–0.3%Sc 550, 1.04 10 1 10 Hot forging and cold rolling 212 [45] × − 5083 Al 300, 3 10 4 1.6–1.8 FSW 550–570 [46] × − Al-4.72Mg-0.35Sc-0.168Zr-0.016Fe-0.012Ti-0.007Si 500, 1 10 2 1.3 FSP 1900 [47] × − 7034 Al 400, 1 10 2 0.3 ECAP >1000 [48] × − AA 5024 450, 5.6 10 1 0.3 ECAP 3300 [49] × −

2.1. Effect of Initial Grain Size on the Superplasticity of Al Alloys In order to achieve the better mechanical properties of the superplastic alloy, the alloys should possess fine grain size and it should remain stable during deformation [50]. A fine-grained microstructure has a greater number of grains and grain boundaries than that of a coarse grain counterpart and is typically less than 10 µm. However, finer microstructure requires high severity processing than coarser ones during SPD, and consequently more energy is stored in the material. This stored energy gives rise to high nucleation density followed by rapid grain growth at elevated temperatures. Therefore, stored energy during SPD process is the driving force for abnormal grain growth at high temperatures. This lowers the maximum temperature where grain boundary sliding (GBS) can operate homogeneously in finer microstructure than for coarser microstructure [51,52]. It is well accepted that grain boundary migration due to extensive GBS results in strain induced grain growth in fine grained polycrystalline materials. Moreover, elevated test temperature and low strain rate can enhance grain growth during superplastic deformation process [53]. In addition, grain growth at high temperature results in strain hardening to stabilize neck free plasticity, determines the limit of ductility and increases the flow stress. It is generally believed that the possible microscopic grain growth mechanism is either grain boundary migration accelerated by superplasticity and/or rotation and coalescence of grains. Masuda et al. [54] proposed that a pair of neighboring grains should slide, rotate, and coalesce with each other in order to align in the tensile axis. Furthermore, change in the shape of grain and dynamic grain growth can also occur to avoid the formation of cavities during superplastic deformation [55]. In order to realize better superplastic property, grain shape should be equiaxed because the grain sliding mechanism cannot operate smoothly in a matrix with an elongated grain structure. Metals 2020, 10, x FOR PEER REVIEW 4 of 26 axis. Furthermore, change in the shape of grain and dynamic grain growth can also occur to avoid the formation of cavities during superplastic deformation [55]. MetalsIn2020 order, 10, 77 to realize better superplastic property, grain shape should be equiaxed because4 ofthe 26 grain sliding mechanism cannot operate smoothly in a matrix with an elongated grain structure. Grain refinement not only increases the range of strain rate but also decreases the temperature for GrainGBS mechanism refinement [56]. not onlyLikewise, increases to achieve the range better of strain superp ratelasticity, but also grain decreases boundaries the temperature should be forof high GBS mechanismangle character, [56]. because Likewise, low to achieveangle boundaries better superplasticity, are not able grain to slide. boundaries However, should in besome of highcases angle low character,angled grain because boundaries low angle can be boundaries changed to are high not by able either to slide. by a static However, discontinuous in some cases recrystallization low angled grainprocess boundaries prior to severe can be plastic changed deformation to high by or either through by a statica continuous discontinuous recrystallization recrystallization process process in the priorearly tostages severe of plasticsuperplastic deformation deformation or through [57–59]. a continuous With reduction recrystallization in grain size, process strain in therate early sensitivity stages ofincreases, superplastic and optimum deformation superplastic [57–59]. elongation With reduction can be in realized grain size, at comparatively strain rate sensitivity lower strain increases, rate andand/or optimum temperature. superplastic Further, elongation it is well canknown be realizedthat deformation at comparatively occurs mainly lower at strain boundaries rate and and/or temperature.decreasing the Further, grain itsize is wellwill knownincrease that the deformation grain boundaries, occurs mainly which at leads boundaries to higher and elongation. decreasing theNevertheless, grain size at will relatively increase larger the grain grain boundaries,size, transition which to GBS leads and to dislocation higher elongation. creep does Nevertheless, not occur at athigh relatively strain rate, larger as grain a result size, superplastic transition to property GBS and cannot dislocation be realized creep does [60] not. Further occur atinformation high strain on rate, a asrange a result of aspects superplastic of grain property size and cannot grain growth be realized may [60 also]. Further be found information in grimes et on al. a range[38], Masuda of aspects et al. of grain[54], Li size et al. and [61], grain and growth Tan et mayal. [60]. also be found in grimes et al. [38], Masuda et al. [54], Li et al. [61], and TanAn etequally al. [60 ].significant aspect of superplastic deformation process is abnormal grain growth. GrainAn growth equally during significant superplastic aspect ofdeformation superplastic pr deformationocess is generally process ascribed is abnormal to grain grain boundary growth. Grainsliding growth or dislocation during superplasticcreep or diffusion deformation creep. processSince grain is generally boundary ascribed migration, to grain diffusion boundary creep, sliding and ordislocation dislocation creep creep are or temperature diffusion creep. dependent, Since grain grain boundary grows migration, with an increase diffusion in creep, temperature. and dislocation Grain creepgrowth are at temperaturehigh temperature dependent, results grainin strain grows hardening, with an consequently increase in temperature. increasing the Grain flow stress growth [55]. at highHowever, temperature abnormal results grain in growth strain hardening, is a process consequently through which increasing few theenergetic flow stress grains [55 grow]. However, at the abnormalexpense of grain the finer growth matrix is a grains. process In through general, which abnormal few energeticgrain growth grains is growmore atlikely the expenseto occur ofwhen the finerthe usual matrix grain grains. growth In general, of the matrix abnormal grain grain stagnate growths, when is more the likelysecond to phase occur whenparticles the are usual unstable grain growthand when of thethere matrix is strong grain texture stagnates, [62,63]. when Charit the secondet al. [64] phase claimed particles that arethe unstablereduction and of pinning when there force is strongdue to texturedissolution [62,63 of]. Charitparticles et al.and [64 anisotropy] claimed that in grain the reduction boundary of pinningenergy and force mobility due to dissolution promotes ofabnormal particles grain and anisotropygrowth. Abnormal in grain grain boundary growth energy becomes and mobilitya possibility promotes when abnormalthe pinning grain parameter, growth. AbnormalZ, lies within grain the growth range 0.25 becomes < Z < a1 possibility(Z = 3 FvR/d when, where the Fv pinning is the volume parameter, fractionZ, lies of particles, within the R rangeis the 0.25average< Z grain< 1 (Z radius,= 3 FvR and/d, whered is theF vaverageis the volume particle fractiondiameter). of particles,Charit et al.R is [65] the found average that grain FSP 7475Al radius, anddid not d is display the average superplastic particle diameter).elongation due Charit to etabnormal al. [65] foundgrain thatgrowth. FSP Figure 7475Al 1 did elucidates not display the superplasticrelation between elongation pinning due parameter to abnormal Z and grain size growth. ratio of Figure FSP7475Al1 elucidates alloy. It the can relation be inferred between from pinningthe Figure parameter 1 that ZZ andvalue grain decreases size ratio continuously of FSP7475Al with alloy. an It increase can be inferred in temperature from the Figureup to 1450 that °C,Z value decreases continuously with an increase in temperature up to 450 C, owing to the coarsening owing to the coarsening and dissolution of MgZn2 precipitates. The authors◦ suggested that abnormal andgrain dissolution growth must of MgZn be restricted2 precipitates. to enhance The authors superplastic suggested elongation. that abnormal Moreover, grain inhomogeneous growth must be restrictedpattern during to enhance FSP also superplastic promotes elongation. the abnormal Moreover, grain growth inhomogeneous during superplastic pattern during deformation FSP also promotesprocess. Second the abnormal phase grainparticles growth are duringeffective superplastic in inhibiting deformation the abnormal process. grain Second growth, phase which particles is aredescribed effective in indetail inhibiting in Section the abnormal2.6. grain growth, which is described in detail in Section 2.6.

Figure 1.1. A microstructural stability map demonstrating th thee transition of microstructure from no growth to abnormal grain growth, reproducedreproduced fromfrom [[65],65], withwith permissionpermission fromfrom Elsevier,Elsevier, 2002. 2002.

Mahidhara [66] compared the ductility of 7475 aluminum alloy with grain sizes ranging between 4 1 9 and 35 µm at 457 ◦C and 517 ◦C at the same strain rate of 10− s− . They found that at temperature Metals 2020, 10, x FOR PEER REVIEW 5 of 26

Mahidhara [66] compared the ductility of 7475 aluminum alloy with grain sizes ranging Metals 2020, 10, 77 5 of 26 between 9 and 35 µm at 457 °C and 517 °C at the same strain rate of 10−4 s−1. They found that at temperature 517 °C, ductility decreases with increase in grain size until grain size reached 20 µm, beyond517 ◦C, ductilitythis, there decreases was no withappreciable increase drop in grain in du sizectility until whereas grain sizeat temperature reached 20 µ457m, beyond°C, initially this, ductilitythere was increases no appreciable with an drop increase in ductility in grain whereas size until at grain temperature size reached 457 ◦C, 14 initiallyµm; beyond ductility this increasesductility decreasedwith an increase with increase in grain in size grain until size. grain This size anomalous reached 14behaviorµm; beyond of alloy this at ductility457 °C is decreased due to a high with nucleationincrease in rate grain of size.cavities. This In anomalous another experiment, behavior of Ma alloy et al. at [67] 457 looked◦C is due into to the a high effect nucleation of grain size rate on of Al-4Mg-1Zrcavities. In another alloy at experiment, 175 °C and Ma 0.7–1.6 et al. [µm.67] lookedThe results into thesuggested effect of grainthat a size decrease on Al-4Mg-1Zr in grain sizes alloy improvesat 175 ◦C andthe ductility 0.7–1.6 µ m.of the The alloy results as suggesteddisplayed thatin Figure a decrease 2. Similarly, in grain Caballero sizes improves et al. the[68] ductility analyzed of Al-Zn-Mg-Cuthe alloy as displayed alloy at intwo Figure different2. Similarly, cooling Caballero rates and et al.grain [ 68 ]sizes analyzed (0.5 Al-Zn-Mg-Cuµm and 0.2 µm). alloy At at twothe −2 −1 temperaturedifferent cooling 400 °C rates and and strain grain rate sizes 10 (0.5 s , µthem anduncooled 0.2 µm). material At the with temperature a grain size 400 of◦C 0.5 and µm strain reached rate 2 1 maximum10− s− , the elongation uncooled to material failure with511%, a grainwhereas size the of 0.5cooledµm reachedmaterial maximumwith the grain elongation size of to 0.3 failure µm reached511%, whereas same maximum the cooled elongation material with to failure the grain at 350 size °C. of Increase 0.3 µm reached in maximum samemaximum elongation elongation to failure at to relativelyfailure at 350small◦C. grain Increase size in confirms maximum the elongation dependence to failure of elongation at relatively on smallthe grain grain size size of confirms the grain the boundarydependence sliding of elongation equation. on the grain size of the grain boundary sliding equation.

Figure 2. Variation of elongation with strain rate and grain size, reproduced from [67], with permission Figure 2. Variation of elongation with strain rate and grain size, reproduced from [67], with from Elsevier, 2005. permission from Elsevier, 2005. 2.2. Effect of Temperature on the Superplasticity of Al Alloys 2.2. Effect of Temperature on the Superplasticity of Al Alloys Superplastic deformation temperature plays a vital role in enhancing the mechanical property of superplasticSuperplastic alloy. deformation Research temperature suggests that plays superplastic a vital role deformation in enhancing temperature the mechanical should property be more ofthan superplastic the homologous alloy. Research temperature suggests to achieve that su betterperplastic mechanical deformation properties temperature [69,70]. Withshould an be increase more thanin temperature, the homologous the viscous temperature force within to achieve the grain better boundary mechanical decreases properties gradually, [69,70]. which With makesan increase grain inboundaries temperature, unstable. the viscous This causesforce within GBS, whichthe grai isn the boundary basic requirement decreases gradually, for superplasticity. which makes However, grain boundariesan excessive unstable. increase This in temperature causes GBS, softens which theis the grain basic boundary requirement and grainfor superplasticity. boundary binding However, force, anwhich excessive declines increase the elongation in temperature [71]. Alhamidi softens the et at.grain [72 ]boundary studied the and superplastic grain boundary behavior binding of AA2024 force, which declines the elongation [71]. Alhamidi et at. [72] studied the superplastic behavior of AA2024 alloy at 400 ◦C and concluded that elongation to failure increases with an increase in temperature alloytill it at reaches 400 °C itsand peak, concluded after which that elongation an increase to fail in temperatureure increases decreaseswith an increase the elongation in temperature to failure. till itThey reaches claimed its peak, that the after former which increase an increase in elongation in temperature with temperature decreases is the due elongation to GBS but atto afailure. temperature They claimed that the former increase in elongation with temperature is due to GBS but at a temperature higher than 400 ◦C, the contribution of GBS to total strain decreases because of grain growth and a higherdecrease than in grain400 °C, boundary the contribution fraction. of Figure GBS 3to further total strain supports decreases the interrelation because of betweengrain growth maximum and a decreaseelongation in tograin failure boundary and temperature fraction. Figure at a specific 3 further strain supports rate. the Dramatic interrelation reduction between in elongation maximum to elongationfailure in Figure to failure3a is dueand totemperature the presence at ofa excessivespecific strain liquid rate. phase Dramatic at grain reduction boundaries, in whichelongation causes to failuregrain boundary in Figure separation3a is due to under the presence tension. of In excessive another experiment liquid phas conductede at grain byboundaries, Li et al. [73 which], the authorscauses grain boundary separation under tension. In another experiment conducted by Li et al.6 [73],2 the1 examined the superplastic behavior of Al-4.5Mg at 450–570 ◦C and strain rates from 10− –10− s− . −6 −2 authorsThrough examined their observations, the superplastic it was behavior concluded of thatAl-4.5Mg elongation at 450–570 increases °C and with strain temperature. rates from However, 10 –10 −1 sthe. maximumThrough their elongation observations, achieved it was was only concluded 282% due that to cavitation. elongation Furthermore, increases with Caballero temperature. et al. [51]

Metals 2020, 10, x FOR PEER REVIEW 6 of 26 Metals 2020, 10, 77 6 of 26 However, the maximum elongation achieved was only 282% due to cavitation. Furthermore, Caballerofound that et flow al. [51] stress found decreases that flow with stress increase decreases in temperature with increase in Al7075 in temperature alloy, which in can Al7075 be inferred alloy, whichfrom Figure can be3 b.inferred from Figure 3b.

FigureFigure 3. 3. ((aa)) Relation Relation between between temperature temperature and and elongation elongation to to failure failure Al-6Mg-0.3Sc Al-6Mg-0.3Sc (Left) (Left) reproduced reproduced b fromfrom [[70],70], with with permission permission from from Elsevier, Elsevier, 1998; (1998;) True (b stress-true) True stress-true strain curves strain at di curvesfferent temperaturesat different for the overaged 7075 alloy, reproduced from [51], with permission from Elsevier, 2017. temperatures for the overaged 7075 alloy, reproduced from [51], with permission from Elsevier, 2017. An equally significant aspect of aluminum superplasticity is low temperature superplasticity, An equally significant aspect of aluminum superplasticity is low temperature superplasticity, which is normally less than 300 C. Noda et al. [74] carried out a low-temperature experiment on which is normally less than 300 °C.◦ Noda et al. [74] carried out a low-temperature experiment on Al-Mg alloy by multi-axial alternative forging, which resulted in a grain size of 0.8 µm. Through Al-Mg alloy by multi-axial alternative forging, which resulted in a grain size of 0.8 µm. Through their observations, elongation of 340% was found at temperature 200 C and 2.8 10 3 s 1 strain rate. their observations, elongation of 340% was found at temperature 200◦ °C and 2.8× × 10−−3 s−1 strain rate. It was inferred from the microstructure evaluation that intragranular deformation due to dislocation It was inferred from the microstructure evaluation that intragranular deformation due to dislocation movement and GBS were the main deformation mechanism of superplastic deformation at 200 C. movement and GBS were the main deformation mechanism of superplastic deformation at 200 °C.◦ Ota et al. [75] conducted a series of experiments employing ECAP process on three different Al-3%Mg Ota et al. [75] conducted a series of experiments employing ECAP process on three different alloys containing 0.2% Sc, 0.2% Fe, and 0.1% Zr separately at 250 and 300 C. Al-3%Mg-0.2%Sc alloy Al-3%Mg alloys containing 0.2% Sc, 0.2% Fe, and 0.1% Zr separately◦ at 250 and 300 °C. yielded maximum elongation of 640% when tested at 250 C and 3.3 10 4 s 1 while 1280% when Al-3%Mg-0.2%Sc alloy yielded maximum elongation of 640%◦ when tested× −at 250− °C and 3.3 × 10−4 s−1 tested at 300 C and 1 10 2 s 1. Such a large elongation at 300 C was possible due to the stable while 1280% when◦ tested× at −300 −°C and 1 × 10−2 s−1. Such a large elongation◦ at 300 °C was possible due fine-grained structure at high temperatures. to the stable fine-grained structure at high temperatures. 2.3. Effect of Strain Rate on the Superplasticity of Al Alloys 2.3. Effect of Strain Rate on the Superplasticity of Al Alloys Strain rate significantly influences the superplastic deformation behavior of Al alloys. Optimum Strain rate significantly influences the superplastic deformation 4behavior1 of Al alloys.3 1 Optimum superplastic behavior is generally observed at strain rates from 10− s− to 5 10− s− , and tensile −4 −1 × −3 −1 superplasticelongation tends behavior to decrease is generally at lower observed and higher at strain strain rates rates from [76 ].10 Generally, s to 5 × GBS 10 ands , and dislocation tensile elongationcreep contribute tends significantlyto decrease at to lower deformation and higher at low strain strain rates rates, [76]. but Generally, these processes GBS and are dislocation too slow to creep contribute significantly to deformation at low strain rates, but these processes are too slow to contribute to total deformation at high strain rates. Consequently, at high strain rate deformation contribute to total deformation at high strain rates. Consequently, at high strain rate deformation occurs mainly due to dislocation creep [60]. According to Equation (1), true stress is dependent on the occurs mainly due to dislocation creep [60]. According to Equation (1), true stress is dependent on strain rate, which infers that stain rate also plays a key role in the superplasticity of Al alloys. the strain rate, which infers that stain rate also plays a key role in the superplasticity of Al alloys. . m 𝜎=𝐾𝜀σ = Kε (1)(1) . Where σ𝜎 is is truetrue stress, ε𝜀 is is strain rate, rate, mm isis strain strain rate rate sensitivity, sensitivity, and and K isK theis the material material parameter. parameter. In FigureIn Figure 4a,4 a,it is it shown is shown that that the thesigmoidal sigmoidal relation relation exists exists between between flow flow stress stress and andstrain strain rate, rate, i.e., maximumi.e., maximum flow flow stress stress declines declines with with decrease decrease in instrain strain rate rate [72,77,78]. [72,77,78 ].Likewise, Likewise, Ma Ma et et al. al. [79] [79] concludedconcluded that that Al-Mg-Zr Al-Mg-Zr alloy alloy shows shows superplastic superplastic behavior behavior even even at at a higher strain rate. It It can can be be inferredinferred from FigureFigure4 b4b that that elongation elongation increases increases with with an an increase increase in strain in strain rate rate and and reaches reaches a peak a peak over overan intermediate an intermediate range ofrange strain of rate strain but rate decreases but decreases with further with increase further in strainincrease rate in [70 strain]. Elongation rate [70]. is 2 1 −2 −1 Elongationlow at strain is rateslow at< 10strain− s− ratesdue < to 10 relatively s due longtimeto relatively for dynamiclongtime recrystallization,for dynamic recrystallization, which causes 2 1 −2 −1 whichgrain growth. causes grain Likewise, growth. elongation Likewise, decreases elongation at strain decreases rate > 10 at− strains− because rate > dynamic10 s because recrystallization dynamic recrystallizationdoes not occur due does to shortnot occur time due of deformation to short time that of makesdeformation grain boundarythat makes di grainfficult boundary to slide. Moreover, difficult to slide. Moreover, Le et al. [80] compared the grain sizes after fracture at different strain rates and

Metals 2020, 10, 77 7 of 26

Metals 2020, 10, x FOR PEER REVIEW 7 of 26 Le et al. [80] compared the grain sizes after fracture at different strain rates and confirmed that grain confirmedsize decreases that withgrain an size increase decreases in initial with strain an rate.increase To some in initial extent microstructurestrain rate. To after some deformation extent microstructurecan be determined after deformation by strain rates. can Low be straindetermined rates couldby strain result rates. into coarseLow strain microstructure rates could while result high intostrain coarseMetals rates microstructure 2020 lead, 10,to x FOR relatively PEER while REVIEW fine high microstructure, strain rates le attributedad to relatively to recovery fine mi andcrostructure, dynamic recrystallization attributed7 of 26 to recoveryprocess. and dynamic recrystallization process. confirmed that grain size decreases with an increase in initial strain rate. To some extent microstructure after deformation can be determined by strain rates. Low strain rates could result into coarse microstructure while high strain rates lead to relatively fine microstructure, attributed to recovery and dynamic recrystallization process.

FigureFigure 4. ( 4.a) (Flowa) Flow stress stress as asa function a function of ofstrain strain rate, rate, reproduced reproduced from from [78], [78 with], with permission permission from from

Elsevier,Elsevier, 2016; 2016; (b)( bElongation) Elongation as a as function a function of strain of strain rate rate reproduced reproduced from from [70], [70 with], with permission permission from from Elsevier,Elsevier,Figure 1998. 1998. 4. (a) Flow stress as a function of strain rate, reproduced from [78], with permission from Elsevier, 2016; (b) Elongation as a function of strain rate reproduced from [70], with permission from Generally,Generally,Elsevier, flow flow 1998. stress stress and and rate rate of flowflow hardeninghardening increase increase with with an an increase increase in strainin strain rate. rate. A glance A glanceat Figure at Figure5a reveals 5a reveals true true stress stress rises rises steeply steeply and and then then hold hold constant constant or or decrease decrease after after attaining attaining the peak valueGenerally, with increasing flow stress strain and rate rate. of In flow addition, hardening the increase span length with an of increase strain during in strain which rate. nearlyA the peakglance value at with Figure increasing 5a reveals strain true stress rate. rises In additi steeplyon, and the then span hold length constant of strain or decrease during after which attaining nearly constant flow behavior appears increases with decreasing strain rate. Rapid increase in the stress at constantthe flow peak behavior value with appears increasing increases strain rate.with In decreasing addition, the strain span rate.length Rapid of strain increase during in which the stressnearly at thethe beginning beginningconstant of flow ofthe the deformationbehavior deformation appears is due is increases due to increase to increasewith decreasing in deformation in deformation strain density.rate. density. Rapid Nevertheless, increase Nevertheless, in the constant stress constant at or or decreasedecreasethe in beginning in stress after ofafter the attaining deformationattaining peak peakis valuedue tovalue is increase due is to in dynamicdue deformation to dynamic softening density. arisingsoftening Nevertheless, from arising a recrystallization constant from or a recrystallizationprocess.decrease Moreover, process.in stress flow Moreover, stressafter attaining increases flow peakstress with increasingvalue increases is due strainwith to increasing ratedynamic because softeningstrain of lack rate ofarising because time forfrom of di fflackausion of andtime accommodation recrystallizationfor diffusion and process. process accommodation Moreover, [78,81]. Charit flow process stress et al. [increases82[78,81].] observed Charitwith increasing a monotonicet al. [82] strain observed increase rate because in a cavitymonotonic of lack volume increasefractionof in time withcavity for an volumediffusion increase fraction and in strain accommodation with rate an as increase displayed process in [78,81]. instrain Figure rateCharit5b. as Atet displayed al. low [82] strain observed in rates,Figure a comparativelymonotonic 5b. At low increase in cavity volume fraction with an increase in strain rate as displayed in Figure 5b. At low strainsmall rates, stress comparatively is accumulated small at thestress grain is accumula boundariested and at the also grain the stressboundaries has more and time also tothe relax, stress thereby has decreasingstrain rates, the possibility comparatively of cavity small stress nucleation. is accumulated at the grain boundaries and also the stress has more timemore to timerelax, to therebyrelax, thereby decreasing decreasing the possibilitythe possibility of ofcavity cavity nucleation. nucleation.

Figure 5. (a)Effect of strain rates on true strain-true stress curves, reproduced from [81], with permission Figure 5. (a) Effect of strain rates on true strain-true stress curves, reproduced from [81], with from Elsevier, 2009. (b) Variation of cavity volume fraction with strain rate, reproduced from [82], permission from Elsevier, 2009. (b) Variation of cavity volume fraction with strain rate, reproduced Figure 5. (a) Effect of strain rates on true strain-true stress curves, reproduced from [81], with with permissionfrom [82], with from permission Cambridge from University Cambridge Press,University 2004. Press, 2004. permission from Elsevier, 2009. (b) Variation of cavity volume fraction with strain rate, reproduced from [82], with permission from Cambridge University Press, 2004.

Metals 2020, 10, 77 8 of 26

Strain rate interval at which superplasticity normally occurs (<1 10 2 s 1) is often too slow from × − − the viewpoint of industrial applications. Strain rate > 1 10 2 s 1 is suitable for industrial applications × − − and would satisfy the current industrial manufacturing speed [83]. Mikhaylovskaya et al. [84] evaluated the impacts of high strain rate on two Al-Zn-Mg-Zr-Sc alloys distinguished by the presence and absence of Al9FeNi particles. Alloy with Al9FeNi particles displayed elongation to failure up to 915% while alloy without Al9FeNi particles exhibited maximum elongation of 310% at strain rates up to 1 10 2 s 1 and a temperature of 480 C. The partially recrystallized structure was observed in alloy × − − ◦ without Al9FeNi particles and superplastic indicators were significantly lower. Charit and Mishra [85] evaluated the effect of high strain rate on AA2024 alloy prepared by FSP. Superplastic experiments 2 1 conducted at a strain rate of 10− s− and 430 ◦C and resulted in a maximum elongation of 525%. It is 1 1 important to note that, even at a high strain rate of 10− s− , the elongation to failure was >280%. Very fine grain size (3.9 µm) and high angle disorientation of grain boundaries were largely responsible for the enhanced superplastic response at high strain rate. Straumal et al. [86] claimed that prewetting or premelting of the grain boundary is responsible for high strain rate superplasticity in Al-Mg alloys. It is, however, important to note that the highest strain rate to obtain superplasticity in Al alloys 1 is roughly 1 s− . Musin et al. [77] achieved an elongation to failure of about 500% at a strain rate of 1 1.2 s− in Al-Mg-Sc alloy. Such high elongation was attributed to the ultrafine grain structure (1 µm) produced by Equal Channel Angular Extrusion (ECAE) process and the presence of coherent Al3Sc dispersoids. Likewise, Ma et al. [79] attained a maximum elongation of~ 450% in FSP Al-1Mg-4Zr alloy ascribed to fine microstructure with a grain size of 1.5 µm and uniform distribution of fine Al3Zr 0 3 1 dispersoids. It can be seen from the above analysis that high strain rate superplasticity (10 –10 s− ) might be interesting and would satisfy the current industrial manufacturing speed.

2.4. Effect of Strain Rate Sensitivity on the Superplasticity of Al Alloys Strain rate sensitivity (m) is an indicator of a material of its superplastic potential and its value is derived from applied stress and corresponding strain rate [87,88]. For a material to exhibit superplastic properties, strain rate sensitivity should be more than or equal to 0.3. The higher value of strain rate sensitivity indicates that the material is more stable against the local strain rate increase and thus exhibits higher superplastic elongation. Experimental studies have shown that the value of strain rate sensitivity depends on temperature, strain rate, and strain [77,89]. Arieli et al. [90] analyzed the relationship between strain rate sensitivity and strain rate and concluded that strain rate sensitivity decreases rapidly at low and high strain rates as the strain rate increases, whereas the rate of decrease is slower at intermediate strain rates. Similarly, Friedman et al. [91] evaluated the influence of strain rate sensitivity and strain rate on AA5083 alloy at 5% elongation and found that deformation at lower strain rate is controlled by diffusional accommodation at grain boundaries but the deformation at relatively higher strain rate is controlled by thermally assisted dislocation motion. However, at the intermediate strain rate, where the ‘m’ value peaks, it is presumed that both diffusional accommodation at grain boundaries and thermally assisted dislocation mechanisms are associated, as shown in Figure6a. Likewise, 6b elucidate that strain rate sensitivity increases with an increase in temperature. Moreover, Figure7 illustrates that strain rate sensitivity shows significantly higher value of m at the strain rates of 1 10 4 and 1 10 5 than at 1 10 3 and × − × − × − 1 10 2. With increasing the strain, strain rate sensitivity decreases due to grain growth as well as × − damage accumulation in the form of cavity nucleation. Metals 2020, 10, x FOR PEER REVIEW 9 of 26 Metals 2020, 10, 77 9 of 26 Metals 2020, 10, x FOR PEER REVIEW 9 of 26

Figure 6. (a) Relation between strain rate sensitivity and strain rate for Superplastic Forming (SPF) Figure 6. (a) Relation between strain rate sensitivity and strain rate for Superplastic Forming (SPF) 5083 5083 alloys in longitudinal orientation, reproduced from [91], with permission from Springer Nature, alloysFigure in 6. longitudinal (a) Relation orientation, between strain reproduced rate sensitivity from [91 ],an withd strain permission rate for fromSuperplastic Springer Forming Nature, 2004;(SPF) 2004; (b) variation of strain rate sensitivity at different temperatures and strain rates for the (b5083) variation alloys in of longitudinal strain rate sensitivity orientation, at reproduced different temperatures from [91], with and permission strain rates from for the Springer Al-6Mg-0.3Sc Nature, Al-6Mg-0.3Scalloy,2004; reproduced (b) variation alloy, from reproduced of [ 70strain], with ratefrom permission sensitivity [70], with from permissionat Elsevier,different 1998.from temperatures Elsevier, 1998. and strain rates for the Al-6Mg-0.3Sc alloy, reproduced from [70], with permission from Elsevier, 1998.

Figure 7. Strain rate sensitivity as a function of true strain at 475 °C◦ C[ [91].91]. 2.5. Effect of MicrostructureFigure 7. RefinementStrain rate sensitivity Techniques as on a function the Superplasticity of true strain of Alat 475 Alloys °C [91]. 2.5. Effect of Microstructure Refinement Techniques on the Superplasticity of Al Alloys Superplastic deformation mechanism and mechanical properties differ with change in 2.5. SuperplasticEffect of Microstructure deformation Refinement mechanism Techniques and on mechanical the Superplasticity properties of Al Alloysdiffer with change in microstructure refinement techniques. Alloys with almost similar composition exhibit different microstructure refinement techniques. Alloys with almost similar composition exhibit different mechanicalSuperplastic properties deformation because of dimechanismfferent refinement and mechanical techniques. properties Loucif et al.differ [92] preparedwith change AA7075 in mechanical properties because of different refinement techniques. Loucif et al. [92] prepared AA7075 alloymicrostructure by HPT while refinement Caballero techniques. et al. [51] prepared Alloys AA7075 with almost alloy by similar FSP and composition they performed exhibit superplastic different alloymechanical by HPT properties while Caballero because of et different al. [51] refinemepreparednt AA7075techniques. alloy Loucif by etFSP al. and[92] preparedthey performed4 AA70751 deformation test in the temperature range from 200–450 ◦C and strain rate from 1 10− –1 10−−4 superplasticalloy2 by HPT deformation while Caballero test in the et temperature al. [51] prepared range fromAA7075 200–450 alloy °C by and FSP strain 1and rate2 ×they from performed 1 × 10 – s− . Loucif−1 −2 et al. attained maximum elongation up to 400% at 350 ◦C and 10− s− while−1Caballero −2 1 × 10 s . Loucif et al. attained maximum elongation up to 400% at 350 °C and 10 s while−4 etsuperplastic al. obtained deformation a maximum test elongation in the temperature of 126% at range the same from temperature 200–450 °C and and strain strain rate rate. from In another1 × 10 – Caballero−1 et−2 al. obtained a maximum elongation of 126% at the same temperature and strain−1 rate.−2 In experiment,1 × 10 s . ChentoufLoucif et et al. al. attained [93] analyzed maximum the superplastic elongation behavior up to of400% AA5083 at 350 by hot°C and cold10 preform,s while another experiment, Chentouf et al. [93] analyzed the superplastic behavior of AA5083 by hot and whoseCaballero average et al. grain obtained size was a maximu determinedm elongation to be 8.32 of and 126% 7.95 atµ them respectively.same temperature They concluded and strain that rate. the In cold preform, whose average grain size was determined to be 8.32 and 7.95 µm respectively. They numberanother ofexperiment, cavities in theChentouf hot preform et al. case[93] analyzed were less thanthe superplastic in the cold preformbehavior case of AA5083 and hot preformedby hot and concluded that the number of cavities in the hot preform case were less than in the cold preform case samplescold preform, led to anwhose increase average in the grain nucleation size was of determined subsurface discontinuities.to be 8.32 and 7.95 Nevertheless, µm respectively. the average They and hot preformed samples led to an increase in the nucleation of subsurface discontinuities. cavityconcluded size ofthat hot the preformed number of sample cavities was in the higher hot pr thaneform cold case preformed were less than sample. in the Large cold preform cavity sizes case Nevertheless, the average cavity size of hot preformed sample was higher than cold preformed areand susceptible hot preformed to the samples formation led of to large an sizedincrease voids in andthe cracking.nucleation Furthermore, of subsurface Smolej discontinuities. et al. [47] sample. Large cavity sizes are susceptible to the formation of large sized voids and cracking. comparedNevertheless, superplastic the average behavior cavity of Al-4.5Mg-0.35Sc-0.15Zrsize of hot preformed alloysample by was rolling higher and FSPthan and cold claimed preformed that Furthermore, Smolej et al. [47] compared superplastic behavior of Al-4.5Mg-0.35Sc-0.15Zr alloy by insample. the temperature Large cavity range sizes from are 350–500 susceptibleC and strainto the rate formation from 1 of10 large2 s 1 –1sized s 1, thevoids elongation and cracking. of the rolling and FSP and claimed that in the◦ temperature range from× 350–500− − °C and− strain rate from 1 × FSP-processedFurthermore, Smolej alloy is et 7, al. 2.5 [47] times compared higher than superpla thosestic of thebehavior rolled of alloy. Al-4.5Mg-0.35Sc-0.15Zr Rolled samples underwent alloy by 10rolling−2 s−1–1 and s−1, FSPthe elongationand claimed of thatthe FSP-processedin the temperatur alloye range is 7, 2.5from times 350–500 higher °C than and thosestrain ofrate the from rolled 1 × 10−2 s−1–1 s−1, the elongation of the FSP-processed alloy is 7, 2.5 times higher than those of the rolled

Metals 2020, 10, 77 10 of 26 Metals 2020, 10, x FOR PEER REVIEW 10 of 26 incipientalloy. Rolled hardening samples followed underwent by softening incipient at higher harden strainsing whereasfollowed FSPby samplessoftening exhibited at higher smooth strains and risingwhereas hardening FSP samples during theexhibited initial smooth superplastic and rising flow. Superiorhardening elongation during the of initial FSP samples superplastic was possibleflow. becauseSuperior of stableelongation microstructure of FSP samples at high temperature,was possibl whiche because was ensuredof stable by microstructure the addition of at Sc high and Zr. temperature,Wang et al. which [94] was claimed ensured exceptionally by the addition high of elongationSc and Zr. of 3250% using Al-Zn-Mg-Cu alloy. The alloyWang was et prepared al. [94] claimed by FSP, exceptionally which resulted high in elongation an equiaxed of 3250% grain using size ofAl-Zn-Mg-Cu 6.2 µm. The alloy. tensile The test alloy was prepared by FSP, which resulted in an equiaxed grain size of 6.2 µm.2 The1 tensile test was was done in the temperature range of 500–535 ◦C and at a strain rate of 10− s− , which resulted in done in the temperature range of 500–535 °C and at a strain rate of 10−2 s−1, which resulted in elongation of 3250% at 535 C. Such high elongation was obtained due to the presence of a small amount elongation of 3250% at 535◦ °C. Such high elongation was obtained due to the presence of a small of liquid phase at the grain boundaries, which helped remove the stress concentration and suppress amount of liquid phase at the grain boundaries, which helped remove the stress concentration and the appearance of cavities during deformation. Further, relatively stable grain size at high temperature suppress the appearance of cavities during deformation. Further, relatively stable grain size at high duetemperature to the pinning due ofto athe high-density pinning of Cra high-density bearing dispersoids Cr bearing was dispersoids responsible was for responsible high elongation. for high Park et al.elongation. [95] studied Park the et superplasticityal. [95] studied of the a commercial superplasticity Al-Mg of alloya commercial subjected Al-Mg to ECAP alloy and subjected ECAP + topost rolling.ECAP Theand high ECAP strain + post rate rolling. superplastic The high elongation strain rate was superplastic remarkably elongation enhanced duringwas remarkably Post-ECAP rolling,enhanced as shown during in FigurePost-ECAP8. ECAP rol+ling,post as rolled shown samples’ in Figure microstructure 8. ECAP consisted+ post rolled of elongated samples’ band structuremicrostructure delineated consisted by lamellar of elongated boundaries band and st deformationructure delineated was dominated by lamellar by GBS boundaries whereas and ECAP sampledeformation was governed was dominated by dislocation by GBS viscous whereas glide. ECAP Post sample rolling was after governed ECAP enhancedby dislocation elongation viscous by increasingglide. Post the rolling portion after of high ECAP angle enhanced boundaries elonga whichtion changedby increasing the deformation the portion mechanismof high angle from dislocationboundaries viscous which glidechanged to GBS. the deformation mechanism from dislocation viscous glide to GBS.

FigureFigure 8. (8.a) ( Equala) Equal channel channel angular angular pressing pressing (ECAP)-processed (ECAP)-processed sample, sample, (b) ECAP (b) ECAP+ post + rolled post samples,rolled reproducedsamples, reproduced from [95], withfrom permission[95], with permission from Elsevier, from 2005.Elsevier, 2005.

SPDSPD techniques techniques provide provide an an opportunity opportunity toto achieveachieve remarkable remarkable grain grain refinement refinement by by imposing imposing largelarge strain strain without without any any significant significant change change in in the the overall overall dimensions dimensions of of the the sample, sample, which which leads leads to to the occurrencethe occurrence of superplasticity of superplasticity not only not at only low temperaturesat low temperatures but also but at high also strainat high rates. strain SPD rates. techniques SPD havetechniques the capability have ofthe altering capability the grainof altering orientation the grain by transforming orientation theby lowtransforming angle boundaries the lowto angle a large proportionboundaries of highto a large angle proportion grain boundaries. of high Nevertheless,angle grain boundaries. the mechanisms Nevertheless, underlying the mechanisms different SPD processingunderlying techniques different are SPD di ffprocessingerent. The evolutiontechniques of are dislocation different. cell The structures evolution and of itsdislocation transformation cell intostructures a new grain and its structure transformation with a large into a proportion new grain ofstructure high angle with grain a large boundaries proportion in of the high process angle of straininggrain boundaries is also diff inerent the forprocess different of straining SPD techniques is also different [96]. ECAP for different technique SPD increases techniques the [96]. mechanical ECAP strengthtechnique of the increases metallic the material mechanical by pressing strength it through of the metallic a die constrained material by within pressing a channel it through bent througha die a sharpconstrained angle nearwithin the a channel center of bent the through die [97]. a Insharp FSP, an thegle friction near the between center of the the rotating die [97]. pin In FSP, and metalthe surfacefriction results between in a stirredthe rotating zone withpin and fine grainmetal sizesurface [98, 99results]. HPT in involves a stirred the zone application with fine of grain a very size high hydrostatic[98,99]. HPT pressure involves on thethe material, application which of a plays very ahigh major hydrostatic role in grain pressure refinement on the [100 material,,101]. Moreover, which plays a major role in grain refinement [100,101]. Moreover, special rolling techniques for grain special rolling techniques for grain refinement like ARB, cryorolling, asymmetric rolling are affected by refinement like ARB, cryorolling, asymmetric rolling are affected by the amount of thickness the amount of thickness reduction in each pass and the level of rolling friction [102]. ARB comprises reduction in each pass and the level of rolling friction [102]. ARB comprises deformation and deformation and bonding process, which on repetition can produce SPD of bulky materials [103,104]. bonding process, which on repetition can produce SPD of bulky materials [103,104]. Asymmetric Asymmetric rolling encompasses compressive and shear strain to maintain a high degree of friction

Metals 2020, 10, 77 11 of 26

Metals 2020, 10, x FOR PEER REVIEW 11 of 26 between the sheet and the rolls. It not only decreases the rolling force and rolling torque but also improvesrolling theencompasses formability compressive of the material and shear [105– 107strain]. Furthermore, to maintain a cryorollinghigh degree producesof friction ultrafine between grainthe microstructuresheet and the in rolls. the light It not metals only decreases and alloys the that rolling require force a comparatively and rolling torque lower but load also to improves induce severe the formability of the material [105–107]. Furthermore, cryorolling produces ultrafine grain strain for producing the sub-microcrystalline structure [108–110]. microstructure in the light metals and alloys that require a comparatively lower load to induce 2.6.severe Effect ofstrain Addition for producing of Trace Elements the sub- inmicrocrystalline Alloys on the Superplasticity structure [108–110]. of Al Alloys

2.6.In Effect general, of Addition single of phase Trace materialsElements in do Alloys not on exhibit the Superplasticity superplastic of behaviorAl Alloys because grain grows rapidly at high temperatures. However, the addition of trace elements like Sc, Zr, Mg, Li, Cu [81] form In general, single phase materials do not exhibit superplastic behavior because grain grows nanoscale coherent Al Zr, Al Sc, Al Sc Zr precipitates at grain boundaries and within the grain rapidly at high temperatures.3 3 However,3 x the1-x addition of trace elements like Sc, Zr, Mg, Li, Cu [81] interiors to resist grain growth during recrystallization at elevated temperature. Thermally stable form nanoscale coherent Al3Zr, Al3Sc, Al3ScxZr1-x precipitates at grain boundaries and within the secondgrain phase interiors particles to resist stabilize grain graingrowth boundaries, during recr subgrainystallization boundaries, at elevated and temperature. dislocation by Thermally the Zener pinningstable e ffsecondect [111 phase,112 ].particles Trace elements stabilize havegrain strongbounda interactionsries, subgrain with boundaries, dislocations and anddislocation have ability by the to increaseZener the pinning dislocation effect [111,112]. production Trac ratee elements during deformation, have strong interactio which leadsns with to higher dislocations strain and hardening have andability subsequently to increase to anthe increased dislocation ductility. production Trace rate elements during not deformation, only interact which with leads all types to higher of structural strain defectshardening like void and formation,subsequently stacking to an increased faults, dislocations ductility. Trace but elements also modify not only the interact collective with behavior all types of theseof defectsstructural [113 defects]. The like addition void formation, of trace elements stacking not faults, only dislocations influences the but process also modify of lattice the dislocationcollective incorporationbehavior of intothese the defects grain [113]. boundary The addition but also of can trace change elements rate ofnot boundary only influences sliding the through process grain of boundarylattice dislocation segregation incorporation [114]. To achieve into the better grain superplasticity, boundary but sizealso ofcan the change second rate phase of boundary should be finesliding (<1 µ m)through and its grain distribution boundary should segregation be uniform. [114]. To Furthermore, achieve better the superplasticity, second phase mustsize of be the able to deformsecond withphase the should matrix be in fine order (<1 to µm) avoid and stress its dist concentrationribution should and earlybe unif fractureorm. Furthermore, [57]. In addition, the segregationsecond phase of impurity must be atomsable to atdeform grain with boundaries the matrix not in only order contribute to avoid stress to grain concentration boundary slidingand early but alsofracture influence [57]. the In cavitation addition, segregation by decreasing of impurity the surface atoms and at grain grain boundary boundaries energy, not only further contribute details to can be foundgrain inboundary [115]. Al sliding alloys canbut providealso influence good high-temperature the cavitation by properties decreasing using the coherent surface intermetallicand grain precipitatesboundary for energy, strengthening. further details can be found in [115]. Al alloys can provide good high-temperature propertiesThe addition using of coherent Mg in Al intermetalli as an alloyingc precipitates element for helps strengthening. to improve the mechanical properties like The addition of Mg in Al as an alloying element helps to improve the mechanical properties like work hardening characteristics of Al. In an experiment conducted by Kishchik et al. [116], mechanical work hardening characteristics of Al. In an experiment conducted by Kishchik et al. [116], behavior of Al-Mg-Mn-Cr was analyzed by varying the Mg content and stipulated that grain size mechanical behavior of Al-Mg-Mn-Cr was analyzed by varying the Mg content and stipulated that decreases with an increase in Mg content from 3 to 6.8%, beyond this, grain size increases due to grain size decreases with an increase in Mg content from 3 to 6.8%, beyond this, grain size increases formation conglomerates of chromium-manganese particles. Decrease in grain size was attributed due to formation conglomerates of chromium-manganese particles. Decrease in grain size was to increaseattributed in to the increase amount in ofthe magnesium amount of oxidemagnes enteringium oxide the entering melt upon the fusion,melt upon which fusion, serves which as a substrateserves foras a the substrate nucleation for the of newnucleation grains of upon new recrystallization.grains upon recrystallization. Likewise, Kotov Likewise, et al. Kotov [117] carriedet al. out[117] an experiment carried out onan Al-Zn-Mg-Cu-Ni-Zrexperiment on Al-Zn-Mg-Cu-Ni alloy and-Zr the alloy experimental and the experimental result suggested result that suggested with an increasethat with in the an amount increase of in Zn the and amount Mg from of Zn 0–4%, and grain Mg from size as 0–4%, well asgrain yield size stress as well decreases as yield as stress shown in Figuredecreases9a while as shown elongation in Figure to 9a failure while increases.elongation Theto failure addition increases. of Zn The and addition Mg not of only Zn and reduces Mg not the averageonly sizereduces of recrystallized the average size grains of recrystallized but also prevented grains thebut reconfiguration also prevented ofthe the reconfiguration dislocation structure of the by thedislocation formation structure of Al3 Zrby dispersedthe formation particles. of Al3Zr dispersed particles.

Figure 9. (a)Effect of concentration of Mg and Zn in alloys, (b) effect of addition of Sc in Al-Mg Figure 9. (a) Effect of concentration of Mg and Zn in alloys, (b) effect of addition of Sc in Al-Mg alloy alloy [117,118]. [117,118].

Metals 2020, 10, 77 12 of 26 Metals 2020, 10, x FOR PEER REVIEW 12 of 26

PengPeng et et al. al. [ 119[119]] compared compared thethe superplasticsuperplastic deformation behavior behavior of of two two aluminum aluminum Al-Mg-Mn Al-Mg-Mn alloysalloys with with Sc Sc and and Zr Zr addition addition and and it wasit was concluded concluded that that alloy alloy with with Sc andSc and Zr exhibitsZr exhibits lower lower flow flow stress, higherstress, m higher value, m refined value, grainrefined structure, grain structure, high elongation high elongation and restrict and grainrestrict growth. grain growth. Addition Addition of Sc and Zrof in Sc Al-Mg and Zr alloys in Al-Mg form alloys Al3Si andform Al Al3(Sc3Si1 andxZr Alx),3(Sc which1−xZr notx), which only refines not only the refines grain structurethe grain butstructure also pin − thebut sub-grain also pin boundaries the sub-grain and dislocations,boundaries and restricts dislocations, grain growth restricts and stabilizegrain growth deformation and stabilize structure atdeformation high temperature, structure that at high causes temperature, the excellent that causes superplasticity the excellent as demonstratedsuperplasticity inas demonstrated Figure9b [ 118 ]. Leein etFigure al. [112 9b ][118]. performed Lee et experimental. [112] performed on three experiment different Al-3%Mg on three alloys,different which Al-3%Mg contained alloys, 0.2% which Sc or 0.2%contained Zr or 0.2% 0.2% Sc Sc+ or0.12% 0.2% Zr.Zr or ECAP 0.2% method Sc + 0.12% was Zr. applied ECAP formethod grain was refinement applied andfor grain the final refinement grain size wasand in the the final range grain of 0.2–0.3 size wasµm forin the all threerange alloys. of 0.2–0.3 They µm concluded for all three that Al-Mg–Zralloys. They alloy concluded did not exhibitthat Al-Mg–Zr alloy did not exhibit superplastic ductility because its grain was not stable at high superplastic ductility because its grain was not stable at high temperatures due to the non-uniform temperatures due to the non-uniform distribution of Al3Zr, which is essential for superplastic flow. distribution of Al3Zr, which is essential for superplastic flow. Al-Mg–Sc and Al-Mg–Sc–Zr alloys both Al-Mg–Sc and Al-Mg–Sc–Zr alloys both exhibited better superplastic properties at 300 °C and 400 °C exhibited better superplastic properties at 300 C and 400 C whereas at temperature 500 C only whereas at temperature 500 °C only Al-Mg–Sc–Zr◦ alloy ◦exhibited excellent superplasticity,◦ as Al-Mg–Sc–Zr alloy exhibited excellent superplasticity, as displayed in Figure 10. Al-Mg–Sc displayed displayed in Figure 10. Al-Mg–Sc displayed better superplastic behavior at 300 °C and 400 °C due to better superplastic behavior at 300 ◦C and 400 ◦C due to Al3Sc precipitates which prevents grain Al3Sc precipitates which prevents grain growth but Al-Mg–Sc–Zr showed excellent superplasticity growth but Al-Mg–Sc–Zr showed excellent superplasticity at 500 ◦C because Al3(ZrxSc1-x) precipitates at 500 °C because Al3(ZrxSc1-x) precipitates appeared to be more stable than Al3Sc precipitates at 500 appeared°C [120]. to be more stable than Al3Sc precipitates at 500 ◦C[120].

FigureFigure 10. 10.Elongation Elongation toto failurefailure vs. strain rate rate reproduc reproduceded from from [112], [112 ],with with permission permission from from Elsevier, Elsevier,

2002:2002: ( a()a) Elongation Elongation to to failurefailure versusversus initial stra strainin rate rate for for the the alloys alloys tested tested at at 400 400 °C;◦C; (b ()b Elongation) Elongation to to failurefailure versus versus initial initial strain strain rate rate forfor thethe Al-Mg-Sc-ZrAl-Mg-Sc-Zr and and Al-Mg-Sc Al-Mg-Sc alloys alloys tested tested at at 500 500 °C.◦C.

TheThe addition addition of Cu as as an an alloying alloying element element improv improveses the heat the heattreatability treatability of alloy of and alloy at the and same at the sametime timedecreases decreases the eutectic the eutectic temperature temperature as well as as wellmelting as meltingpoint of pointthe alloy. of the Furthermore, alloy. Furthermore, copper copperforms formsCuAl2 CuAlphase2 andphase many and other many intermetallic other intermetallic compou compounds,nds, which improves which improves the strength the strengthof the ofcasting the casting parts parts[121–124]. [121 –Hossain124]. Hossain et at. [125] et at.studied [125] the studied effect theof addition effect of of addition Cu in Al-6Si-0.5 of Cu in Mg Al-6Si-0.5 alloy Mgand alloy authors and authorsreported reportedan increase an increasein tensile in strength tensile strengthand decrease and decreasein ductility in ductilityof the alloy. of the They alloy. Theyfurther further elaborated elaborated that thatan addition an addition of 2% of 2%of ofCu Cu showed showed maximum maximum strength. strength. Increase Increase in intensile tensile strengthstrength is is attributed attributed toto thethe precipitationprecipitation of copper rich rich precipitates precipitates and and decrease decrease in in ductility ductility to tothe the formationformation of of void void and and its its coalescence.coalescence. Moreover, Watanabe Watanabe et et al. al. [126], [126], in in another another test, test, put put emphasis emphasis onon finding finding superplasticsuperplastic behavior of of Al Al alloys alloys with with the the addition addition of Copper. of Copper. Their Theirwork revealed work revealed that finely dispersed particles in copper containing alloy inhibit subgrain growth during continuous that finely dispersed particles in copper containing alloy inhibit subgrain growth during continuous recrystallization, which in turn results in finely recrystallized grain structures. The addition of 0.6% recrystallization, which in turn results in finely recrystallized grain structures. The addition of 0.6% of of copper in Al-5% Mg resulted in an increase in the value of m, enhanced the corrosion resistivity copper in Al-5% Mg resulted in an increase in the value of m, enhanced the corrosion resistivity and and increased total elongation by about 500%. increased total elongation by about 500%. Irina and Haiou [127] evaluated the influence of Mn and Fe on Al-Mg-Mn-Fe alloy by changing Irina and Haiou [127] evaluated the influence of Mn and Fe on Al-Mg-Mn-Fe alloy by changing the composition of Mn and Fe. The addition of Mn and Fe was in the range of 1.5% and 0.05% the composition of Mn and Fe. The addition of Mn and Fe was in the range of 1.5% and 0.05% respectively for grain refinement. Figure 11a indicates that Mn content can be increased beyond 1.5%

Metals 2020, 10, x FOR PEER REVIEW 13 of 26 Metals 2020, 10, 77 13 of 26 butMetals it is 2020 difficult, 10, x FOR to PEER prepare REVIEW it by casting due to the risk of precipitation of primary intermetallic13 of 26 crystals whereas Figure 11b illustrates that formability reduces with an increase in Fe content. respectivelybut it is difficult for grain to prepare refinement. it by Figurecasting 11 duea indicates to the risk that of Mn precipitation content can of beprimary increased intermetallic beyond Highest elongation to failure of Al-4.5 Mg-1.5 Mn-0.05 Fe-<0.1 Si alloy was 400% at 475 °C and 10−3 1.5%crystals but itwhereas is difficult Figure to prepare 11b illustrates it by casting that due formability to the risk reduces of precipitation with an of increase primary in intermetallic Fe content. s−1. During casting, coarse Al6Mn constituent particles were formed and fine dispersoids Al6Mn crystalsHighest whereas elongation Figure to 11failureb illustrates of Al-4.5 that Mg-1.5 formability Mn-0.05 reduces Fe-<0.1 with Si analloy increase was 400% in Fe at content. 475 °C Highest and 10−3 particles during homogenization and hot rolling, which in the presence of Fe formed Al6 (Fe, 3Mn).1 elongations−1. During to casting, failure ofcoarse Al-4.5 Al Mg-1.56Mn constituent Mn-0.05 Fe-particles<0.1 Si were alloy formed was 400% andat fine 475 dispersoids◦C and 10− Als6−Mn. These fine Al6 (Fe, Mn) particles inhibited the grain growth at high temperature by zener pinning Duringparticles casting, during coarse homogenization Al6Mn constituent and hot particles rolling, were which formed in the and presence fine dispersoids of Fe formed Al6Mn Al6 particles (Fe, Mn). effect, which is essential and beneficial to superplasticity. Likewise, Henager et al. [114] performed duringThese homogenizationfine Al6 (Fe, Mn) and particles hot rolling, inhibited which the in grain the presence growth ofat Fehigh formed temperature Al6 (Fe, by Mn). zener These pinning fine an experiment on Al-Mg-Mn-Sc-Sn alloy and suggested that the addition of Sn reduces cavitation by Aleffect,6 (Fe, Mn)which particles is essential inhibited and thebeneficial grain growth to superp at highlasticity. temperature Likewise, by Henager zener pinning et al. [114] effect, performed which is allowing a redistribution of stress at critical hetero-junctions (sites at which cavities nucleate) in the essentialan experiment and beneficial on Al-Mg-Mn-Sc-Sn to superplasticity. alloy and Likewise, suggeste Henagerd that the et al.addition [114] performedof Sn reduces an experimentcavitation by alloys. onallowing Al-Mg-Mn-Sc-Sn a redistribution alloy andof stress suggested at critical that hetero-junctions the addition of (sites Sn reduces at which cavitation cavities bynucleate) allowing in the a redistributionalloys. of stress at critical hetero-junctions (sites at which cavities nucleate) in the alloys.

Figure 11. Total elongation vs. ( a) Mn and ( b) Fe content [127]. [127].

3. Superplastic Superplastic Deformation Figure Mechanism 11. Total elongation of Al Alloys vs. (a) Mn and (b) Fe content [127].

3. SuperplasticSuperplastic Deformationdeformation Mechanismcan be defineddefined of Al by Alloys twotwo S-shapedS-shaped andand peakpeak shapedshaped curves.curves. S-shaped curve describes the relationship between logarithmic flow flow stress log(σ) and logarithmic strain rate . Superplastic deformation can be defined by two S-shaped and peak shaped curves. S-shaped loglog((εε) ), whereas, whereas the the peak-shaped peak-shaped curve curve describes describes the the relationship relationship between between strain strain rate rate sensitivity sensitivity m and m curve. describes the relationship between logarithmic flow stress log(σ) and logarithmic strain rate andlog( εlog(). Figureε ) . Figure 12 shows 12 shows the comparative the comparative effects effects of temperature, of temperature, grain size,grain and size, strain and rate.strain rate. log(ε ) , whereas the peak-shaped curve describes the relationship between strain rate sensitivity m and log(ε ) . Figure 12 shows the comparative effects of temperature, grain size, and strain rate.

(a) (b)

Figure 12.12.( a()a Variation)Variation of of strain(a ) strain rate, rate, flow stressflow andstress strain and rate strain sensitivity rate sensitivity (m)(b at) different (m) temperaturesat different temperaturesand grain sizes, and reproduced grain sizes, fromreproduced [36], with from permission [36], with permission from Elsevier, from 1969; Elsevier, (b) relation1969; (b) between relation Figure 12. (a)Variation of strain rate, flow stress and strain rate sensitivity (m) at different betweenflow stress, flow strain stress, rate strain sensitivity, rate sensitivity, and temperature, and temperature, reproduced reprod fromuced [ 128from], with[128], permissionwith permission from temperatures and grain sizes, reproduced from [36], with permission from Elsevier, 1969; (b) relation fromElsevier, Elsevier, 1982. 1982. between flow stress, strain rate sensitivity, and temperature, reproduced from [128], with permission Figurefrom Elsevier,1212aa demonstratesdemonstrates 1982. that that with with a a decrease decrease in in grain grain size, size, the the flow flow stress stress decreases, decreases, and and strain strain rate ratesensitivity sensitivity increases increases whereas whereas experimental experimental results re ofsults Figure of 12Figureb shows 12b that shows the flowthat stressthe flow decreases stress Figure 12a demonstrates that with a decrease in grain size, the flow stress decreases, and strain withdecreases an increase with an in increase the temperature in the temperature and the maximum and the value maximum of strain value rate of sensitivity strain rate is obtainedsensitivity at is a rate sensitivity increases whereas experimental results of Figure 12b shows that the flow stress decreases with an increase in the temperature and the maximum value of strain rate sensitivity is

MetalsMetals 2020,2020 10, x, 10 FOR, 77 PEER REVIEW 14 of14 26 of 26 obtained at a critical temperature (Tc). Lowest flow stress and the highest sensitivity at critical temperaturecritical temperature make it the (T bestc). Lowest working flow temperature stress and the [128]. highest sensitivity at critical temperature make it the best working temperature [128]. Superplastic performance, properties, and deformation mechanism are affected by the Superplastic performance, properties, and deformation mechanism are affected by the microstructural changes occurring during the superplastic forming as shown in Figure 13. microstructural changes occurring during the superplastic forming as shown in Figure 13.

Figure 13. Dependence of the microstructural evolution process on different factors. Figure 13. Dependence of the microstructural evolution process on different factors. The microstructural process at the surface depends on various factors, such as strain, strain rate, temperature,The microstructural back pressure, process and at strain the surface path. Morphological depends on changes various are factors, determined such byas strain, phase or strain grain rate, temperature,growth, phase back spatial pressure, distribution and strain and phasepath. shapeMorphological changes. Crystal changes geometric are determined changes depend by phase on or graintexture growth, changes phase and spatial grain boundary distribution changes. and Similarly, phase cavitationshape changes. and dislocation Crystal structure geometric are factors changes dependresponsible on texture for defect changes accumulation and grain [129 ].boundary changes. Similarly, cavitation and dislocation structure are factors responsible for defect accumulation [129]. 3.1. Superplastic Deformation Mechanisms 3.1. SuperplasticPlenty De offormation research hasMechanisms been done to find dominant microstructural features and results of those research have indicated that GBS is the most important feature of aluminum alloys superplasticityPlenty of research [130, 131has]. been However, done dislocation to find domina motion,nt dimicrostructuralffusion in grains, features and near-grain and results boundary of those researchregion have are indicated also major that features GBS of is superplasticity. the most impo Thertant theoretical feature of models aluminum for GBS alloys are divided superplasticity into [130,131].two categories, However, namely dislocation intrinsic motion, GBS anddiffusion extrinsic in GBS.grains, Grain and boundariesnear-grain slideboundary freely region in intrinsic are also majorGBS features and are of not superplasticity. hindered by any The external theoretical factors suchmodels as the for triple GBS grainare divided junctions, into grain two boundary categories, namelyledges, intrinsic and theGBS second and extrinsic phase at GBS. grain Grain boundaries. boundaries Generally, slide GBSfreely in in bicrystal intrinsic is intrinsicGBS and GBS. are not hinderedLikewise, by any in extrinsic external GBS, factors the slidingsuch as at the grain trip boundariesle grain junctions, is hindered grain by triple boundary grain junctions ledges, andand the secondother phase structural at grain factors boundaries. [132]. GBS Generally, must be GBS accommodated in bicrystal to is prevent intrinsic the GBS. formation Likewise, of a voidin extrinsic in GBS,the the microstructure. sliding at grain Accommodation boundaries is hindered mechanisms by cantriple be grain divided junctions into three and groups other as structural (i) diffusion factors accommodation; (ii) accommodation by dislocation motion, and (iii) combination of dislocation and [132]. GBS must be accommodated to prevent the formation of a void in the microstructure. diffusional accommodation [76]. Accommodation mechanisms can be divided into three groups as (i) diffusion accommodation, (ii) When tensile stress is applied to polycrystalline metal, it may deform either by dislocation accommodationprocesses occurring by dislocation within the grainsmotion, or by and the stress(iii) flowcombination of vacancies, of whichdislocation is known and as di diffusionalffusion accommodationcreep. Langdon [76]. [133 ] presented an extrinsic sliding model and claimed that dislocation is moved by a combinationWhen tensile of glidestress and is climb applied along to the polycrystalline GB leading to sliding metal, of theit may grains deform along the either boundary. by dislocation It was processesalso concluded occurring in within [133] that the climb grains velocity or by is the slower stress than flow that of of vacancies, glide and it which controls is theknown GBS rate.as diffusion Lin creep.and Langdon McLean [[133]134] presentedpresented evidence an extrinsic which sliding suggested model that dislocationand claimed primarily that dislocation originates within is moved the by a combinationgrains and entersof glide the and boundary climb regionalong bythe slip. GB Furthermore,leading to sliding Crossman of the and grains Ashby along [135] assumedthe boundary. that It was thealso grain concluded creep obeys in [133] the power-lawthat climb creep velocity and grainis slower boundaries than that slide of in glide a Newtonian and it controls viscous way.the GBS rate. ThroughLin and theirMcLean calculations, [134] presented it was displayed evidence that which at high suggested stress or strain-rate, that dislocation grain deforms primarily rapidly originates and the contribution of GBS is a negligible fraction to the total deformation. However, GBS contribution within the grains and enters the boundary region by slip. Furthermore, Crossman and Ashby [135] increases with decrease in stress and the deformation become non-uniform. SEM micrographs of the assumed that the grain creep obeys the power-law creep and grain boundaries slide in a Newtonian FSP 7075Al alloy deformed at 490 C and 3 10 3 s 1 strain rate, resulted in elongation of 200% and ◦ × − − viscous400% way. as shown Through in Figure their 14 calculations,a,b, respectively. it was It can disp belayed seen in that Figure at 14higha that stress surface or grainsstrain-rate, A, B, C,grain deformsD, and rapidly E move and apart the due contribution to GBS. With of further GBS increaseis a negligible in strain tofraction an elongation to the of total 400%, deformation. the GBS However,became GBS more distinctcontribution and the increases grains were with elongated decrease along thein tensilestress directionand the [136 deformation]. become non-uniform. SEM micrographs of the FSP 7075Al alloy deformed at 490 °C and 3 × 10−3 s−1 strain rate, resulted in elongation of 200% and 400% as shown in Figure 14a,b, respectively. It can be seen in Figure 14a that surface grains A, B, C, D, and E move apart due to GBS. With further increase in strain to an elongation of 400%, the GBS became more distinct and the grains were elongated along the tensile direction [136].

MetalsMetals 20202020,, 1010,, x 77 FOR PEER REVIEW 1515 of of 26 26 Metals 2020, 10, x FOR PEER REVIEW 15 of 26

(a) (b) (a) (b) Figure 14. SEM micrographs showing surface morphologies of FSP 7075Al superplastically deformedFigure 14.14. toSEM (SEMa) an micrographs elongationmicrographs showing of 200%showing surfaceand (surfaceb) morphologiesan elongation morpho logies of FSP400% 7075Alof [136]. FSP superplastically 7075Al superplastically deformed todeformed (a) an elongation to (a) an elongation of 200% and of (200%b) an and elongation (b) an elongation of 400% [136 of]. 400% [136]. Ball and Hutchison [137] proposed the first model to explain GBS accommodated by dislocation movement.Ball and Ball Hutchison and Hutchison [137] [137] proposed proposed the that first first during modelmodel tosuperplastic explain GBS deformation accommodated a group by dislocation of grains, whosemovement. boundaries Ball Ball and and are Hutchison Hutchison suitably alignedprop proposedosed tends that that to during during slide assuperplastic superplastic a unit until deformation deformation unfavorably a a group grouporiented of of grains, grains obstructwhose boundariesboundaries the process. are are suitably Assuitably shown aligned aligned in tendsFigure tends to slide 15a,to sl asidedislocation a unitas a until unit pile unfavorably until up unfavorably at grain oriented boundaries oriented grains obstruct grains and continuationtheobstruct process. the As ofprocess. shownsliding inrequiresAs Figure shown climb15a, in dislocation by Figure the leadin 15a, pileg up dislocation at grain boundaries towardpile up annihilation at and grain continuation boundaries sites. Soon of sliding afterand therequirescontinuation Ball and climb Hutchison of by sliding the leading requires model, dislocation Mukherjeeclimb by towardthe [138] leadin annihilationputg dislocationforward sites.a modificationtoward Soon annihilation after theof Ball Ball sites.and and Hutchison HutchisonSoon after modelmodel,the Ball thatMukherjee and explained Hutchison [138 the] model, putfact forward that Mukherjee grains a modification slide [138] indi putvidually of forward Ball rather and a Hutchisonmodification than in group model of andBall that grain and explained Hutchisonboundary the ledgesfactmodel that canthat grains act explained as slide dislocation individually the fact sources. that rather grains In thanMukherjee’ slide in indi groupviduallys model, and grain ratheradjacent boundary than grains in ledgesgroup need canand not act grainhave as dislocation toboundary slide as groupssources.ledges canand In act the Mukherjee’s as distance dislocation climbed model, sources. by adjacent dislocation In Mukherjee’ grains at needthes model, grain not haveboundary adjacent to slide grainsfor asthe groupsn purposeeed not and haveof annihilation the to distance slide as isclimbedgroups likely and byto be dislocationthe of distance the order at climbed the of grain the by grain boundary dislocation diameter for at the. theLater purpose grain Gifkins boundary of annihilation [139] forpresented the is purpose likely a slightly to of be annihilation of different the order modelofis likely the grainknown to be diameter. asof the coreorder Later and of Gifkins themantle grain [139 model ]diameter presented to expl. Later aain slightly theGifkins mechanism diff [139]erent presented model of GBS. known Ina slightlycore as theand different core mantle and model,mantlemodel known modelgrain was toas explainthe considered core the and mechanism asmantle the core model of and GBS. to theexpl In corereaingions andthe adjacentmechanism mantle model,to theof GBS. graingrain In wasboundary core considered and mantleas the as mantle.themodel, core grainGifkins and thewas considered regions considered adjacent the as factthe to the thatcore grain slidingand boundary the takes regions asplace theadjacent by mantle. the to motion Gifkinsthe grain of considered boundarygrain boundary the as factthe dislocationsthatmantle. sliding Gifkins takeswhich considered place pile byup theat thethe motion triplefact ofthat point. grain sliding boundaryShear takes stress dislocations place on the by whole the which motiongroup pile upconcentratesof atgrain the triple boundary on point. any grainSheardislocations that stress obstructs onwhich the wholepilethe movementup group at the concentrates triple of the point. group on Shear and any isstress grain relaxed thaton theby obstructs dislocation whole thegroup movementmotion concentrates in the of theblocking on group any grainsandgrain is that relaxedas illustratedobstructs by dislocation the in movementFigure motion 16a. of A in the themodel group blocking wa ands presented grainsis relaxed as illustrated byin dislocationwhich ingrain Figure motion boundary 16a. in A the modelsliding blocking was is accommodatedpresentedgrains as illustrated in which by the grain in deformation Figure boundary 16a. fold. slidingA model Figure is accommodated wa15bs illustratespresented the byin dislocationwhich the deformation grain mechanisms boundary fold. Figure slidinginvolved 15 isb inillustratesaccommodated the relaxation the dislocation byof stressthe deformation concentrations mechanisms fold. involvedresulting Figure 15b infrom theillustrates grain relaxation boundary the ofdislocation stress sliding concentrations mechanismsproposed by resultinginvolvedBall and Hutchison,fromin the grain relaxation boundaryMukherjee of stress sliding and concentrations Gifkins. proposed by resulting Ball and from Hutchison, grain boundary Mukherjee sliding and Gifkins. proposed by Ball and Hutchison, Mukherjee and Gifkins.

(a) (b) (a) (b) FigureFigure 15.15.( a()a Illustration) Illustration of the of Ball–Hutchison the Ball–Hutchison model ofmodel grain boundaryof grain sliding boundary (GBS) accommodatedsliding (GBS) accommodatedbyFigure dislocation 15. (a movement,) byIllustration dislocation [140 ].of movement, ( bthe) Slip Ball–Hutchison accommodation [140]. (b) Slip ofmodel GBSaccommodation proposedof grain by boundary (I)of Gifkins,GBS proposed sliding (II) Mukherjee, by(GBS) (I) Gifkins,andaccommodated (III) (II) Ball Mukherjee, and by Hutchinson dislocation and (III) [76 movement, Ball]. and Hutchinson [140]. (b) [76].Slip accommodation of GBS proposed by (I) Gifkins, (II) Mukherjee, and (III) Ball and Hutchinson [76].

Metals 2020, 10, 77 16 of 26 Metals 2020, 10, x FOR PEER REVIEW 16 of 26

(a) (b)

FigureFigure 16. (a )16. GBS (a and) GBS its accommodationand its accommodation proposed pr byoposed Gifkins by [139 Gifkins]. (b) A unified[139]. ( modelb) A unified for Rachinger model for GBS originatedRachinger GBS by Langdon originated [141 by]. Langdon [141].

PolycrystallinePolycrystalline material material under under the actionthe action of external of external stress stress may may also also deform deform by theby flowthe flow of of vacanciesvacancies known known as di asffusion diffusion creep. creep. Lifshitz Lifshitz [142 ][142] proposed proposed a theory, a theory, which which is also is also known known as Lifshitz as Lifshitz sliding,sliding, that that individual individual grains grains become become elongated elongated and an grainsd grains are movedare moved with with respect respect to each to each other other so so that sharpthat sharp offsets offsets become become visible. visible. However, However, Rachinger Rachinger [143] claimed [143] claimed that grains that retain grains essentially retain essentially their originaltheir shape original and shape are displaced and are withdisplaced respect with to eachrespect other to toeach contribute other to increase contribute in theirincrease number in their alongnumber the tensile along axis. the tensile Langdon axis. developed Langdon adeveloped unified model a unified for Rachingermodel for Rachinger sliding at allsliding grain at sizes all grain by employingsizes by employing a single modela single for model both for the both large the and larg smalle and grainsmall sizegrain ranges size ranges as demonstrated as demonstrated in in FigureFigure 16b [16b141 ].[141]. The The difference difference between between Lifshitz Lifshitz GBS GBS and and Rachinger Rachinger GBS GBS is that is that Lifshitz Lifshitz sliding sliding is is itselfitself an accommodation an accommodation process process for conventional for convention diffusional diffusion creep whereas creep Rachinger whereas slidingRachinger requires sliding accommodationrequires accommodation by intragranular by intragranular slip. slip. FigureFigure 16b shows16b shows the process the process of GBS, whereof GBS,d and whereλ are d grain and sizeλ are and grain subgrain size size, and respectively. subgrain size, Coarserespectively. grain samples Coarse at high grain temperatures samples at high get divided temperat intoures arrays get divided of subgrain into having arrays anof averagesubgrain size having of λ.an When averageλ > sized, grains of λ. moveWhen andλ > d rotate, grains in move response and to rotate external in response forces and to external the stress forces concentration and the stress is atconcentration triple point C dueis at to triple GBS. point This stressC due concentration to GBS. This is accommodatedstress concentration by intragranular is accommodated slip in by the adjacentintragranular grain slip but in in the the adjacent absence grain of any but subgrain, in the absence dislocation of any subgrain, moves to dislocation grain boundary moves at to D. grain Whenboundaryλ < d, grain at D. exhibits When λ small < d, grain displacement exhibits withsmall respect displacement to the adjacent with respect grain to and the sliding adjacent makes grain a and minorsliding contribution makes toa theminor total elongationcontribution [30 to,141 the]. GBS total is controlledelongation and [30,141]. accompanied GBS is by controlled dislocation and motionaccompanied with the grains by dislocation [36,139]. Furthermore,motion with the creep grains behavior [36,139]. of Al-MgFurthermore, solid solution creep behavior comprises of Al-Mg of glidesolid on slip solution planes comprises and climb ofover glide physical on slip planes obstacles. and Thisclimb process over physical is a sequential obstacles. process This process and its is a overallsequential creep rate process is determined and its overall by the creep slower rate process is determined for sequential by the glide slower and process climb asfor expressed sequential in glide Equationand climb (2): as expressed in Equation (2): 1 1 1 . = . + . (2) εt 11εc εg 1 = + (2) . . . εεε where, εt, εg, and εc represent overall creep rate,tcg climb controlled creep rate, and glide controlled creep rate respectively [144]. Ashby and Verrall [145] proposed a model to describe the process of superplasticityε ε as a transitionε between the diffusion-accommodated flow at low strain rates, where, t , g , and c represent overall creep rate, climb controlled creep rate, and glide and dislocation creep at high strain rates. This model gave vivid description on experimentally controlled creep rate respectively [144]. Ashby and Verrall [145] proposed a model to describe the observed switching of equiaxed grains throughout deformation. Ashby and Verrall claimed that grains process of superplasticity as a transition between the diffusion-accommodated flow at low strain switch their neighbors and do not change their shape much during diffusion accommodated flow. rates, and dislocation creep at high strain rates. This model gave vivid description on experimentally The equation below is a general constitutive law for lattice-diffusion controlled dislocation creep in observed switching of equiaxed grains throughout deformation. Ashby and Verrall claimed that polycrystalline metals of high stacking fault energy metals. grains switch their neighbors and do not change their shape much during diffusion accommodated flow. The equation below is a general. constitutive. . law for lattice-diffusion controlled dislocation εT = εslip + εGBS (3) creep in polycrystalline metals of high stacking fault energy metals.

Metals 2020, 10, 77 17 of 26

. . . where εT is total deformation rate, εslip is dislocation creep strain rate and εGBS is strain rate due to . . grain boundary sliding. Substituting the value of εslip and εGBS, Equation (3) can be written in the form

. ε 1011  σ 5 2 109  σ 2 T = + (4) 2 × 2 DL b E L E where DL is lattice diffusivity, b is the Burger’s vector, L is the mean linear intercept grain size, and E is the modulus of elasticity [146]. In Equation (4), the first term on the right side is independent of grain size whereas the second term is dependent on grain size. The decrease in the value of L increases the second term on the right side, which in turn increases total deformation. Thus, grain size refinement improves superplastic mechanisms.

! !p . DGb b  σ n ε = A (5) kT d G

Equation (5) is the phenomenological equation for high temperature creep where A is a constant that depends on the material and the dominant deformation mechanism, D is a diffusion coefficient  Qs  (=Do exp , where Do is a frequency factor, Qs is the activation energy for the appropriate diffusion − RT process), G is shear modulus, b is the magnitude of the Burgers vector, d is the grain size, p is the grain-size exponent, R is the universal gas constant, n is stress exponent (=1/m, where m is strain . rate sensitivity), and ε is strain rate. In order to establish a constitutive relationship for a material, it is essential to compute the value of the material parameters like p, n, and Qs, which can help to identify the rate controlling mechanism. The grain size exponent, n, is in the range of 2–3 under GBS creep, 0–1 under power law creep, and 0 under solute drag creep. Likewise, stress exponent, n, is around 2 under GBS creep, close to 5 under power law creep, and solute drag creep has an average n value of about 3 [76,147,148]. Moreover, the superplastic deformation mechanism can also be predicted by calculating the activation energy (Q). Q value of 84 kJ/mol indicates GBS, 123–135 kJ/mol denotes solute drag mechanism and 142 kJ/mol represents lattice diffusion [149]. McNelley et al. [56] 1 1 reported that the Solute Drag Creep mechanism is dominant at higher strain rate (~10− s− ) whereas 1 1 governing deformation mechanism at lower strain rate (~10− s− ) is GBS. Further information on a range of aspects of superplastic deformation mechanism may also be found in Grimes et al. [38], Stevens et al. [150], Prabu et al. [151], Mukherjee et al. [152], and Sergueeva et al. [153].

3.2. Constitutive Equations of Superplasticity Constitutive equations play a vital role in predicting and analyzing the superplastic behavior as well as reflect the dynamic response to the thermal parameters in the superplastic processing. Moreover, constitutive equations are essential for commercial applications and can be used for the process as well as product design through superplastic forming processes [28,154]. Different constitutive equations are proposed corresponding to different situations, some of the typical forms are reviewed and listed in Table2. The superplastic constitutive equation, which is shown in Equation (6) is a ffected by strain hardening exponent [155]. Constitutive equation is analyzed at a relatively lower temperature by Rossard [156] (Equation (7)) and at high temperature by Backofen et al. [157] (Equation (8)). The constitutive equation at lower temperature indicates that the process combines strain hardening with strain rate hardening whereas equation at higher temperature indicates that the process is mainly influenced by strain rate sensitivity exponent m.

σ = Kεn (6)

. m σ = Kε (7) . m σ = Kεnε (8) Metals 2020, 10, 77 18 of 26

. where σ is the true stress, ε is the true strain, ε the strain rate, n is the strain-hardening exponent, and K is a parameter for the material. The value of m decreases with an increase in grain size, which in turn decreases the superplastic behavior whereas decreasing the grain size increases the ‘m’ value and also moves to higher strain rate [158]. Larger the strain rate sensitivity, greater the superplastic elongation will be [156] and its value more than 0.3 is considered to be superplastic [159]. Likewise, small grain size and particle size helps to minimize the cavitation [160]. Ma et al. [161] performed an experiment on Al 7075 alloy to investigate the relationship between grain size and cavitation. Through their observation, it was concluded that a decrease in grain size results in a decrement of size, density and volume fraction of cavities. However, density, size and volume fraction of cavities increased with an increase in initial strain rates from 1 10 2 to 1 10 1 s 1 but cavity density was decreased with an × − × − − increase in temperature from 450 to 510 ◦C. Nieh et al. found that presence of a small amount of liquid phase at grain boundary at higher temperature reduces the cavitation by assisting GBS accommodation and thus delays the appearance of internal cavitation [94,162].

Table 2. Superplastic Constitutive Equations.

Equations Key Points Ref.

. m It is used in the simple stress state without any σ = Kε Backofen et al. [157] strain effects

. m It involves the combined effect of the strain rate σ = Kε εn Rossard et al. [163] sensitivity (m) and the strain hardening (n)   q   . . ln 10 π 2η mm σe It consists of equations with varying m based ε = εm exp × tg m m 1 log Song et al. [164] 2η √mm/mk 1 π m k k f on Backofen equation × − × −  x Backofen equation was modified to consider . f ” m z σ = A ε L d the effect of size and volume of particles of the Churyumov et al. [165] second phase It describes the superplasticity in matrix  n p . σ σth b ε = A −G d D composite (for aluminum matrix composite Mabuchi and Higashi [166] with Si3N4 whiskers (20%), n = p = 2)

 . m  0.5 Evaluates the deformation and internal damage σ = Cε + ko + R 1 ξ Khaleel et al. [167] − of AA5083 alloy N ! ! P ci i B1 B2 Phenomenological constitute equation for = k c + + . σ exp o ln ε i ε −lgε xo ln 10 Guan et al. [168] − Al–Zn–Mg–Zr alloy i=1 1+e ∆x

. m . m In Table2, values of m and n in σ = Kε [157] and σ = Kε εn [163] are regarded as constant but they should vary with the microstructural change, so they cannot precisely represent the superplastic behavior. Song et al. [164] proposed an equation with varying m and n based on Backofen equation for superplastic flow. Churyumov et al. [165] further modified the Backofen equation and included the factor that characterizes the influence of particles of excessive phase on yield stress. Similarly, Mabuchi and Higashi [166] developed an equation to represent the superplastic behavior of aluminum matrix composite with Si3N4 whiskers and concluded that stress-strain relation for the composites and alloy without crackers was almost the same. Likewise, the bottom two equations [167,168] explain the superplastic behavior of the respective alloy at different conditions.

4. Summary and Prospect Al alloys superplasticity not only reduces manufacturing cost but also has an ability to form parts of intricate geometry which would be difficult to fabricate by other means. The processing of Al alloys through the application of SPD leads to exceptional grain refinement and provides new opportunities for achieving excellent superplastic properties. Of late, several SPD techniques have been developed, however, it has been found that two procedures have received the maximum interest, i.e., ECAP and HPT. Considering the superplastic behavior of Al alloys, some investigations were performed to analyze the factors that influence the superplasticity of Al alloys. The results showed that alloys with almost similar composition exhibit different mechanical properties due to different refinement techniques. Besides, it was also observed that lower the size of the grain, higher is the Metals 2020, 10, 77 19 of 26 elongation. Furthermore, elongation to failure is higher if the temperature is more than the homologous temperature and the strain rate is in the range of 1 10 5 s 1–1 10 1 s 1. Elongation increases with × − − × − − increase in temperature and strain rate, reaches a peak over an intermediate range but decreases with further increase in temperature and strain rate. Fine, equiaxed, and high angle grain structures that remains stable during deformation enhances the superplastic property. Moreover, to achieve better superplasticity, the size of the second phase particles should be fine and their distribution should be uniform. Generally, the superplastic deformation mechanism in Al alloy is operated by GBS, where a group of grains whose boundaries are suitably aligned tends to slide as a unit. Moreover, the dominant deformation mechanism can be predicted by constitutive equations. In addition to basic research, if adequate research is carried out in the domain of advancement of computer modeling and simulation, it would lead to an intelligent adaptive tool to analyze the deformation process in detail. With further research on high strain rate, superplasticity will significantly contribute to industrial applications in the future. Rapid progress in the superplastic field can be achieved by suitable blending of material research and mechanical engineering. Further, there is a need to develop low-temperature and high strain rate superplastic deformation techniques and materials. Cryogenic deformation technique can also be used to produce ultrafine-grained materials, whose superplastic deformation capability should be studied.

Author Contributions: Conceptualization, H.Y.; formal analysis, L.B.; investigation, L.B.; writing—original draft preparation, L.B., H.Y.; writing—review and editing, A.P., A.P.Z., P.T., C.K., H.Y.; supervision, H.Y.; project administration, H.Y.; funding acquisition, A.P., A.P.Z., P.T. and H.Y. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by National Natural Science Foundation of China (Grant number: 51674303), National Youth Thousand Plan Program of China, Huxiang High-Level Talent Gathering Project of HUNAN Province (Grant number: 2018RS3015), Innovation Driven Program of Central South University (Grant number: 2019CX006), the Research Fund of the Key Laboratory of High Performance Complex Manufacturing at Central South University, the Ministry of Science and Higher Education of Russia Federation for financial support (Grant number: 14.Z50.31.0043), and the Department of Science & Technology, Ministry of Science and Technology, Government of India (Grant number: INT/RUS/RFBR/P-349). Conflicts of Interest: The authors declare no conflict of interest.

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