metals

Article Development of New -Free Maraging with Superior Mechanical Properties via Electro-Pulsing Technology

Dong Pan, Yuguang Zhao *, Yitong Wang, Xiaofeng Xu and Xueying Chong

Key Laboratory of Automobile Materials, Ministry of Education and School of Materials Science and Engineering, Jilin University, No. 5988 Renmin Street, Changchun 130022, China; [email protected] (D.P.); [email protected] (Y.W.); [email protected] (X.X.); [email protected] (X.C.) * Correspondence: [email protected]; Tel.: +86-431-8509-4481

 Received: 11 November 2019; Accepted: 26 November 2019; Published: 2 December 2019 

Abstract: The ductility of cobalt-free maraging steel is unsatisfactory because of the high content of Ti. Traditional heat treatment can not effectively improve the ductility of this kind steel. In this contribution, high-energy electro-pulsing is adopted in a T250 steel to solve this problem efficiently. It is found that the EPS treatment (electro-pulsing treatment under water-cooling condition) can accelerate the formation of retained and nano stacking faults. Meanwhile, the microstructure is also refined by EPS treatment. Then, taking the EPS sample as the initial state, nano-reverted austenite combined with finer η-Ni3Ti precipitates are formed during EPA treatment (electro-pulsing treatment under air-cooling condition), compared with TA (traditional aging) treatment. The results of mechanical properties indicate that the strength and elongation are both enhanced by electro-pulsing treatment. Consequently, the electro-pulsing treatment can be a promising technology to devise cobalt-free maraging steel with better properties.

Keywords: T250 steel; electro-pulsing; retained austenite; stacking faults; mechanical properties

1. Introduction Since the 1960s, maraging have become irreplaceable materials in navigation and aviation fields by virtue of their excellent strength, toughness, and resistance [1–3]. Generally, compared with the quenched steel, the maraging steels usually possess better ductility because of their high content [4]. The pre- treatment will dissolve alloying elements into the matrix and after the water-cooling procedure, the matrix will be transformed into soft martensitic laths. Whereas, the soft matrix is only the foundation of the advanced mechanical properties of maraging steels and appropriate aging regime must be taken into force to guarantee a huge increment in strength. During the aging process, some inter-metallic compounds, such as Ni3Ti and Ni3Mo with hexagonal crystal structure will be formed, which will lead to the increment in strength [5]. In order to further enhance the strengthening effect, the element cobalt is added to maraging steel for the dynamic interaction between cobalt and elements, i.e., the prior positions of molybdenum atoms will be occupied by cobalt atoms so that the precipitation of Ni3Mo compounds will be promoted [6]. However, accompanied with the gradually increased price and decreased resource reserve, cobalt is becoming a strategic resource around the global, and under this background, cobalt-free maraging steels are proposed [7]. Although the production costs of cobalt-free maraging steels decrease significantly, another problem also come out. To attain the same strength level as the maraging steels containing cobalt, more is added to cobalt-free maraging steel [8]. In other words, less Ni3Mo and more Ni3Ti will be

Metals 2019, 9, 1299; doi:10.3390/met9121299 www.mdpi.com/journal/metals Metals 2019, 9, 1299 2 of 15

formed after aging regime is applied. Whereas, due to the higher c/a (lattice constant) value of Ni3Ti, the lattice coherence between Ni3Ti and the matrix is also lower than that between Ni3Mo and the matrix [9,10]. Thus, compared with maraging steels with cobalt content, the ductility of cobalt-free maraging steels is unsatisfactory. Recently, plenty of research has concentrated on the enhancement of ductility of maraging steels. Wang et al. [11] reported that during aging treatment, layered reverted austenites will be formed. As the reverted austenite can reduce the {111} fiber in [12] and prevent the crack propagation during tensile deformation, the ductility of maraging steels will be improved [13,14]. However, the content of reverted austenites cannot be easily controlled so that the toughening (ductility improvement) effect is unstable. Besides, Pereloma [15] and Li [16] proposed that by rapid heating treatment, the strength is remarkably enhanced because of the formation of co-clusters and in contrast to maraging steels under traditional treatment, there is almost no loss in ductility. However, the R/b (radius/burgers vector) value of the co-cluster (known as the prior series of GP zone containing Mn, Ni, Ti, and Al) is very sensitive to treating parameters (temperature and treating procedure). If this value is higher than 15, there will be almost no toughening effect, and from the viewpoint of developing maraging steels with superior properties, no loss in ductility is insufficient. Moreover, Lu et al. [17] demonstrated that the ductility of maraging steels can be modified by the formation of Ni(Al,Fe) precipitations of which the lattice misfit is much lower than Ni3Ti. Whereas, the mass percentage of aluminum in this steel even reaches up to 3%. Such a high content of aluminum will lead to the increment in viscosity of molten steel, and the production cost will also increase. On the other hand, the grain refinement is considered to be the most effective method to toughening maraging steels. Unfortunately, there exist some problems for traditional austenitic grain (γ) refinement technology of maraging steels. Firstly, some severe plastic deformation technology, such as equal channel angular process (ECAP) [18,19] and high pressure torsion (HPT) can effectively reduce the grain (γ) size. However, a large amount of crystal defects will also be introduced so that the toughening effect is weakened. Then, even though the cyclic γ α transformation can also ↔ 0 refine the grains (γ), there will exist intergranular oxidation because the high content of titanium enriched at the unstable grain boundaries, and so, the effect ductility improvement will be weakened. Finally, for recrystallization of maraging steels, not only the processing is time consuming, but also the parameters (temperature and deformation amount) are rigorous [20]. As a result, a new and more efficient toughening method is urgently needed for maraging steels. Currently, as a rapid and effective treating technology, high-energy electro-pulsing processing has been widely utilized in many fields, such as structural relaxation in amorphous alloys [21], self-repairing of damage crack [22] and microstructural refinement [23]. For industrial manufacturing, the electro-pulsing current is also applied on electro-plastic drawing [24] and auxiliary molding of Ni-based alloys [25]. For steels, many studies have demonstrated the obviously improved strength and ductility induced by electro-pulsing current [26,27]. Moreover, our previous works also indicated that some substructures, such as sub-grains, nano-twins, and clusters, which are hard to be formed in steels by traditional treating strategy and beneficial for the ductility, can be obtained via electro-pulsing treatment [28,29]. In other words, the electro-pulsing treatment could be a suitable method for the ductility enhancement of maraging steels. Thus, this work is aimed at the gradually optimization of solution and aging treatment of T250 steel by the comparison between traditional heat treatment and electro-pulsing heat treatment. Besides, the microstructure-property relationship of T250 steels under different treatment will also be closely investigated. It is expected to develop cobalt-free maraging steels with superior properties more efficiently. Metals 2019, 9, 1299 3 of 15

2. ExperimentalMetals 2019, 12, x FOR PEER REVIEW 3 of 15

Metals 2019, 12, x FOR PEER REVIEW 3 of 15 2.1. Materials2.1. Materials and and Methods Methods The2.1. Materials materialsThe materials and used Methods used in this in this work work is commercialis commercial T250 T250 steel steel and and thethe initial state state is is annealed annealed (Figure (Figure 1a, 1a, containing ferrite, Laves phase (compounds with Ti, Mo, and Al) and γ phase (FeNi3)), and the containingThe ferrite, materials Laves used phase in this (compoundswork is commercial with T250 Ti, Mo,steel and the Al) initial and stateγ phase is annealed (FeNi 3(Figure)), and the chemical composition (weight percentage: wt.%) of the T250 steel is listed in Table 1. The T250 steel chemical1a, containing composition ferrite, (weight Laves phase percentage: (compounds wt.%) with of the Ti, T250Mo, and steel Al) is and listed γ phase in Table (FeNi1.3 The)), and T250 the steel was machined into slices (50 × 10 × 2.5 mm3) by CNC wire cutting machine (Jiangsu Dongqing CNC was machinedchemical composition into slices (weight (50 10 percentage:2.5 mm 3wt.%)) by CNC of the wire T250 cutting steel is listed machine in Table (Jiangsu 1. The Dongqing T250 steel CNC Machine Tool Co., LTD, Taizhou× × , China).3 Then, all the Samples will be traditionally and electro- Machinewas Toolmachined Co., LTD,into slice Taizhou,s (50 × China).10 × 2.5 mm Then,) by all CNC the Sampleswire cutting will machine be traditionally (Jiangsu Dongqing and electro-pulsing CNC Machinepulsing treat Tooled Co.,, respectively, LTD, Taizhou and, theChina). treating Then, flow all isthe shown Samples in Figurewill be 1 btraditional. As is seen,ly and the electrostate of- treated, respectively, and the treating flow is shown in Figure1b. As is seen, the state of samples for pulsingsamples treatfor aginged, respectively, treatment isand optimized the treating according flow is to shown the mechanical in Figure properties1b. As is seen, of samples the state after of aging treatment is optimized according to the mechanical properties of samples after solution heat samplessolution forheat aging treatment. treatment Traditional is optimized solution according heat treatment to the mechanical (TS) was carried properties out atof 820samples °C for after 1 h treatment.solutionunder Traditionalwater heat- coolingtreatment. solution condition. Traditional heat While treatment solution the corresponding (TS) heat was treatment carried electro (TS) out-pulsing atwas 820 carried ◦treatmentC for out 1 h at under(EPS) 820 °Cwas water-cooling for taken 1 h into force via using a self-made machine containing thyristor circuit (Figure 2b) under water-cooling condition.under Whilewater-cooling the corresponding condition. While electro-pulsing the corresponding treatment electro (EPS)-pulsing was treatment taken into (EPS) force was via taken using a self-madeintoconditi force machineon. via using containing a self-made thyristor machine circuit containing (Figure thyristor2b) under circuit water-cooling (Figure 2b) under condition. water-cooling condition. Table 1. The nominal composition of commercial T250 steel (wt.%). Table 1. The nominal composition of commercial T250 steel (wt.%). Element Ni Table 1.Cr The nominalMo composition Ti of commercialAl T250 steelSi (wt.%). Mn Fe Element Ni Cr Mo Ti Al Si Mn Fe Elementwt.% 18.86Ni 0.37Cr 3.06Mo 1.20Ti 0.11Al 0.01Si Mn0.01 Bal.Fe wt.% 18.86 0.37 3.06 1.20 0.11 0.01 0.01 Bal. wt.%Note: The18.86 content of C,0.37 Co, S, and 3.06P are all less1.20 than 0.003%,0.11 the content0.01 of O and N0.01 in the moltenBal. steel are less than 20 ppm. Note:Note: The content The content of C, Co, of S,C, andCo, PS, are and all P less are than all less 0.003%, than the 0.003%, content the of content O and N of in O the and molten N in steelthe molten are less than 20 ppm. steel are less than 20 ppm.

Figure 1. The microstructure of T250 steel at annealed state (a) and the Schematic diagram showing the treating flow of this work (b). FigureFigure 1. The 1. The microstructure microstructure of T250of T250 steel steel at at annealed annealed state state ( a(a)) and and thethe SchematicSchematic diagram diagram showing showing the the treating flow of this work (b). treating2.2. Technological flow of this Parameters work (b ).and the Electro-Pulsing Apparatus 2.2. Technological Parameters and the Electro-Pulsing Apparatus

Figure 2. The physical map of the electro-pulsing apparatus (a) and the sketch map of the thyristor circuit (b). Metals 2019, 9, 1299 4 of 15

2.2. Technological Parameters and the Electro-Pulsing Apparatus The electro-pulsing apparatus and thyristor circuit are shown in Figure2, the samples are fastened on the Cu–Zr electrodes by the fastening bolts, and the distance between the two electrodes can be adjusted by the sleeves. The electro-pulsing parameters were set up by an EPT software, and the detailed parameters (current and period) are 8.15 107 A/m2 and 360 ms, respectively. × For aging process, the parameters of traditional treatment (TA) and electro-pulsing treatment (EPA) under air-cooling conditions were 480 C, 2 h and 3.40 108 A/m2, 220 ms and 280 ms, respectively. ◦ × The temperature during electro-pulsing treatment is measured by an infrared gun (TBSC).

2.3. Microstructural Analysis and Property Measurement Samples for optical microscopy (OM) was polished and then etched with ethanol +6.25 wt.% trichloride +20 vol.% hydrochloric acid solution. The Axio Imager M2M optical microscopy (Carl Zeiss AG, Oberkochen, Germany) was used for grain boundary and microstructure observation. Samples for transmission electron microscopy (TEM) test were polished to about 30 µm thickness and then the twin-jet electro-polishing in a 20 vol.% perchloric ethanol electrolyte at 20 C under a voltage of 20 V − ◦ was adopted. The JEM-2100F transmission electron microscopy (JEOL LTD, Tokyo, Japan) is adopted for substructure and precipitation detection. The X-ray diffraction (XRD) was conducted by using a D/Max 2500PC XRD machine (Scanning range was 20◦–110◦ and scanning rate was 2◦/min) (Rigaku Coporation, Tokyo, Japan). The tensile test was conducted at room temperature by using a MTS-810 performance testing machine (MTS Industrial System (China) Co., LTD, Shenzhen City, China), and 1 the tensile speed is 0.003 s− .

3. Results and Discussion

3.1. Optimization of Solid-Solution Treatment Figure3 shows the microstructure and prior austenitic grain boundary of samples under di fferent kinds of solution heat treatment. The microstructure of TS sample is composed of lath-like martensite. As for the EPS sample, the corresponding microstructure is also mainly comprised of lath-like martensite (the temperature is measured to be 807 ◦C, which is above Ac3 temperature of T250 steel [30]). Whereas, the width of martensitic laths in the EPS sample is obviously smaller than that of the TS sample. Besides, the prior austenitic grains of EPS sample are also finer than the TS sample (Figure3c,d). The above mentioned results indicate that the martensitic laths and prior austenitic grains of T250 steel are refined by electro-pulsing treatment, and the refined martensite is a consequence of the refined prior austenitic grains [31]. This can be attributed to the enhanced nucleation rate of austenitic transformation induced by electro-pulsing current [32]. Furthermore, the growth of austenitic grains are also inhibited due to the extremely short treating procedure of EPS treatment. In this way, finer microstructure can be inevitably obtained by electro-pulsing treatment. Metals 2019, 9, 1299 5 of 15 Metals 2019, 12, x FOR PEER REVIEW 5 of 15

Figure 3. OOpticalptical microscopy microscopy (OM) (OM) images images showing showing the the microstructure microstructure of the traditional traditional solution heat treatment ( (TS)TS) sample ( a)),, the electro-pulsingelectro-pulsing treatment ( (EPS)EPS) sample ( b)),, the prior austenitic grain boundary of the TS sample (c),), and the EPS sample (d).).

In order order to deeply deeply investigate investigate the substructure substructure of martensite in the EPS EPS sample, sample, TEM TEM observation observation waswas conducted. As depicted in Figure 44,, thethe widthwidth ofof martensiticmartensitic lathslaths inin thethe TSTS samplesample isis largerlarger thanthan the EPS sample (Figure 44a,e),a,e), whichwhich isis consistentconsistent withwith thethe resultsresults ofof FigureFigure3 .3. In In addition, addition, compared compared with the the TS TS sample, sample, the the EPS EPS presents presents severer severer dislocation dislocation pile-ups pile- (Figureups (Figure4b,f). Furthermore,4b,f). Furthermore, the results the resultsof Figure of4 Figured demonstrate 4d demonstrate that there that also there exist also some exist stacking some stacking faults in faults the EPS in the sample. EPS sample. It is worth It is worthnoting noting that some that layered some layered retained retained austenite austenite is also detected is also indetected EPS sample in EPS (Figure sample4a,c), (Figure and the 4a, crystalc), and theorientation crystal betweenorientation retained between austenite retained and austenite martensite and is martensit the K–S relationshipe is the K–S [ 33 relationship]: (011)α//(111) [33]:γ, (011)[–100]α//(111)α//[1–10]γ, γ[–.100 whereas]α//[1– in10] theγ. whereas TS sample, in almostthe TS no sample, retained almost austenite no can retained be distinguished. austenite can To get be distinguished.rid of the scale limitation To get rid of of TEM the observation scale limitation in the of detection TEM obs ofervation retained austenite in the detection in different of retainedsamples, austeniteXRD detection in different is necessary samples, to beXRD conducted. detection is necessary to be conducted.

Metals 2019, 9, 1299 6 of 15 Metals 2019, 12, x FOR PEER REVIEW 6 of 15

Figure 4. 4.Transmission Transmission electron electron microscopy microscopy (TEM) (TEM) images images showing showing the martensitic the martensitic laths (a), dislocations laths (a), indislocations martensitic in lathsmartensitic (b), retained laths (b austenite), retained ( ca),ustenite nano stacking (c), nano faults stacking (d) faults in the ( EPSd) in samplethe EPS and sample the martensiticand the martensitic laths (e), laths dislocations (e), dislocations in martensitic in martensitic laths (f) inlaths the ( TSf) in sample; the TS the sample; insert the maps insert show maps the patternsshow the or patterns HRTEM or image HRTEM ofthe image selected of the area selected in the area corresponding in the corresponding figures. figures.

The results of the XRD measurement are shown in Figure5 5.. CharacteristicCharacteristic peakspeaks ofof austeniteaustenite areare found in the XRD patterns of the EPS sample. However, no characteristic peaks of retained austenite can be detected in the TS sample. The XRD results imply that the electro electro-pulsing-pulsing treatment promotes the formationformation ofof retainedretained austenite. austenite. For For the the formation formation mechanisms, mechanisms, the the homogeneously homogeneously distributed distributed Ni contentNi content in samples in samples at initial at initial state, state, the crystalthe crystal defects defects in austenitic in austenitic grains grains and theand grain the grain refinement refinement effect aftereffect EPS after treatment EPS treatment should should be taken be into taken consideration. into consideration. Firstly, because Firstly, thebecause treating the period treating is extremelyperiod is short,extremely there’s short, no suthere’sfficient no time sufficient for Ni time content for toNi homogeneously content to homogeneously dissolve into dissolve the matrix. into Becausethe matrix. the NiBecause is an austeniticthe Ni is an stabilizing austenitic element, stabilizing the element, retained the austenite retained iseasier austenite to be isformed easier to in be the formed Ni enriched in the areaNi enriched in the EPS area sample in the EPS [34]. sample Secondly, [34]. under Secondly, the intensely under the impact intensely of theimpact electron of the wind, electron there wind, will existthere morewill exist crystal more defects crystal in austeniticdefects in grainsaustenitic during grains EPS during treatment, EPS andtreatme as ant, result, and theas a martensitic result, the transformationmartensitic transformation will be inhibited will [be35 ].inhibited Finally, because[35]. Finally, the average because grain the sizeaverage of the grain EPS samplesize of the (9.7 EPSµm, Figuresample3 d,(9.7 calculated μm, Figure by 3 Imaged, calculated Pro Plus by 6.0Image software) Pro Plus (6.0 6.0 version, software) Media (6.0 version, Cybernetics, Media MD, Cybernetics, USA) is remarkablyMD, USA) is smaller remarkably than thatsmaller of the than TS that sample of the (38.2 TS sampleµm, Figure (38.23 c,μm, calculated Figure 3 byc, calculated Image Pro by Plus Image 6.0 Pro Plus 6.0 software), the interface energy of EPS sample between austenite and martensite is higher

Metals 2019, 9, 1299 7 of 15

Metals 2019, 12, x FOR PEER REVIEW 7 of 15 software), the interface energy of EPS sample between austenite and martensite is higher than the TS sample, thus, the growth of martensitic in the EPS sample will stop earlier than the TS sample when than the TS sample, thus, the growth of martensitic in the EPS sample will stop earlier than the TS the growth is about to attach the prior austenitic grain boundary. As a result, more retained austenite sample when the growth is about to attach the prior austenitic grain boundary. As a result, more is formed in the EPS sample [36]. retained austenite is formed in the EPS sample [36].

FigureFigure 5. 5. XX-ray-ray diffraction diffraction (XRD) (XRD) patterns patterns of of samples samples under under different different kinds kinds of of treatm treatment.ent.

GenerallyGenerally speaking, speaking, the the stacking stacking faults faults are are hard hard to to form form in in ferrous ferrous materials materials with with bcc bcc structure structure duedue to to the the relatively relatively higher higher stacking stacking fault fault energy. energy. As As for for the the formation formation mechanisms mechanisms of of nano nano stacking stacking faultsfaults in in the the EPS EPS sample sample (Figure (Figure 44d)d),, the authors believe that the thermal compressive stress induced byby electro electro-pulsing-pulsing is is nece necessaryssary to to be be investigated investigated,, based based on on the the previous previous research research [ 29]].. During During EPS EPS treatment,treatment, due toto thethe extremely extremely rapid rapid heating heating speed, speed, the the thermal expansion will bewill delayed be delayed [37], which [37], whichwill result will inresult the generationin the generation of thermal of thermal compressive compressive stress (stressFT) in ( theFT) EPSin the sample. EPS sample. The value The FvalueT of can FT ofbe can estimated be estimated by [38 ]:by [38]: FT = Eλ∆T (1) FT  EλΔT (1) where λ is the thermal expansion coefficient, ∆T refers to the temperature rise, and E refers to Young’s where λ is the thermal expansion coefficient, ΔT refers to the temperature rise, and E refers to Young’s modulus. Whereas, in fact, there exist a critical force (FC) for the formation of stacking faults, and its modulus. Whereas, in fact, there exist a critical force (FC) for the formation of stacking faults, and its value is governed by [39]: value is governed by [39]: 2 2µαbp µa = + FC 2 (2) 2μαDbp 24μaπωbp FC   (2) where D means the grain size of EPS sample,Dbp is the24πω partialbp dislocation vector, µ refers to , a denotes the lattice constant, and ω is the width of nano stacking faults in Figure4d. If the where D means the grain size of EPS sample, bp is the partial dislocation vector, μ refers to shear actually driven force is smaller than this critical force, no stacking faults will be formed. During EPS modulus, a denotes the lattice constant, and ω is the width of nano stacking faults in Figure 4d. If the treatment, the thermal compressive stress can be regarded as the driven force for the formation of actually driven force is smaller than this critical force, no stacking faults will be formed. During EPS stacking faults. According to Equations (1) and (2), the calculated thermal compressive stress is about treatment, the thermal compressive stress can be regarded as the driven force for the formation of 1.79 GPa, which is obviously higher than the calculated critical force (0.54 GPa). Therefore, it can be stacking faults. According to Equations (1) and (2), the calculated thermal compressive stress is about concluded that the formation of nano-stacking faults is attributed to the thermal compressive stress 1.79 GPa, which is obviously higher than the calculated critical force (0.54 GPa). Therefore, it can be during EPS treatment. concluded that the formation of nano-stacking faults is attributed to the thermal compressive stress during EPS treatment. The tensile properties of solution heat treated samples are shown in Figure 6. The strength and elongation of TS sample are 794 MPa and 15.8%. For the EPS sample, the corresponding values are 1045 MPa and 19.8%, which are apparently higher than that of the TS sample. The higher tensile

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The tensile properties of solution heat treated samples are shown in Figure6. The yield strength and elongation of TS sample are 794 MPa and 15.8%. For the EPS sample, the corresponding values are 1045 MPa and 19.8%, which are apparently higher than that of the TS sample. The higher tensile properties of the EPS sample are attributed to its refined prior austenitic grains. Based on the Metals 2019, 12, x FOR PEER REVIEW 8 of 15 Hall–Petch effect, with the decrease of prior austenitic grain size, the yield strength is bound to increase. Furthermore,properties theof the deformation EPS sample processare attributed will also to its become refined prior more austenitic homogeneous grains. becauseBased on ofthe the Hall refined– priorPetch austenitic effect, grains.with the Moreover, decrease of exceptprior austenitic for the refinedgrain size, grain the size,yield thestrength stacking is bound faults to increase. and retained austeniteFurthermore are also, favorablethe deformation for the process better will tensile also properties. become more Under homogeneous the pinning beca effuseect of of the stacking refined faults on dislocationprior austenitic movement, grains. Moreover the strength, except will for be the enhanced. refined grain As size, for thethe stacking effect of faults retained and retained austenite on mechanicalaustenite properties are also favorable of the EPSfor the sample, better tensile it can properties. be reflected Under by t thehe pin work-hardeningning effect of stacking rate curvesfaults (Θ, on dislocation movement, the strength will be enhanced. As for the effect of retained austenite on (dσtrue/dεtrue)/σtrue, σtrue is true stress and εtrue is true strain). As is depicted in Figure6b, because of mechanical properties of the EPS sample, it can be reflected by the work-hardening rate curves (Θ, the finer grain size and martensitic laths, the dislocation are easier to be entangled with each other (dσtrue/dεtrue)/σtrue, σtrue is true stress and εtrue is true strain). As is depicted in Figure 6b, because of the in thefiner EPS grain sample. size and Therefore, martensitic the laths, initial the work-hardening dislocation are easier rate to ofbe theentangled EPS sample with each is higherother in thanthe the TS sample.EPS sample. Moreover, Therefore, in contrastthe initial towork the-hardening disappeared rate o work-hardeningf the EPS sample is abilityhigher than in the the TS TS samplesample. at a true strainMoreover, of 0.038, in contrast the EPS to the sample disappeared possesses work more-hardening significant ability continuous in the TS sample work-hardening at a true strain capability of (Figure0.038,6b). the This EPS can sample be attributed possesses more to the significant prevention continuous of crack work propagation-hardening in capability the EPS (Figure sample 6b). during tensileThis deformation, can be attributed which to is the better prevention for the improvement of crack propagation of ductility in the [ 40 EPS]. During sample this during process, tensile when the crackdeformation, extends which to the layeredis better retainedfor the improvement austenite, the of martensiticductility [40 transformation]. During this process, will be when promoted the so that thecrack further extends plastic to the deformation layered retained will austenite, be guaranteed the martensitic and the work-hardeningtransformation will can be bepromoted continued so [40]. that the further plastic deformation will be guaranteed and the work-hardening can be continued The above-mentioned effect of retained austenite on mechanical properties of the EPS sample is known [40]. The above-mentioned effect of retained austenite on mechanical properties of the EPS sample is as the transformation induced plasticity (TRIP) effect. known as the transformation induced plasticity (TRIP) effect.

FigureFigure 6. Tensile 6. Tensile properties properties of samples of samples under diunderfferent different kinds of kinds solid-solution of solid-solution treatment: treatment: (a) engineering (a) tensileengineering curves, (b tensile) normalized curves, ( work-hardeningb) normalized work rate-hardening curves. rate curves.

3.2. Optimization3.2. Optimization of Aging of Aging Treatment Treatment In orderIn order to develop to develop T250 T250 steel steel with with higherhigher strength and and ductility, ductility, the the EPS EPS sample sample with with better better propertiesproperties is taken is taken as theas the initial initial state state of of the the agingaging treatment, according according to the to theresults results of Section of Section 3.1. 3.1. The microstructure of the EPS + EPA samples with different treating period is given in Figure 7. After The microstructure of the EPS + EPA samples with different treating period is given in Figure7. 220 ms of the EPA treatment, a large number of rod-like η-Ni3Ti precipitates with a length of 32 nm After 220 ms of the EPA treatment, a large number of rod-like η-Ni Ti precipitates with a length of were found. Based on the SAED (Selected Area Electron Diffraction) results3 (the left insert in Figure 32 nm7a), were retained found. austenite Based is on also the found SAED in the (Selected EPS + EPA Area (220 Electron ms) sample. Diffraction) In contrast results to the (the EPS leftsample insert in Figure(Figure7a), retained 4c), the austenite width of is retained also found austenite in the in EPS the +EPSEPA + (220EPA ms) (220 sample. ms) sample In contrast was smaller. to the EPS sampleFurthermore, (Figure4c), there the are width almost of no retained characteristic austenite peaks in of the retaine EPSd austenite+ EPA (220 in the ms) corresponding sample was XRD smaller. Furthermore,patterns, which there indicates are almost the no decomposition characteristic of retained peaks of austenite retained during austenite 220 ms in of the the corresponding EPA treatment XRD patterns,were which found. indicates For the EPS the +decomposition EPA (280 ms) sample, of retained the η- austeniteNi3Ti precipitates during 220are also ms ofdetected the EPA and treatment the average length was about 47 nm. This indicates that with the increase in the EPA treating period, the were found. For the EPS + EPA (280 ms) sample, the η-Ni3Ti precipitates are also detected and the averageη-Ni length3Ti precipitates was about present 47 nm. significant This indicates growth. thatWhereas, with it the is worth increase noting in thethat EPA some treating nano-globular period, the phase with a size less than 10 nm are observed in the EPS + EPA (280 ms) sample (Figures 7b–f). The η-Ni3Ti precipitates present significant growth. Whereas, it is worth noting that some nano-globular results of SAED patterns presented in Figure 7c,d, and f demonstrate that the globular phase is austenite. Also, it is seen from the results of XRD patterns (Figure 5), characteristic peaks of austenite

Metals 2019, 9, 1299 9 of 15 phase with a size less than 10 nm are observed in the EPS + EPA (280 ms) sample (Figure7b–f). The results of SAED patterns presented in Figure7c,d, and f demonstrate that the globular phase is austenite.Metals 2019 Also,, 12, itx FOR is seen PEER from REVIEW the results of XRD patterns (Figure5), characteristic peaks of9 austenite of 15 appear again when the EPA treating time arrives 280 ms. Consequently, the nano austenite in the appear again when the EPA treating time arrives 280 ms. Consequently, the nano austenite in the EPS EPS + EPA (280 ms) sample can be considered as nano-reverted austenite. + EPA (280 ms) sample can be considered as nano-reverted austenite.

FigureFigure 7. TEM 7. TEM images images of of EPS EPS+ +EPA EPA (220 (220 ms)ms) sample ( (aa) )and and EPS EPS + EPA+ EPA (280 (280 ms) ms) (b–f ();b –Thef); Theinsert insert mapsmaps show show the SAEDthe SAED patterns patterns of theof the selected selected area area in in the the corresponding corresponding figures; figures; TheThe ((ff)) isis thethe darkdark field field image of (e); REA—retained austenite, RTA—reverted austenite. image of (e); REA—retained austenite, RTA—reverted austenite. The results of Figure 5 have shown that the characteristic peaks of austenite disappeared in the The results of Figure5 have shown that the characteristic peaks of austenite disappeared in the EPS + EPA (220 ms) sample. Whereas, after the EPA treatment for 280 ms, the characteristic peaks of EPS + EPA (220 ms) sample. Whereas, after the EPA treatment for 280 ms, the characteristic peaks austenite appeared again. This indicates that during EPA treatment for 220 ms, there exists the of austenitedecomposition appeared of the again. layered This retained indicates austenite that in during EPS sample. EPA As treatment for the EPA for treatment 220 ms, therefor 280 exists ms, the decompositionthere also exists of the the layered formation retained of nano austenite-reverted in EPS austenite. sample. Even As though for the the EPA crystal treatment structure for 280 of ms, thereretained also exists austenite the formation and reverted of nano-reverted austenite are austenite. crystallographically Even though identical, the crystal it is still structure necessary of retained to austenitediscuss and the reverted formation austenite mechanism are crystallographically of nano-reverted austenite identical, because it is still of necessary its totally to different discuss the formationmorphology mechanism and size of from nano-reverted the retained austenite austenite. becauseAt first, the of local its totally enrichment different of Ni morphology should be firstly and size fromdiscussed. the retained Compared austenite. with At the first, EPS the + EPA local (220 enrichment ms) sample, of the Ni η should-Ni3Ti precipitates be firstly discussed. in the EPS + Compared EPA (280 ms) sample show more significant growth. This needs higher local enrichment of Ni in the with the EPS + EPA (220 ms) sample, the η-Ni3Ti precipitates in the EPS + EPA (280 ms) sample show matrix. Then, because the Ni is austenitic stabilization element, there will be more reverted austenite more significant growth. This needs higher local enrichment of Ni in the matrix. Then, because the Ni in the EPS + EPA (280 ms) sample. In other words, the chemical conditions of α’→γ reverse is austenitic stabilization element, there will be more reverted austenite in the EPS + EPA (280 ms)

Metals 2019, 9, 1299 10 of 15 sample. In other words, the chemical conditions of α’ γ reverse transformation in the EPS + EPA → (280 ms)Metals sample 2019, 12, x are FOR more PEER REVIEW sufficient than the EPS + EPA (220 ms) sample. 10 of 15 The dislocation morphology, retained austenite and nano stacking faults in the EPS sample are also responsibletransformation for in the the formation EPS + EPA of (280 nano-reverted ms) sample are austenite, more sufficient and the than detailed the EPS mechanisms + EPA (220are ms) given sample. in Figure8. As shown in Figure8a,b, the dimension characteristic of a single dislocation line is The dislocation morphology, retained austenite and nano stacking faults in the EPS sample are anisotropic,also responsible and aff ectedfor the byformation this, the of nano second-reverted phase austenite, will grow and into the detailed a rod-like mechanisms shape [41 are]. given Whereas, the dislocationin Figure 8. pile-upsAs shown are in Figure more isotropic8a and b, the than dimension the dislocation characteristic line, of the a morphologysingle dislocation of theline secondis phaseanisotropic, will be globular and affected under by this this, condition the second [41 phase]. Based will ongro thew into results a rod of-like Figure shape4b,f, [41]. the Whereas, nano-reverted the austenitedislocation will finally pile-ups grow are intomore a isotropic globular than shape. the dislocation Moreover, line, considering the morphology the reduction of the second in the phase energy of austeniticwill be reverse globular transformation, under this condition the existence [41]. Based of retained on the austeniteresults of Figureand stacking 4b,f, the faults nano can-reverted accelerate the formationaustenite wi ofll nano-reverted finally grow into austenite. a globular Owing shape. to Moreover, the same considering crystal structure the reduction of retained in the austenite energy and revertedof austenitic austenite, reverse the reverted transformation, austenite the tends existence to nucleate of retained on the austeniteretained austenite. and stacking In this faults regard, can the nucleationaccelerate energy the formation is relatively of nano lower-reverted than nucleate austenite. in Owing other regionsto the sam soe thatcrystal the structure nucleation of retained on retained austenite and reverted austenite, the reverted austenite tends to nucleate on the retained austenite. austenite is easier to occur. This can also be verified by the results of Figure7c, the distribution of In this regard, the nucleation energy is relatively lower than nucleate in other regions so that the nano-reverted austenite at layered retained austenite is clearly detected. More important, the α’ γ nucleation on retained austenite is easier to occur. This can also be verified by the results of Figure → reverse7c, the transformation distribution of willnano generally-reverted austenite undergo at twice-latticelayered retained shear austenite [42]. is As clearly depicted detected. in FigureMore 8d, becauseimportant, the nano the stacking α’→γ reverse faults transformation have already possessedwill generally the undergo a/6<111 > twiplane,ce-lattice which shear is [42]. the same As as the hexagonaldepicted in closely Figure arranged8d, because surface the nano after stacking the first faults lattice have shear, already the possessed reverse transformationthe a/6<111> plane, energy can alsowhich be is reduced the same [42 as]. the Thereupon, hexagonal theclosely existence arranged of surface stacking after faults the isfirst also lattice a favorable shear, the factor reverse for the formationtransformation of nano-reverted energy can austenite. also be reduced The above [42]. Thereupon, mentioned the discussion existence isof mainlystacking related faults is the also eff aect of the substructurefavorable factor on the for distribution the formation and of nano morphology-reverted ofaustenite. nano-reverted The above austenite. mentioned As discussion for the nano is size and intensivemainly related distribution the effect of of reverted the substructure austenite, on itthe is distribution related with and enhanced morphology nucleation of nano- ratereverted [32] and austenite. As for the nano size and intensive distribution of reverted austenite, it is related with extreme-treating time for growth induced by the EPA treatment. enhanced nucleation rate [32] and extreme-treating time for growth induced by the EPA treatment.

FigureFigure 8. Sketch 8. Sketch maps maps showing showing the the e ffeffectsects of of dislocationdislocation morphology morphology (a,b (a,b), retained), retained austenite austenite (c) and (c) and nanonano stacking stacking faults faults (d) ( ond) on the the formation formation of of nano-reverted nano-reverted austenite austenite..

Metals 2019, 9, 1299 11 of 15

Metals 2019, 12, x FOR PEER REVIEW 11 of 15 For the EPS + TA sample, the existence of η-Ni3Ti precipitates with a length of 54 nm is also detectedFor (Figure the EPS9a,b). + TA Because sample, thethe existenceη-Ni3Ti precipitatesof η-Ni3Ti precipitates have enough with time a length for growthof 54 nm during is also TA treatment,detected their (Figure size 9a,b). is also Because larger the than η- thatNi3Ti in precipitates the EPS + EPAhave sample. enough Andtime also,for growth compared during with TA the EPStreatment,+ EPA sample, their size the is overgrowth also larger ofthan Ni that3Ti in in the the EPS EPS+ + TAEPA sample sample. will And lead also, to morecompared Ni consumption with the soEPS that + the EPA local sample, enrichment the overgrowth of Ni does of Ni not3Tiexist in the in EPS the + EPS TA sample+ TA sample. will lead Consequently, to more Ni consump the revertedtion austeniteso that the cannot local beenrichment detected of in Ni EPS does+ notTA exist sample in the (Figures EPS + TA5 and sample.9a). What’sConsequently, more, accordingthe reverted to austenite cannot be detected in EPS + TA sample (Figures 5 and 9a). What’s more, according to Figure Figure9c,d, some globular Fe 2Mo precipitates were also observed. The Fe2Mo precipitate is an overaging9c,d, some product. globular During Fe2Mo theprecipitates early precipitation were also observed. stage, the T structurehe Fe2Mo of precipitateη precipitate is an is overaging a core-shell product. During the early precipitation stage, the structure of η precipitate is a core-shell structure structure [43], i.e., the growth of Ni3Ti core of can be restricted by the Ni3Mo shell. Whereas, with the [43], i.e., the growth of Ni3Ti core of can be restricted by the Ni3Mo shell. Whereas, with the extension extension of aging time, the shell (Ni3Mo) will be gradually dissolved into the matrix so that the size of of aging time, the shell (Ni3Mo) will be gradually dissolved into the matrix so that the size of the core the core (Ni3Ti) will become larger (the so called overaging state). Meanwhile, the dissolved Ni3Mo (Ni3Ti) will become larger (the so called overaging state). Meanwhile, the dissolved Ni3Mo shell will shell will also be transformed into Fe2Mo precipitates [43]. also be transformed into Fe2Mo precipitates [43].

Figure 9. TEM images of precipitates (a), dark field image of Ni3Ti (b), dark field image of Fe2Mo (c) Figure 9. TEM images of precipitates (a), dark field image of Ni3Ti (b), dark field image of Fe2Mo (c) andand HRTEM HRTEM image image of of precipitates precipitates (d(d)) inin EPSEPS ++ TA sample.sample. The The insert insert maps maps show show the the SAED SAED or orFFT FFT patternspatterns of of the the selected selected area area in in the the corresponding corresponding figures.figures.

The traditional peak aging regime for maraging steel at TS state is holding at 480 °C for 2–3 h The traditional peak aging regime for maraging steel at TS state is holding at 480 ◦C for 2–3 h [28]. Whereas,[28]. Whereas, compared compared with TS sample,with TS the sample, EPS sample the EPS possesses sample higher possesses energy higher (higher energy entanglement (higher degreeentanglement of dislocation degree andof disloc theation existence and the of existence nano stacking of nano faults) stacking for faults) thefollowing for the following aging aging process. process. Therefore, in contrast to traditional peak aging regime, the peak aging time should be less Therefore, in contrast to traditional peak aging regime, the peak aging time should be less than 2–3 than 2–3 h. Thus, the EPS + TA sample is in the overaging state. Besides, the measured temperature h. Thus, the EPS + TA sample is in the overaging state. Besides, the measured temperature of the of the EPS + EPA (220 and 280 ms) sample are 295 °C and 393 °C, which are lower than the EPS + EPA (220 and 280 ms) sample are 295 C and 393 C, which are lower than the temperature of the temperature of the traditional peak aging regime.◦ Based◦ on classical nucleation theory, it seems that traditional peak aging regime. Based on classical nucleation theory, it seems that the thermal conditions the thermal conditions for the formation of η-Ni3Ti precipitates in the EPS + EPA sample are for the formation of η-Ni3Ti precipitates in the EPS + EPA sample are inadequate. Whereas, in fact, the inadequate. Whereas, in fact, the nucleation sites of η-Ni3Ti precipitates must be some crystal defects η nucleation(such as vacancy sites of or-Ni interstitial3Ti precipitates atom). Thus, must due be someto thecrystal higher defectsresistivity (such of these as vacancy crystal defects, or interstitial the atom).temperature Thus, due around to the the higher nucleat resistivityion sites of will these be crystalhigher than defects, the themeasured temperature values around (295 and the 393 nucleation °C) in sitesthe will EPS be+ EPA higher sample than [44]. the In measured a word, even values though (295 andthe temperature 393 ◦C) in theof the EPS whole+ EPA samples sample are [ 44lower]. In a word,than even the temperature though the temperatureof traditional of peak the aging whole regime, samples the are temperature lower than of the nucleation temperature area of of traditional η-Ni3Ti

Metals 2019, 9, 1299 12 of 15 Metals 2019, 12, x FOR PEER REVIEW 12 of 15 peak aging regime, the temperature of nucleation area of η-Ni Ti will be even more higher than the will be even more higher than the samples so that the precipitation3 of η-Ni3Ti still possesses sufficient samplesthermal soconditions. that the precipitation Moreover, due of η to-Ni the3Ti extremely still possesses short su treatingfficient thermalperiod of conditions. the EPA treatment, Moreover, there due to the extremely short treating period of the EPA treatment, there is no sufficient time for the growth of is no sufficient time for the growth of η-Ni3Ti. Consequently, compared with the EPS + TA sample, η-Ni Ti. Consequently, compared with the EPS + TA sample, the size of η-Ni Ti precipitate is smaller the size3 of η-Ni3Ti precipitate is smaller in the EPS + EPA sample. 3 in theThe EPS tensile+ EPA properties sample. of aging treated samples are displayed in Figure 10. The yield strength and Theelongation tensile of properties the EPS of+ TA aging sample treated are samples1870 MP area and displayed 16.1%. As in Figurefor the 10electro. The-pulsing yield strength treated andsample, elongation the corresponding of the EPS + valuesTA sample are 1987 are MPa, 1870 MPa17.5% and, and 16.1%. 2123 MPa, As for 20.1% the electro-pulsing for the EPS + EPA treated (220 sample,ms) and the EPS corresponding + EPA (280 ms) values samples, are 1987 respectively. MPa, 17.5%, These and 2123results MPa, indicate 20.1% that for theunder EPS the+ EPA effect (220 of ms)electro and-pulsing, EPS + EPA the (280strength ms) andsamples, ductility respectively. of aging Thesetreated results sample indicate are improved that under simultaneously. the effect of electro-pulsing,Take the precipitation the strength strengthening and ductility mechanism of aging into treated consideration, sample are improvedthe contribution simultaneously. of precipitates Take the precipitation strengthening mechanism into consideration, the contribution of precipitates to the to the yield strength (Δσp) is governed by [45]: yield strength (∆σp) is governed by [45]: 0.4MGb ln(2r / b) Δσp 0.4MGb ln(2r/b) , (3) ∆σp = π 1υ λP , (3) π √1 υ• λP where, M is the Taylor factor, G is the shear modulus,− b means the Burgers vector, ν is Poisson’s ratio, where, M is the Taylor factor, G is the shear modulus, b means the Burgers vector, ν is Poisson’s ratio, and λp refers to the average spacing of the rod-like precipitates. Because the size of η-Ni3Ti η andprecipitatesλp refers ( tor, considered the average as spacing a sphere, of the increase rod-like with precipitates. the increment Because in the the length size of of rod-Ni-3likeTi precipitates precipitate) (inr, consideredthe EPS + EPA as a sample sphere, is increase smaller withthan thethat incrementin the EPS in + TA the sample, length of therefore, rod-like the precipitate) contribution in the of EPSprecipitates+ EPA sample to yield is smaller strength than in that the in theEPS EPS + EPA+ TA sample,sample therefore, is higher the than contribution the EPS + of TA precipitates sample. toConsequently, yield strength the in EPS the + EPS EPA+ sampleEPA sample possesses is higher higher than yield the strength. EPS + TA On sample. the other Consequently, hand, due to thethe EPS + EPA sample possesses higher yield strength. On the other hand, due to the bigger size of η-Ni Ti, bigger size of η-Ni3Ti, the lattice mismatch between precipitates and matrix in the EPS + TA sample3 theis higher lattice mismatchthan that betweenin the EPS precipitates + EPA sample, and matrix which in will the EPSlead+ toTA crack sample initiation is higher at thanthe interface that in the of EPSprecipitate+ EPA sample,and matrix which [46]. will Consequently, lead to crack the initiation ductility at of the the interface EPS + EPA of precipitatesample is relatively and matrix higher. [46]. Consequently,It is worth noting the ductilitythat, the oftensile the EPS properties+ EPA sampleof the EPS is relatively + EPA (280 higher. ms) sample It is worth are notingalso better that, than the tensilethat of properties the EPS + of EPA the EPS (220+ ms)EPA sample. (280 ms) This sample can are be also attributed better than to the that existence of the EPS of+ nanoEPA- (220reverted ms) sample.austenite This in the can EPS+EPA be attributed (280 toms) the sample. existence Compared of nano-reverted with the EPS austenite + EPA in (220 the ms) EPS sample,+EPA (280 despite ms) sample. Compared with the EPS + EPA (220 ms) sample, despite the relatively bigger size of η-Ni Ti in the relatively bigger size of η-Ni3Ti in the EPS + EPA (280 ms) sample, the nano-reverted austenite3 is thealso EPS an + effectiveEPA (280 strengthening ms) sample, the phase. nano-reverted Hence, the austenite EPS + EPA is also (280 an ems)ffective sample strengthening possesses phase.higher Hence,strength. the Moreover, EPS + EPA the (280 TRIP ms) sample effect induced possesses by higher nano- strength.reverted Moreover, austenite are the alsoTRIP helpful effect induced for the bymodification nano-reverted of ductility. austenite Therein, are also compared helpful for with the modificationthe EPS + EPA of ductility.(220 ms), Therein,the EPS compared+ EPA (280 with ms) thepossesses EPS + EPAbetter (220 ductility. ms), the EPS + EPA (280 ms) possesses better ductility.

FigureFigure 10.10. TensileTensile propertiesproperties ofof samplessamples underunder didifferentfferent kindskinds ofof agingaging treatment: treatment: (a(a)) EngineeringEngineering tensiletensile curves,curves, ((bb)) Normalized Normalized work-hardeningwork-hardening rate rate curves. curves.

Figure 9b shows the work-hardening rate curves of aging treated samples. As is seen, due to the relatively bigger size of austenitic grains, the work-hardening rate of EPS + TA sample is the lowest. Besides, the EPS + EPA (280 ms) sample possesses the highest initial work-hardening rate, which is

Metals 2019, 9, 1299 13 of 15

Figure9b shows the work-hardening rate curves of aging treated samples. As is seen, due to the relatively bigger size of austenitic grains, the work-hardening rate of EPS + TA sample is the lowest. Besides, the EPS + EPA (280 ms) sample possesses the highest initial work-hardening rate, which is induced by the co-strengthening effect of η-Ni3Ti and nano-reverted austenite. Furthermore, unlike the continuously decreased work-hardening rate of EPS + TA and EPS + EPA (220 ms) samples, and peak is observed in work-hardening rate curve of EPS + EPA (280 ms) sample. This interesting phenomenon is related with the TRIP effect induced by nano-reverted austenite in EPS + EPA (280 ms) sample [36]. During this process, the gradually concentrated local stress around the nano-reverted austenite will lead to the increase in work-hardening rate [40]. Whereas, after the γ α’ transformation, → the concentrated local stress will be released so that the work-hardening rate will decrease again [40]. Based on the above mentioned results, the optimized electro-pulsing treatment can simultaneously enhance the strength and ductility of T250 steel very efficiently. However, there still exist some deficiencies of this technology. For example, the formation mechanism of retained austenite in EPS is related with the heterogeneously distributed Ni element. Whereas, if this technology is applied in the microstructure refinement of materials at semi-solid state or as cast state, there may exist elemental segregation, which is harmful to the mechanical properties. Besides, the grain refinement effect is sensitive to the electro-pulsing parameters. If the parameters are not appropriate, there will be no refinement effect or the grains will be coarser. So, the authors will continue to study the detailed effect of electro-pulsing current on the structure-property relationship of metallic materials so that the application of electro-pulsing technology will be more universal and more advanced.

4. Conclusions In summary, high-energy electro-pulsing current is applied in cobalt-free maraging steel to obtain more advanced mechanical properties. Compared with TS treatment, after EPS treatment within only 360 ms, substantial retained austenite and nano-stacking faults are formed. Moreover, finer prior austenitic grains and martensitic lath are also acquired. Based on this, the following EPA treatment can promote the formation of finer η-Ni3Ti precipitates and a large amount of nano-reverted austenite within only 280 ms. Under the same initial state, the tensile strength and elongation are both modified via electro-pulsing treatment. Eventually, the best tensile properties are optimized to be 2123 MPa (Yield strength) and 20.1% (Total elongation, TE%) by EPS + EPA (280 ms) treatment. To summarize, the electro-pulsing treatment can be a novel and effective method to developing new cobalt-free maraging steel with superior properties.

Author Contributions: Conceptualization, D.P.; methodology, Y.Z. and X.X.; formal analysis, Y.W., and Y.Z.; investigation, D.P.and Y.W.; data curation, D.P.and X.C.; writing—original draft preparation, D.P.; writing—review and editing, Y.W. and X.C.; visualization, Y.W. and X.X.; supervision, Y.Z.; project administration, X.X. and Y.Z. Funding: This research was supported by the National Natural Science Foundation of China (Grant No. 51701080) and Talent Development Program-Outstanding Youth Fund of Jilin Province (Grant No. 20190103053JH). Acknowledgments: TEM observation by Yitong Wang is sincerely acknowledged. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Guo, Z.; Sha, W.; Vaumousse, D. Microstructural evolution in a PH 13–8 after aging. Acta Mater. 2003, 51, 101–116. [CrossRef] 2. Luo, H.; Shi, J.; Wang, C.; Gao, W.; Sun, X.; Dong, H. Experimental and numerical analysis on formation of stable austenite during the intercritical of 5Mn steel. Acta Mater. 2011, 59, 4002–4014. [CrossRef] 3. Liu, H.W.; Liu, J.W.; Luo, C.P.; Liu, Z.J. Microstructure, crystallography of phase transformation and multiple precipitations in PH 15-7 Mo stainless steel. J. Alloys Compd. 2016, 672, 386–392. [CrossRef] 4. Xu, W.; Rivera-Díaz-del-Castillo, P.E.J.; Yan, W.; Yang, K.; San Martín, D.; Kestens, L.A.I.; van der Zwaag, S. A new ultra-high strength stainless steel strengthened by various coexisting nanoprecipitates. Acta Mater. 2010, 58, 4067–4075. [CrossRef] Metals 2019, 9, 1299 14 of 15

5. Rajkumar, K.V.; Rao, B.P.C.; Sasi, B.; Kumar, A.; Jayakumar, T.; Raj, B.; Ray, K.K. Characterization of aging behaviour in M250 grade maraging steel using eddy current non-destructive methodology. Mater. Sci. Eng. A 2007, 464, 233–240. [CrossRef] 6. Tharian, K.T.; Sivakumar, D.; Ganesa, R.; Balakrishnan, P.; Sinha, P.P. Development of new low nickel, cobalt free maraging steel. Mater. Sci. Technol. 1991, 7, 1082–1088. [CrossRef] 7. Li, K.; Wei, L.; An, B.; Yu, B.; Misra, R.D.K. Aging phenomenon in low lattice-misfit cobalt-free maraging steel: Microstructural evolution and strengthening behavior. Mater. Sci. Eng. A 2019, 739, 445–454. [CrossRef] 8. Squire, R.; Wilson, E.A. Effect of cobalt on impact toughness of steels. Mater. Sci. Technol. 1994, 10, 52–55. [CrossRef] 9. Li, Y.C.; Yan, W.; Cotton, J.D.; Ryan, G.J.; Shen, Y.F.; Wang, W.; Shan, Y.Y.; Yang, K. A new 1.9 GPa maraging stainless steel strengthened by multiple precipitating species. Mater. Des. 2015, 82, 56–63. [CrossRef] 10. Shekhter, A.; Aaronson, H.; Miller, M.; Ringer, S.; Pereloma, E. Effect of aging and deformation on the microstructure and properties of Fe-Ni-Ti maraging steel. Metall. Mater. Trans. A 2004, 35, 973–983. 11. Wang, C.C.; Zhang, C.; Yang, Z.G. Austenite layer and precipitation in high Co-Ni maraging steel. Micron 2014, 67, 112–116. [CrossRef][PubMed] 12. Masoumi, M.; de Barrous, I.F.; Herculano, L.F.G.; Coelho, H.L.F.; de Abreu, H.F.G. Effect of microstructure and crystallographic texture on charpy impact test for maraging 300 steel. Mater. Charact. 2016, 120, 203–209. [CrossRef] 13. Du, P.J.; Yang, D.P.; Bai, M.K.; Xiong, X.C.; Wu, D.; Wang, G.D.; Yi, H.L. Austenite stabilisation by two step partitioning of manganese and carbon in a Mn-TRIP steel. Mater. Sci. Technol. 2019, 17, 2084–2091. [CrossRef] 14. Harding, I.; Mouton, I.; Gault, B.; Raabe, D.; Kumar, K.S. Carbon partitioning and microstructure evolotion during of an Fe-Ni-C steel. Scripta Mater. 2019, 172, 38–42. [CrossRef] 15. Pereloma, E.; Shekhter, A.; Miller, M.; Ringer, S. Aging behaviour of an Fe-20Ni-1.8Mn-1.6Ti-0.59Al (wt.%) maraging : Clustering, precipitation and hardening. Acta Mater. 2004, 52, 5589–5602. [CrossRef] 16. Li, K.; Yu, B.; Misra, R.D.K.; Han, G.; Liu, S.; Shang, C.J. Strengthening of cobalt-free 19Ni3Mo1.5Ti maraging steel through high-density and low lattice-misfit nanoscale precipitates. Mater. Sci. Eng. A 2018, 715, 174–185. [CrossRef] 17. Jiang, S.H.; Wang, H.; Wu, Y.; Liu, X.J.; Chen, H.H.; Yao, M.J.; Gault, B.; Ponge, D.; Raabe, D.; Hirata, A.; et al. Ultrastrong steel via minimal misfit and high-density nanoprecipitation. Nature 2017, 544, 460–465. [CrossRef] 18. Yang, M.X.; Yang, G.; Liu, Z.D.; Wang, C.; Huang, C.X. Significant enhancement of strength in a lamellar-type nanostructured maraging steel subjected to equal-channel angular pressing for 12 passes. Mater. Sci. Eng. A 2012, 550, 429–433. [CrossRef] 19. Yang, M.X.; Yang, G.; Liu, Z.D.; Wang, C.; Hu, C.; Huang, C.X. Effect of equal-channel angular pressing on the microstructures and properties of 18Ni(C250) maraging steel. Acta Metall. Sin. 2012, 48, 164–169. [CrossRef] 20. Zhang, W.; Zou, D.N.; Qiao, G.J. Hot deformation behavior and microstructural evolution of as-cast co containing maraging stainless steel. Mater. Res. Express 2019, 6, 086586. [CrossRef]

21. Mizubayashi, H.; Okuda, S. Structural relaxation induced by passing electric current in amorphous Cu50Ti50 at low temperature. Phys. Rev. B 1989, 40, 8057–8060. [CrossRef][PubMed] 22. Song, H.; Wang, Z.J.; He, X.D.; Duan, J. Self-healing of damage inside metal triggered by electropulsing stimuli. Sci. Rep. 2017, 7, 7097. [CrossRef][PubMed] 23. Zhao, Y.G.; Zhang, J.T.; Tan, J.; Ma, B.D. Microstructure refinement and property improvement of metastable austenitic manganese steel induced by electro-pulsing. J. Iron Steel Res. Int. 2014, 21, 685–689. [CrossRef] 24. Zhou, Y.; Chen, G.Q.; Fu, X.S.; Zhou, W.L. Effect of electropulsing on deformation behavior of Ti-6Al-4V alloy during cold drawing. Trans. Nonferrous Met. Soc. China 2014, 24, 1012–1021. [CrossRef] 25. Zhang, X.; Li, H.W.; Zhan, M. Mechanism for macro and micro behaviors of the Ni-based superalloy during electrically-assisted tension: Local Joule of heating effect. J. Alloys Compd. 2018, 742, 480–489. [CrossRef] 26. Zhao, Y.G.; Ma, B.D.; Guo, H.C.; Ma, J.; Yang, Q.; Song, J.S. Electropulsing strengthened 2GPa boron steel with good ductility. Mater. Des. 2013, 43, 195–199. [CrossRef] 27. Ma, B.D.; Zhao, Y.G.; Bai, H.; Ma, J.; Zhang, J.T.; Xu, X.F. Gradient distribution of mechanical properties in the high induced by the detour effect of the pulse current. Mater. Des. 2013, 49, 168–172. [CrossRef] Metals 2019, 9, 1299 15 of 15

28. Pan, D.; Zhao, Y.G.; Xu, X.F.; Wang, Y.T.; Jiang, W.Q.; Chong, X.Y. A novel strengthening and toughening strategy for T250 maraging steel: Cluster-orientation governed higher strength-ductility combination induced by electropulsing. Mater. Des. 2019, 169, 107686. [CrossRef] 29. Pan, D.; Zhao, Y.G.; Wang, Y.T.; Xu, X.F.; Yin, P.L.; Chong, X.Y. Formation of ultrafine pearlites in tempered 42CrMo steel induced by electro-pulsing. Mater. Lett. 2019, 251, 73–77. [CrossRef] 30. Gui, B.C.; Yang, Z.Y.; Jiang, Y.H.; Ding, Y.L. Effect of solution temperature on mechanical properties of corrosion-resistant maraging steel. J. Iron Steel Res. 2012, 24, 45–48. 31. Loewy, S.; Rheingans, B.; Meka, S.R.; Mittemeijer, E.J. Modulated martensite formation behavior in Fe-Ni-based alloys; athermal and thermally activated mechanisms. J. Mater. Res. 2015, 30, 2101–2107. [CrossRef] 32. Pan, D.; Wang, Y.T.; Zhao, Y.G.; Xu, X.F.; Chong, X.Y.; Yin, P.L. Completely wetted α/α grain boundaries in electro-pulsing treated DP600 steel under different cooling conditions. Mater. Lett. 2019, 252, 264–267. [CrossRef] 33. Zhang, M.X.; Kelly, P.M. Accurate orientation relationship between ferrite and austenite in low carbon martensite and granular . Scripta Mater. 2002, 47, 749–755. [CrossRef] 34. Wang, Y.T.; Pan, D.; Zhao, Y.G.; Sun, L.Y.; Xu, X.F.; Yin, P.L.; Chong, X.Y.; Jiang, W.Q.; Wang, Z. Enhanced stability of retained austenites in quenched 25SiMn2MoV steel by electro-pulsing current. Mater. Sci. Eng. A 2019, in press. [CrossRef]

35. Strife, J.R.; Carr, M.J.; Ansell, G.S. The effect of austenite prestrain above the Md temperature on the martensitic transformation in Fe-Ni-Cr-C alloys. Metall. Trans. A 1977, 8, 1471–1484. [CrossRef] 36. Yang, H.S.; Bhadeshia, H.K.D.H. Austenite grain size and the martensite-start temperature. Scr. Mater. 2009, 60, 493–495. [CrossRef] 37. Zhang, W.; Sui, M.L.; Zhou, Y.Z.; He, G.H.; Guo, J.D.; Li, D.X. Electro-pulsing induced evolution of microstructures in materials. Acta Metall. Sin. 2003, 39, 1009–1018. 38. Tang, D.W.; Zhou, B.L.; Gao, H.; He, G.H. Thermal stress relaxation behavior in thin films under transient laser-pulse heating. J. Appl. Phys. 1993, 73, 3749–3752. [CrossRef] 39. Huang, C.X.; Wang, K.; Wu, S.D.; Zhang, Z.F.; Li, G.Y.; Li, S.X. Deformation twinning in polycrystalline copper at room temperature and low strain rate. Acta Mater. 2006, 54, 655–665. [CrossRef] 40. Pan, D.; Zhao, Y.G.; Xu, X.F.; Wang, Y.T.; Jiang, W.Q.; Ju, H. Effect of high-energy and instantaneous electropulsing treatment on microstructure and properties of 42CrMo steel. Acta Metall. Sin. 2018, 54, 1245–1252. 41. Wang, Y.T.; Zhao, Y.G.; Xu, X.F.; Pan, D.; Jiang, W.Q.; Yang, X.H.; Wang, Z. Superior mechanical properties induced by the interaction between dislocations and precipitates in the electro-pulsing treated Al-Mg-Si alloys. Mater. Sci. Eng. A 2018, 735, 154–161. [CrossRef] 42. Zhu, J.; Zhao, Y.W.; Pan, T.X.; Cai, Q.G. Investigation on reverse austenite in 18Ni(250 grade) maraging steel. Iron Steel 1981, 16, 41–45. 43. Tian, J.L.; Li, Y.C.; Wang, W.; Yan, W.; Shan, Y.T.; Jiang, Z.H.; Yang, K. Alloying element segregation effect in a multi-phase strengthened maraging stainless steel. Acta Metall. Sin. 2016, 52, 1517–1526. 44. Zhang, X.; Li, H.W.; Zhan, M.; Shao, G.D.; Ma, P.Y.Extraordinary effect of the δ phase on the electrically-assisted deformation responses of a Ni-based superalloy. Mater. Charact. 2018, 144, 597–604. [CrossRef] 45. Srivatsan, T.S. Dispersion strengthened aluminum alloys. Mater. Manuf. Process. 1991, 6, 565–568. [CrossRef] 46. Ashby, M.F. Work hardening of dispersion-hardened crystals. Philos. Mag. 1966, 14, 1157–1178. [CrossRef]

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