Twinning-induced plasticity (TWIP) steel offers high strength and ductility. However, in this study, deformation-induced detwinning was studied to prove that not all twins significantly benefit the mechanical properties. The quasi in situ observation results indicated that submicron-sized twins were unstable during Charpy impact loadin

Chenchong Wang

Lingyu Wang

Minghao Huang

Mogucheva, A.

Xiaofei Zhu

DSpace at Belgorod State University

BRIEF COMMUNICATIONTwin Instability and Its Effect on the DislocationBehavior of UFG Austenitic Steel Under CharpyImpact TestMINGHAO HUANG, CHENCHONG WANG, XIAOFEI ZHU, LINGYU WANG,ANNA MOGUCHEVA, and WEI XUTwinning-induced plasticity (TWIP) steel offers high strength and ductility. However, in thisstudy, deformation-induced detwinning was studied to prove that not all twins significantlybenefit the mechanical properties. The quasi in situ observation results indicated thatsubmicron-sized twins were unstable during Charpy impact loading. In contrast to the TWIPconcept, analysis of the geometrically necessary dislocation evolution associated withdetwinning revealed that unstable twins have negligible influence on the pile-ups of dislocations,offering a limited strengthening effect.https://doi.org/10.1007/s11661-022-06661-x The Minerals, Metals & Materials Society and ASM International 2022OWING to their combination of strength andtoughness, ultrafine-grained (UFG) austenitic steelshave been widely used in critical applications, such asliquified natural gas (LNG) tanks.[1,2] The superiorproperties of austenitic steel are partially due tomechanical twinning, that is, the twinning-inducedplasticity (TWIP) effect.[3] Deformation-induced twin-ning enables back stress hardening, and twin boundariesefficiently reduce the mean free path of dislocations;both contribute considerably to the flow stress. Tailor-ing the steel microstructure to produce a significantnumber of twins has been regarded as an efficientstrengthening strategy and has received increasingattention.MINGHAO HUANG, CHENCHONG WANG, LINGYUWANG, and WEI XU are with the State Key Laboratory of Rollingand Automation, School of Materials Science and Engineering,Northeastern University, Shenyang 110819, China. Contact e-mails:xuwei@ral.neu.edu.cn, wanglingyu@ral.neu.edu.cn XIAOFEI ZHU iswith Shenyang National Laboratory for Materials Science, Institute ofMetal Research, Chinese Academy of Sciences (CAS), Shenyang110016, China. ANNA MOGUCHEVA is with the Laboratory ofMechanical Properties of Nanoscale Materials and Superalloys,Belgorod State University, Belgorod, Russia 308015.Manuscript submitted January 4, 2022; accepted March 12, 2022.Article published online March 30, 2022METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 53A, JUNE 2022—1921Most previous studies suggested that twins arestable and effectively hinder dislocation gliding, likegrain boundaries. Hence, extensive effort has beendevoted to promoting twin formation before and duringloading tests.[4,5] In contrast to conventional observa-tions, some recent studies revealed that detwinningoccurs under several specific load situations, such astension-compression cycle loading[6–8] and dynamicplastic deformation (DPD).[9] These observations raisean interesting question, as to whether twins are mechan-ically stable throughout loading. Compared to twinning,detwinning has received lesser attention. High strainrates make it difficult to characterize the detwinningbehavior under dynamic loading conditions. Dynamicloading is one of the most extensive applications ofaustenitic steel. The fabrication of UFG alloys is alsorelated to dynamic loading processes, such as severeplastic deformation treatment.[10] On the material fab-rication side, detwinning would be undesirable for grainrefinement. In terms of material strengthening, itremains to be elucidated as to whether the twins aresufficiently mechanically stable and provide an effectivebarrier to hinder dislocation gliding during the dynamictest.To address the aforementioned challenges, theCharpy impact test was selected in this study. It is oneof the most common dynamic loading tests, character-ized by a strain rate in the range of 102–103 s1. In thisstudy, the detwinning behavior of UFG austenitic steelduring interrupted Charpy impact loading was directlyobserved, and its effect on dislocations was quantita-tively studied.A commercial SUS 321 austenitic stainless steel(Fe–17.56Cr–9.34Ni–0.021C–0.578Si–1.38Mn–0.252Ti,wt pct) was selected as the experimental material. Thereceived steel plates were cold-rolled from 12 mm to 3.5mm, followed by reverse annealing at 900 C for 90seconds, resulting in a UFG microstructure, as shown inFigure 1(a). The equiaxed grains were characterized by amean grain size of 1 lm. To interpret the microstructureevolution, a quasi in situ electron backscattered diffrac-tion (EBSD) study was carried out by controlling theCharpy impact energy. Interrupted Charpy testing wasperformed using an MTS impact tester (SANSZBC2452-C) with a 1 J solution. The testing methodand characterization region are schematically shown inFigure 1(b).EBSD mapping was focused on the area 100 lmdirectly above the V-notch, and was performed using ascanning electron microscope (GeminiSEM 300)equipped with an EBSD detector (NordlysNano,Oxford Instruments). The Charpy specimen was care-fully electrolytically polished at  20 C and a voltage of20 V for 20 seconds. The EBSD data were analyzedusing AZtecCrystal 2.0 analysis software. The nominalstrain was calculated as e = (L  L0)/L0, where L0 andL are the distances between two specified points beforeand after Charpy impact, respectively, as reported in ourprevious study.[11]The microstructural evolution during the Charpyimpact tests is shown in Figure 2. Figures 2(a), (b),and (c) show the IPF X maps of austenite, captured after0, 3.24, and 6.78 pct nominal strain, respectively. TheIPF maps indicate the disappearance of twins along witha change in the orientation of the grains (Figure 2(j)).This is consistent with the experimental results of Zhanget al.,[12] who reported that detwinning is related to grainrotation. To verify the detwinning results, Figures 2(d),(e), and (f) show the corresponding IPF Z maps of thesame area. Meanwhile, Figures 2(g), (h), and (i) showthat the misorientation angle changes between 3.24 and6.78 pct nominal strain, as indicated by the black arrowsin Figure 2(e). The detwinning phenomenon is quan-tized, as confirmed in Figures 2(g), (h), and (i). For easeof description, the twins/matrices are denoted as T1/M1,T2/M2, and T3/M3, respectively (Figure 2(e)). In theIPF maps, the thickness of T3 gradually decreases withincreasing strain, but the lamellar morphology isretained. The fragmented T1 is shown in Figure 2(f);notably, a short segment of T1 remains inside the coreregion, which presents an intermediate stage of detwin-ning. The measurements show that among the threeanalyzed twins, T2 nearly completes detwinning. Basedon these observations, it is concluded that detwinningmight be a result of the progressive thinning of twins.The three twins mentioned above represent threeFig. 1—(a) IPF Z map associated with grain boundaries of fabricated UFG steel. (b) Schematic of the loading condition and characterizationregion. RD, TD, and ND represent the rolling, transverse, and normal directions, respectively.1922—VOLUME 53A, JUNE 2022 METALLURGICAL AND MATERIALS TRANSACTIONS AFig. 2—Microstructure evolution during the Charpy impact test. (a, b, c) IPF X maps of austenite with 0, 3.24, and 6.78 pct nominal strain,respectively. (d, e, f) IPF Z maps of austenite with 0, 3.24, and 6.78 pct nominal strain, respectively. The nominal strain was measured by thedistance between two specified points, indicated in (d) with hollow arrows. In addition, the black areas in (c) and (f) represent unindexed pixels.(g, h, i) The misorientation angle changes along the black arrows in (e) at 3.24 and 6.78 pct nominal strain for lines 1, 2, and 3 respectively. (j)Grain rotation during Charpy tests in which deformation occurs by duplex slip. (Note: EBSD tests were performed with a 0.13 lm step size forthe 0 pct nominal strain sample, and 0.05 lm step size for the 3.24 and 6.78 pct nominal strain samples).METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 53A, JUNE 2022—1923statuses: thinning (T3) fi broken (T1) fi disappearance(T2). Besides the three twins shown in Figure 2, there arealso other individual tests that validate the detwinningphenomenon during the Charpy process (as shown inthe appendix). And similar detwinning phenomena havebeen previously investigated in Cu-Al systems usingTEM.[9]In contrast to previous studies, which describeddetwinning in TWIP steels under cyclic loading condi-tions, the present study directly reveals the detwinningbehavior occurring in austenitic steel under dynamicloading conditions.Detwinning behavior in face-centered cubic (fcc)crystals has been reported previously, especially in Cualloys,[13] and several possible mechanisms for thedetwinning behavior have been considered. The firstand most accepted mechanism is detwinning induced bytwin-slip interaction.[14–17] Therefore, it is worth ana-lyzing the grain rotation and dislocation slip behaviorduring Charpy impact. As indicated in Figure 2(j), thegrains rotate from the initial orientation to an orienta-tion close to (112), indicating the occurrence of duplexslip,[18] where intense dislocation slip occurs. Shockleypartial dislocations are the products of twin-slip inter-actions, and glide along the twin boundaries, resulting inlocally thinned twins. The difference in crystal orienta-tion between the twin and matrix leads to varied Schmidfactors (Figure 3(a)). The difference between the Schmidfactors of the twins and matrix causes the concentrationof high-level shear stress in the twin boundaries.[19] Torelease the shear stress and coordinate the shear strain,dislocation reactions with the twin boundaries areinitiated.[20,21] The dislocations promoted by theseinternal strains make the twin plates thinner.[22] Withincreasing shear stress, detwinning occurs until the twinsdisappear. A higher difference in the Schmid factorbetween the matrix and twins indicates a higher shearstress, which is important for detwinning. The lamellaT2 (red-colored), which has the largest misfit of theSchmid factor between the twins and matrix among thethree twins, vanishes quickly compared to the others.The second mechanism proposed previously is attrib-uted to secondary twinning within the former twin,which can transform former twin orientations to matrixorientations, resulting in detwinning.[23] In the presentstudy, the progressive thinning of the twins suggests thatdetwinning is governed by the twin-slip interaction.Thus, Shockley partial dislocation reactions are respon-sible for the observed detwinning. This specific detwin-ning process is schematically illustrated in Figure 3(b).The role of mechanically metastable twins in themovement of dislocations also needs to be considered, asthey influence the flow stress. Previous studies haverevealed that deformation twinning effectively strength-ens steels,[3] first because twin formation induces backstress, and second because twins confine dislocationgliding by reducing the mean free path of dislocations.For UFG steel, grain refinement results in mechanicallymetastable twins with limited thickness,[24] such as T1and T2. The contribution of the metastable twins to thedislocation movement was studied, and is shown inFigure 4. In Figures 4(a), (b), and (c), with increasingnominal strain, the average value of the geometricallynecessary dislocation (GND) density increases steadily.Fig. 3—(a) Schmid factor under 3.24 pct nominal strain of the twins and their respective matrices. (b) Schematic of the evolution of detwinningwith increasing strain.1924—VOLUME 53A, JUNE 2022 METALLURGICAL AND MATERIALS TRANSACTIONS AWhile grain boundaries are strong barriers for disloca-tions, the thin twins T1 and T2 have a negligible effecton the dislocation pile-up. The GND density profilesalong the white lines indicate that T2 has a lesser effecton the dislocation pile-up (Figures 4(g), (h), and (i)). Inthe present study, the contribution of mechanicallymetastable twins to flow stress was limited.UFG austenitic steel typically contains submi-cron-sized twins due to the limitation of an extremelyhigh nucleation stress. The present results indicate thatthese types of twins might be mechanically unstable andhave a limited effect on the strength of the studiedaustenitic steel under Charpy impact loading.This study investigated the detwinning behavior andits contribution to the dislocation pile-up behaviorduring Charpy impact testing. The detwinning behaviorof austenitic steel was directly observed under theone-direction dynamic loading process. The mechani-cally metastable twins did not strengthen the UFG steelduring Charpy impact tests. Based on the presentedresults, the following conclusions were drawn: (1)Induced and existing submicron-sized twins are unsta-ble during Charpy impact loading. (2) Deformation-in-duced detwinning occurs more completely in twins witha higher Schmid factor misfit with the matrix. (3) InFig. 4—(a, b, c) Calculated GND density evolution with increasing nominal strain. (d, e, f) Magnified regions of (a), (b), and (c) respectively. (g,h, i) GND density profiles along the white lines in (d), (e), and (f), respectively.METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 53A, JUNE 2022—1925contrast to the traditional TWIP concept, twins with alimited thickness play a negligible role in work harden-ing, as they fail to obstruct the gliding of dislocations.This research was financially supported by theNational Natural Science Foundation of China (No.U1808208, and 51961130389), and International/Regional Cooperation and Exchange Program ofNSFC (NSFC-RFBR, No. 52011530032).MH: Conceptualization, Methodology, Investiga-tion, Writing—original draft. Chenchong Wang: Writ-ing—review and editing. XZ: Writing—review andediting. LW: Writing—review and editing. AM: Super-vision. WX: Supervision, Project administration, Writ-ing—review and editing, Funding acquisition.The raw data required to reproduce these findingsare available upon reasonable request.The authors declare that they have no known com-peting financial interests or personal relationships thatcould have appeared to influence the work reported inthis paper.APPENDIXSee Fig. AI; Table AI.Fig. AI—Microstructure evolution during the Charpy impact test. (a, d, g) IPF Z maps of austenite under 0 J Charpy energy. (b, e, h) IPF Zmaps of austenite under 3 J Charpy energy. (c, f, i) Schmid factor maps of austenite under 0 J Charpy energy. The loading energy wascontrolled by the Charpy impact tester. In addition, non-indexed pixels are shown in black.1926—VOLUME 53A, JUNE 2022 METALLURGICAL AND MATERIALS TRANSACTIONS AREFERENCES1. H. Wang, J. Zhou, Y. Luo, P. Tang, and Y. Chen: Proc. Eng.,2014, vol. 81, pp. 837–42.2. C.C. Koch: Scr. Mater., 2003, vol. 49, pp. 657–62.3. O. Bouaziz: Scr. Mater., 2012, vol. 66, pp. 982–85.4. R. Kalsar, P. Khandal, and S. Suwas: Metall. Mater. Trans. A.,2019, vol. 50, pp. 3683–96.5. Y.Z. Li, Li, Z.Y. Liang, and M.X. Huang: Int. J. Plast., 2022, vol.150, p. 103198.6. H. Huang, A. Godfrey, W. Liu, and J.P. Zheng: Mater. Lett.,2016, vol. 178, pp. 208–12.7. B.M. Morrow, R.J. McCabe, E.K. Cerreta, and C. Tomé: Metall.Mater. Trans. A., 2014, vol. 45A, pp. 36–40.8. Y. Chen, B. Gou, X. Ding, J. Sun, and E.K.H. Salij: J. Mater. Sci.Technol., 2021, vol. 92, pp. 31–39.9. C.S. Hong, N.R. Tao, X. Huang, and K. Lu: Acta Mater., 2010,vol. 58, pp. 3103–16.10. A. Volokitin, A. Naizabekov, I. Volokitina, S. Lezhnev, and E.Panin: Mater. Lett., 2021, vol. 304, p. 130598.11. M. Huang, J. Yuan, J. Wang, L. Wang, A. Mogucheva, and W.Xu: Mater. Sci. Eng. A., 2022, vol. 831, p. 142319.12. H.J. Zhang, E. Salvati, C. Papadaki, K.S. Fong, X. Song, andA.M. Korsunsky: Key Eng Mater., 2019, vol. 793, pp. 17–22.13. M. Ge, W. Yuan, K. Wang, J. He, and J. Luo: Nanoscale., 2020,vol. 12, pp. 14831–37.14. Z.H. Jin, P. Gumbsch, K. Albe, E. Ma, K. Lu, H. Gleiter, and H.Hahn: Acta Mater., 2008, vol. 56, pp. 1126–35.15. Z.H. Jin, P. Gumbsch, E. Ma, K. Albe, K. Lu, H. Hahn, and H.Gleiter: Scr. Mater., 2013, vol. 54, pp. 1163–68.16. R. Mohammadzadeh: Mater. Sci. Eng. A., 2020, vol. 782, pp.139251.1–139251.8.17. P. Müllner and C. Solenthaler: Mater. Sci. Eng. A., 1997, vol. 230,pp. 107–15.18. G.M. Bellefon and C.J. Duysen: J. Nucl. Mater., 2016, vol. 475,pp. 168–91.19. K.H. Kwon, B.C. Suh, S.I. Baik, Y.W. Kim, and N.J. Kim: Sci.Technol. Adv. Mater., 2013, vol. 14, pp. 014204.1–014204.8.20. L. Wu, S.R. Agnew, D.W. Brown, G.M. Stoica, B. Clausen, A.Jain, D.E. Fielden, and P.K. Liaw: Acta Mater., 2008, vol. 56, pp.3699–3707.21. M.S. Szczerba, S. Kopacz, and M.J. Szczerba: Acta Mater., 2012,vol. 60, pp. 6413–20.22. Y.T. Zhu, X.L. Wu, X.Z. Liao, J. Narayan, L.J. Kecskés, and S.N.Mathaudhu: Acta Mater., 2011, vol. 59, pp. 812–21.23. H. Paul, A. Morawiec, A. Piątkowski, E. Bouzy, J.J. Funden-berger: Metall. Mater. Trans. A., 2004, vol. 35A, pp. 3775–86.24. D. Li, L. Qian, C. Wei, S. Liu, and J. Meng: Mater. Sci. Eng. A,2020, vol. 789, p. 139586.Publisher’s Note Springer Nature remains neutral with regard tojurisdictional claims in published maps and institutional affiliations.Table AI. Summarize the Schmid Factor of Twin/Matrix With Different Extent of DetwinningTwin/Matrix Number Schmid Factor of Twin Schmid Factor of Matrix Misfit of the Schmid Factor Extent of Detwinning1 0.46 0.44 0.02 broken2 0.48 0.34 0.14 disappearance3 0.46 0.45 0.01 thinning4 0.441 0.465 0.024 broken5 0.498 0.453 0.045 disappearance6 0.420 0.477 0.057 disappearance7 0.453 0.464 0.011 broken8 0.478 0.482 0.004 no detwinningMETALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 53A, JUNE 2022—1927

Twin instability and its effect on the dislocation behavior of UFG austenitic steel under Charpy impact test

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Twin instability and its effect on the dislocation behavior of UFG austenitic steel under Charpy impact test

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