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TWINNING-INDUCED plasticity (TWIP) steels are the second-generation advanced high-strength steel for potential automotive body-in-white applications.[1–3] TWIP steels possess high ultimate tensile strength (UTS) and superior uniform elongation ascribed to the high work-hardening rates.[1–3] The formation of deformation twins has been widely accepted as an important mechanism for the high work-hardening rate.[4–6] However, except the contribution of deformation twins, there are several work-hardening mechanisms that can also contribute to the work-hardening rate of TWIP steel such as dynamical strain aging (DSA)[7–9] and forest dislocation hardening.[10–12] In general, dislocation–dislocation interactions are the most important contributions to the work-hardening in the metals. But the contribution of dislocations on the work-hardening rate of TWIP steels has not received enough attention, which could be due to the fact that other work-hardening mechanisms show more Peng ZHOU, PhD Candidate, and MING XIN HUANG, Assistant Professor, are with the Shenzhen Institute of Research and Innovation, University of Hong Kong, Shenzhen, China, and also with the Department of Mechanical Engineering, University of Hong Kong, Pokfulam Road, Hong Kong, China. Contact e-mail: mxhuang@ hku.hk Manuscript submitted September 18, 2014. Article published online August 25, 2015 5080—VOLUME 46A, NOVEMBER 2015

significant effect and can be observed easily.[1] However, in contrast to pure fcc metals, the large dislocation accumulation rate[12] and the planar glide characteristics of dislocations in TWIP steels can also lead to high work-hardening rate.[1,13] DSA phenomenon, which is commonly described as a result of the dynamic interaction between solid solution and dislocations during deformation, usually occurs in high-carbon (>0.5 wt pct) FeMnC alloys in a restricted range of temperatures and strain rates.[14] The effect of DSA on the work-hardening rate of TWIP steel is still debated. Shun et al. reported that DSA is the major cause of work-hardening for Fe-Mn-C(-Al) TWIP steels.[15] However, in other studies, it was concluded that DSA had only limited contribution to the work-hardening rate compared to deformation twinning or dislocation hardening.[9,16] The DSA process is considered to be a reason for the serration in the stress–strain curves of TWIP steel containing carbon.[1,17] Al addition not only reduces the intensity of serration but also increases the critical strain for the serration as Al can suppress the DSA effect.[18–21] TWIP effects are highly related to the stacking fault energy (SFE) of the materials. It was reported that deformation twins can be formed in FeMnC alloys with For SFE smaller than SFE between 18 and 35 mJ/m2[22] . 18 mJ/m2, martensitic transformation may take place during deformation. In previous study,[23] both e

METALLURGICAL AND MATERIALS TRANSACTIONS A

martensitic transformation and a¢ martensitic transformation occur when the SFE [at 300 K (27 C)] is near 0.1 mJ/m2, but only e martensitic transformat