Effects of Stacking Fault Energy on Deformation Mechanisms in Al-Added Medium Mn TWIP Steel
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THE importance of Mn-based twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) steels has increased in the last few years due to improvements in combinations of strength and uniformity in elongation, which has furthered their potential in various applications in automobiles. The synchronous improvement in strength and ductility in Mn-based steel depends on the stacking fault energy and hence, on the associated active deformation mechanisms.[1–3] The TWIP steels have very good ultimate tensile strength and elongation; however, these steels exhibit low yield strength[4,5] and delayed cracking.[6,7] It has been reported that the addition of aluminum leads to an improvement in the yield strength by solid solution strengthening and also prevents delayed cracking in Mn steels.[7,8]
RAJIB KALSAR, PRIYANKA KHANDAL, and SATYAM SUWAS are with the Department of Materials Engineering, Indian Institute of Science, Bangalore 560 012, India. Contact e-mail: [email protected] Manuscript submitted June 3, 2018. Article published online May 22, 2019 METALLURGICAL AND MATERIALS TRANSACTIONS A
The TWIP steels are characterized by low stacking fault energy; therefore, cross-slip is difficult and twinning becomes the predominant deformation mechanism.[9,10] The SFE and the associated deformation micro-mechanisms play an important role in phase stability and twinning-induced hardening in TWIP steels. The most important among these mechanisms is the ‘‘dynamic Hall–Petch’’ effect in austenite grains,[11,12] as reported for Fe-22Mn-0.6C TWIP steels. The austenite grain is progressively sub-divided into semi-coherent twin boundaries, which reduces the mean free path of dislocations and causes strengthening.[9,13,14] The interactions between these twin boundaries and dislocations have a major role in the very high strain hardening of TWIP steels. The deformation twins contain a high density of sessile dislocations, which further act as strong barriers for dislocation movements and hence, contribute to strain hardening.[15,16] In addition, the interactions between manganese-carbon (Mn-C) octahedral clusters and dislocations further increase strain hardening in TWIP steels.[17] However, it is almost established that twins play the most significant role in deformation mechanisms. The nucleation of twins and their propagation and growth in TWIP steels are a function of grain size and grain orientation[18,19] in addition to SFE. The thickness of deformation twins and the distance between them or the
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mean twin-free path directly contribute towards the dynamic Hall–Petch effect. One of the important attributes of Al addition is to increase the SFE of TWIP steels. Chin et al.[6] reported that the addition of 1 wt pct Al increases the SFE by nearly 8 mJ/m2 in Fe-18Mn-0.6C TWIP steels. Therefore, the addition of Al is likely to influence the overall deformation micro-mechanisms in TWIP steels. Besides its effect on the propensity of twinning, Al addition has been reported to increase t
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