Effects of Stacking Fault Energy on Deformation Mechanisms in Al-Added Medium Mn TWIP Steel

In this study, the effect of aluminum (Al) addition to a manganese (Mn) steel Fe-12Mn-0.5C in regard to the change in stacking fault energy (SFE) and the consequent evolution of deformation microstructure and texture were investigated during cold rolling. An analysis of the texture and microstructure was performed to understand the deformation micro-mechanisms. Deformation micro-mechanisms were substantiated by the estimation of dislocation density and the arrangement of dislocations in the deformed microstructure by X-ray line profile analysis, which revealed significant changes in the dislocation structure with the addition of Al. Three stages of deformation mechanism were observed in all Al-added compositions. In the early stages of deformation, slip as well as twinning prevailed. In the intermediate stage, twinning took over completely and at large strains, macroscopic shear bands became the dominant deformation mode. An increase in the propensity of nanometer-sized deformation twins was observed with rolling strain. However, the addition of Al decreased the overall twin fraction in the deformed microstructure. The theoretical twinning stress was calculated to explain the crucial role of SFE on the occurrence of deformation twins in these steels. The deformation texture was predominantly of the brass type for all the Al-added compositions; however, appreciable differences were seen with Al content. The < 111 >//ND -fiber, which develops in Al-free Fe-12Mn-0.5C, completely disappeared in 3wtpct Al-containing material.