Effect of deformation mode on martensitic transformation in medium Mn steel

Abstract

In the present work, we have investigated the effect of deformation modes namely uniaxial loading and plane strain loading on martensite transformation in medium Mn TRIP steels, since it plays an important role in influencing formability. Interrupted deformation experiments showed that the martensitic transformation proceeds continuously with plastic strain and is influenced by the specific deformation mode applied. Uniaxially loaded samples underwent a complete austenite to martensite transformation. On the other hand, under plane strain loading, only 70 % of the austenite transformed into martensite, even after fracture had occurred. Through synchrotron and EBSD experiments, we observed that under uniaxial loading, the transformation sequence followed the γ-ε-α′ pathway, whereas, under plane strain, a direct γ-α′ transformation was observed. The intermediate ε-martensite phase is known to facilitate the transformation from austenite to martensite. Consequently, the presence of ε-martensite resulted in a higher degree of austenite transformation in the uniaxial samples. We rationalize the difference in transformation sequences based on resolved shear stress calculations for the two deformation modes.