Unusual step-like stress flow behavior and mechanisms of a 1.3 GPa grade medium-Mn steel with 51.2 % ductility

Abstract

A Fe-8.9Mn-1.59Si-1.0Al-0.25C medium-Mn steel have achieved a superior combination of strength and ductility, i.e., the product of strength and elongation (PSE) of 66.5 GPa%. Meanwhile, an unusual step-like stress–strain curve due to heterogeneous deformation bands including Lüders band and multiple Portevin–Le Chatelier (PLC) bands was investigated systematacially. The repeated heterogeneous deformation is due to a combination of extrinsic and intrinsic factors. The former refers a small stress concentration near the grip ends of the tensile sample, and the latter is associated with low density of initial mobile dislocations, limited capacity of mobile dislocation multiplication and the presence of meta-stable austenite. As for meta-stable austenite, stress-assisted martensite transformation (SAMT) was confirmed experimentally, which will facilitate strain concentration and the starting of heterogeneous deformation. Moreover, before strain-induced martensite transformation (SIMT), it is the plastic deformation of austenite grains through the slip of partial dislocation and stacking faults (SFs) that contributes to a larger Lüders strain. Finally, we propose that whether it is possible to make the heterogeneous deformation to be a positive side for certain working conditions, which may open new doors for the application of medium-Mn steels. We will continue to do some exploratory studies in this direction next.