Competing deformation mechanisms in an austenite-ferrite medium-Mn steel at different strain rates

The study of strain rate is of particular significance in automotive applications. Thus, we describe here the mechanical behavior and deformation mechanisms in Fe-0.2C–11Mn–2Al medium-Mn steel in the strain rate range of 2 × 10−4–200s−1. The ultimate tensile strength decreased from 1456 MPa to 1086 MPa with increase in strain rate, the strain rate sensitivity index m of flow stress decreased generally from −0.00154 to −0.0146 with the increase of strain, and the uniform elongation decreased from 39.4% to 33.0% with the increase of strain rate (2 × 10−4-2s−1). However, the elongation increased to 37.5% when the strain rate is further increased to 200s−1. Transformation-induced plasticity (TRIP) effect and PLC effect was dominant at low strain rates (2 × 10−4–2 × 10−3s−1), but at medium strain rates (2 × 10−2-2s−1), TRIP effect was significantly suppressed, twining-induced plasticity (TWIP) effect and softening effect increased gradually with strain rate, which compensated for the loss of TRIP effect. At high strain rates (20–200s−1), TRIP effect, TWIP effect and softening effect enhanced simultaneously, but PLC effect was suppressed with increase in strain rate, contributing to increase in ductility with increasing strain rate. According to the fracture microstructure, it was indicated that normal stress was dominant at low strain rates, while shear stress played an increasingly prominent role at medium and especially high strain rates.