Effect of the Strain Rate on the TRIP-TWIP Transition in Austenitic Fe-12 pct Mn-0.6 pct C TWIP Steel

The strain-rate dependence of the plasticity-enhancing mechanisms in Fe-12 pct Mn-0.6 pct C-0.06 pct N steel was investigated. At low strain rates, deformation-induced epsilon-martensite was formed. At high strain rate, the strain-induced formation of epsilon-martensite was inhibited, and mechanical twinning was the dominant plasticity-enhancing deformation mechanism. This transition was associated with an increased work hardening rate and a higher total elongation. Dynamic strain aging (DSA) took place at all strain rates. While propagating type C Portevin Le Chatelier (PLC) bands were observed at low strain rates, isolated propagating type A PLC bands were observed at high strain rates. The critical strain for the occurrence of DSA had an anomalous negative strain-rate dependence at low strain rates and a normal positive dependence at high strain rates. The transition from negative-to-positive strain-rate dependence was associated with a sharp change in the strain-rate sensitivity of the flow stress. Transmission electron microscopy was used to analyze the relationship between the stacking fault energy (SFE), the strain rate, and the plasticity-enhancing mechanisms. The SFE and critical resolved shear stress for the onset of the twinning and the epsilon-martensite transformation were calculated and compared with experimental results. (C) The Minerals, Metals & Materials Society and ASM International 2013