Correlation between deformation behavior and austenite characteristics in a Mn-Al type TRIP steel

We investigate here the correlation between deformation behavior and retained austenite characteristics in a medium-Mn transformation-induced plasticity (TRIP) steel. The sample was characterized by a dual-phase microstructure consisting of ultra-fine grained ferrite and retained austenite with relatively high mechanical stability after annealing at 700 °C for 5 h. Both lath-like and blocky (granular) retained austenite with volume fraction of 38.7% and relatively inhomogeneous grain size was obtained. The tensile specimen exhibited outstanding mechanical properties with yield strength of 745 MPa, tensile strength of 1005 MPa and total elongation of 46%, as well as a distinctive work hardening behavior. The in-depth investigation on deformation behavior demonstrated that the transformation mechanism of retained austenite during deformation was strain-induced and the yielding behavior was controlled mainly by the deformation of soft ferrite phase. As to the multi-peak work hardening behavior, it is believed to be attributed to the inhomogeneous and discontinuous occurrence of TRIP effect, which resulted from the inhomogeneous stability of retained austenite. Moreover, the orientation of retained austenite (Schmid factors) was proved an important factor in determining the mechanical stability of retained austenite upon deformation, in addition to the heterogeneity of grain size. These two factors together resulted in the inhomogeneous stability of retained austenite.