Revealing orientation-dependent martensitic transformation in a medium Mn steel by micropillar compression

Abstract

In this contribution, for the first time, the effect of crystal orientation on the critical stress for martensitic transformation in a medium Mn steel is explicitly revealed by using micropillar compression experiments. Although the martensitic transformation is observed in both [100] and [110] micropillars during compression tests, the average critical stress for martensitic transformation in [100] micropillars (∼483 MPa) is much smaller than that in [110] micropillars (∼700 MPa). The deformation twins can be found in the deformed [100] micropillars while the deformed [110] micropillars are free of deformation twins. The estimated critical stress for twin divergence in the [100] micropillars (∼465 MPa) is close to the corresponding critical stress for martensitic transformation. The theoretical critical stress for martensitic transformation in both [100] and [110] micropillars is estimated to be the same (650–750 MPa). The lower critical stress for martensitic transformations in the [100] micropillars could be ascribed to the formation of deformation twins, which is confirmed by the microstructural observation that the martensite nucleates at the intersections of deformation twin bundles. Two martensitic transformation pathways, namely twin-assisted martensitic transformation and free-standing martensitic transformation are proposed to explain the orientation dependent martensitic transformation observed in the present medium Mn steel.