Deformation of Austenitic CrMnNi TRIP/TWIP Steels: Nature and Role of the ɛ−martensite

High-alloy CrMnNi TRIP/TWIP steels show a martensitic phase transformation, which is frequently described by the transformation sequence from ϒ-austenite (fcc) via ɛ−martensite (hcp) to α−martensite (bcc). During the plastic deformation, deformation bands in austenite are formed, which contain a high density of stacking faults on the lattice planes {111}. The formation of the stacking faults is enabled by the dissociation of perfect dislocations into the partial dislocations. In ɛ−martensite, the stacking faults are highly ordered. Diffraction experiments recognize the periodic arrangement of the stacking faults as a hexagonal phase. In this contribution, the structure of ɛ−martensite was described with the aid of the analysis and the DIFFaX simulation of X-ray diffraction patterns, electron back-scatter diffraction and high-resolution transmission electron microscopy. The faulting of austenite enhances the formation of α−martensite during deformation, thus the work hardening capability is significantly increased.