Formation mechanism of dislocation patterns under low cycle fatigue of a high-manganese austenitic TRIP steel with dominating planar slip mode

Abstract

We investigated the evolution of dislocation patterns under low cycle fatigue in a Fe–17Mn-1.5Al-0.3C (wt. %) steel with transformation-induced plasticity (TRIP). To this end, quasi in-situ electron channelling contrast imaging under controlled diffraction conditions (cECCI) was employed. Cross-correlation based electron backscattering diffraction (CC-EBSD) with high angular resolution was applied to measure the distributions of residual strain/stress and geometrically necessary dislocations (GNDs). Planar slip mode was found to be dominating in this low stacking fault energy (SFE) steel under the small strain amplitudes that were applied here (<0.8%). However, un-expected complex dislocation patterns (typical for materials with high SFE and cross slip), e.g. dislocation tangles and walls, were often observed. These structures were found always to be accompanied with a dense and homogeneous arrangement of slip traces, which can be ascribed to the highly planar slip in conjunction with high passing stresses. The interaction of different slip systems forms junctions, which assist the formation of dislocation dipoles and later on trigger the formation of dislocation walls. These walls consist of primary faulted dipoles in their interior and pile-ups of dissociated dislocations of opposite signs on their two sides. It was found that the dislocations in the walls belong to two major, collinear slip systems. The distribution of stresses and strains over channels and walls corresponds well to a refined composite model of Mughrabi.