Deconvoluting the effects of austenite grain size and shape on the strain-induced martensitic transformation in medium-Mn steels

A key design challenge in advanced high strength steels (AHSS) is to control the mechanical stability of the retained austenite. This is a challenge because of the interplay of several influencing factors such as austenite chemistry, size, shape and surroundings. In addition, this understanding must be developed at both the local grain level, as well as the macroscopic level and this requires a systematic characterisation combining both macroscopic and local measurements. This study used a multi-technique characterisation method coupling quasi in-situ micro-DIC (digital image correlation), electron back-scattered diffraction (EBSD) and in-situ neutron diffraction (ND) to deconvolute austenite grain size and shape effects on its mechanical stability in a medium-Mn steel. The processing of the steel was controlled via intercritical annealing so that the austenite of different sizes and shapes had the same chemistry (confirmed by SEM/TEM EDX). Statistical analyses unambiguously reveal an intrinsic size effect (i.e. smaller grains are more stable), confirming previous reports. Furthermore, we demonstrate, for the austenite size and shape ranges we consider, that the so-called shape effect is simply an ‘effective’ size effect when analysed based on the local strain experienced by austenite grains prior to transformation.