Grain size dependent mechanical behavior and TRIP effect in a metastable austenitic stainless steel

Abstract

The change in the trends of mechanical behavior and martensite formation of austenitic steels versus grain size has not yet been explained for the coarse-grained regime. Moreover, the effect of grain size needs systematic investigation by considering a wide range of grain sizes from the ultrafine-grained (UFG) regime to the coarse-grained one. In response, the effect of a wide range of austenite average grain sizes from 0.5 to 192 µm on the mechanical behavior and transformation-induced plasticity (TRIP) effect in an AISI 304L stainless steel was systematically investigated in the present work. Analysis of the tensile properties revealed that there is a transition grain size range of 34 to 90 µm, where a meaningful change in the trends of the ultimate tensile strength (UTS), total elongation, tensile toughness, work-hardening capacity, and yield ratio was observed. For instance, the total elongation increased with increasing average grain size up to the transition range but decreased at coarser grain sizes. However, the yield stress followed the Hall-Petch relationship of YS(MPa)=134.2+632.6/D(μm) for the whole grain size range, indicating that the presence of the transition grain size range is related to the work-hardening behavior, especially the TRIP effect. The apparent stacking fault energy decreased with increasing the grain size and reached a plateau after the transition grain size range. The same trends were found for the critical strain for the onset of the TRIP effect. Moreover, increasing the grain size up to this transition range promoted the formation of deformation-induced martensite, which was followed by its suppression at coarser grain sizes. The same trends were found for the maximum work-hardening rate related to the TRIP effect. The evolution of mechanical properties and TRIP effect with the grain size up to the transition range was explained based on the dependency of apparent stacking fault energy on the grain size. However, with the constancy of apparent stacking fault energy at coarser grain sizes, the decline in the formation of shear band intersections was found to play a significant role. The latter effect was further supported by Olson-Cohen analysis and investigation of the grain size dependency of the rate of shear-band formation (α) and the probability of generation of the martensite embryo at shear-band intersections (β).