Plastic Deformation Modes in Mono- and Bimodal-Type Ultrafine-Grained Austenitic Stainless Steel

Abstract

An attempt was made to track plastic tensile deformation modes operating in bulk ultrafine-grained austenitic stainless steel with mono- (maximum at ~0.6 μm) and bimodal-type (minimum at ~0.5 μm and maximum at ~1.65 μm) grain size distributions produced by cyclic thermal processing. Post tensile deformation electron backscatter diffraction studies were conducted to analyse the impact of grain size distribution on plastic deformation characteristics. The study revealed extensive strain localisation in the monomodal-type grain size distribution, leading to poor strain-hardening behaviour. On the other hand, the bimodal-type distribution disclosed a conventional dislocation-mediated deformation mechanism operating in the coarse grains, while it was restricted to initial small strains in ultrafine austenite grains. The subsequent deformation process in these ultrafine austenite grains was dictated by nucleation and autocatalytic growth of strain-induced α′-martensite. The observed martensitic transformation of ultrafine austenite grains in preference to coarse grains was attributed to activation of local ‘grain to grain’ interactions.