Significant effects of grain size on mechanical response characteristics and deformation mechanisms of metastable austenitic stainless steel

Abstract

Grain size of a 304 austenitic stainless steel was tuned via deformation-induced martensite transformation and austenite reversion annealing, and the significant effects of grain size on mechanical properties, strain hardening behavior, deformation mechanisms were studied. The results showed that grain refinement strengthening significantly increased the yield strength from 235 MPa (~20 μm) to 1030 MPa (~200 nm), meanwhile, the interaction of Lüders deformation and deformation-induced martensite transformation realized the stable propagation of Lüders bands, leading to a considerable total elongation (above 30%). It was inferred that martensitic transformation was enhanced with a decrease in grain size. The underlying reasons were attributed to the formation of carbide precipitation (M 23 C 6 ) and the change of martensite nucleation sites depending on grain size. In coarse grains, α'-martensite formed at shear bands with or without the aid of ɛ-martensite, whereas α'-martensite mainly nucleated at grain boundaries and deformation twins in ultrafine austenite grains. Besides TRIP effect and dislocation glide, TWIP effect also became one of the dominate deformation mechanism with the decrease in austenite grain size. • Nano/ultrafine-grained (NG/UFG) 304 stainless steels with high strength and good tensile plasticity are achieved. • Deformation-induced martensite transformation is enhanced as austenite grain sizes decrease. • The enhanced strain-hardening caused by martensitic transformation supports the propagation of Lüders bands. • The dominant strain hardening mechanisms change from TRIP effect in CG steels to TRIP and TWIP effects in NG/UFG steels.