Mechanical behavior and deformation mechanism of nano/ultrafine grained austenitic stainless steel at cryogenic, room, and elevated temperatures

Nano/ultrafine-grained (Nano/UFG) austenitic stainless steel (ASS) has attracted considerable attention in recent years due to its excellent combination of ultra-high strength and good ductility. However, few studies have focused on its mechanical response at cryogenic and elevated temperatures. In this study, the mechanical properties and microstructure evolution of Nano/UFG steel during tensile testing at temperatures ranging from −60 to 100 °C were systematically investigated. The aim was to reveal the mechanical response of Nano/UFG steel at different temperatures and the correlation between mechanical behavior and deformation mechanism. The results show that the yield strength of Nano/UFG ASS is almost unaffected by the deformation temperature, but the ultimate tensile strength (UTS) and tensile elongation (TE) vary significantly at different temperatures. When deformed at temperatures lower than 60 °C, Nano/UFG ASS achieves relatively high UTS and TE due to the strong hardening caused by martensitic transformation and deformation twinning. However, when deformed at 100 °C, the tensile ductility decreases dramatically due to the inhibition of dislocation gliding by the small grain size. Microstructural characterization reveals that the deformation mechanism of Nano/UFG ASS is influenced by both the stacking fault energy and grain size. The dominant deformation mechanism changes from deformation-induced γ → α′ martensitic transformation at −60 °C to a combination of martensitic transformation and deformation twining at 20 °C, further to martensitic transformation at 60 °C, and finally to stacking faults at 100 °C. Additionally, the differences in the deformation mechanism between coarse-grained ASS and Nano/UFG ASS at different temperatures are also clarified.