Nanoscale Deformation Behavior of Phase-Reversion Induced Austenitic Stainless Steels: The Interplay Between Grain Size from Nano-Grain Regime to Coarse-Grain Regime

We have used the recently adopted concept of phase reversion to obtain grain size from the nanograined/ultrafine-grained (NG/UFG) to fine grain (FG) regime by varying temperature–time annealing sequence of cold deformed metastable austenite. The phase-reversion induced NG/UFG structure was characterized by high strength-high ductility combination. The concept of phase reversion involves severe cold deformation of metastable austenite to generate strain-induced martensite. Upon annealing, martensite transforms back to austenite through a diffusional reversion mechanism with NG/UFG, sub-micron grains (SMG) or FG structure, depending on the annealing condition. Depth-sensing nanoindentation experiments were combined with electron microscopy to elucidate the dependence of grain size from nanograin/ultrafine-grain (NG/UFG) to coarse grain (CG) regime on the deformation mechanisms. There was distinct transition in the deformation mechanism from intense mechanical twinning and stacking faults in NG/UFG structure to strain-induced martensite formation at the intersection of shear bands in the CG structure. The transition in the deformation mechanism is discussed in terms of increase in austenite stability with decrease in grain size.