Nanoscale deformation experiments on the strain rate sensitivity of phase reversion induced nanograined/ultrafine-grained austenitic stainless steels and comparison with the coarse-grained counterpart

We have recently adopted the concept of phase-reversion to obtain nanograined/ultrafine-grained (NG/UFG) structure with high strength–high ductility combination in some austenitic Cr–Ni stainless steels. The controlled deformation-annealing schedule involves cold deformation (∼45–77%) of metastable austenite to generate strain-induced martensite that on annealing transforms back to austenite with nano (NG) or ultrafine grain (UFG) size. In the present study, we describe the first report on systematic experiments carried out to investigate the strain rate sensitivity of deformation in an Fe–17Cr–7Ni steel characterized by NG/UFG structure and compare with the conventional coarse grained (CG) counterpart, using nanohardness measurements and extensive transmission electron microscopy of the plastically deformed nanoindented regions. The observations reveal that the NG/UFG structure exhibited greater strain rate sensitivity and smaller activation volume compared to that of the CG structure, under identical test conditions. Furthermore, there was clear distinction and fundamental transition in the deformation mechanism from 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 stacking fault energy and grain size.