Deformation mechanisms in meta-stable and nitrogen-stabilized austenitic stainless steel during severe surface deformation

AISI 304L stainless steel in austenitized and in solution nitrided condition was severely mechanically deformed by surface roller burnishing. High-temperature solution nitriding was applied to achieve a nitrogen-concentration depth profile, leading to a depth-gradient in the austenite stability. X-ray diffraction, electron microscopy and hardness indentation were applied for characterization of the graded microstructures obtained by combining a composition profile and a deformation profile. While severe plastic surface straining of an austenitized specimen leads to a deformation-induced transformation of austenite into martensite, the solution nitrided specimen remains austenitic upon deformation, even in the region where nanocrystallization occurs. The deformation mechanisms operable in the nitrogen-stabilized austenitic stainless steel, i.e. twinning or dislocation glide, depend on the combination of applied plastic strain/strain rate, and the nitrogen-concentration dependent stacking fault energy.