The influence of stacking fault energy on plasticity mechanisms in triode-plasma nitrided austenitic stainless steels: Implications for the structure and stability of nitrogen-expanded austenite

Austenitic stainless steels (ASSs), especially AISI type 304 and 316 ASSs, have been extensively studied after thermochemical diffusion treatments (e.g. nitriding, carburising) to resolve the anomalous lattice expansion after supersaturation of interstitial elements under paraequilibium conditions. The known issues are i) plastic deformation of surfaces under nitrogen-introduced strain at low treatment temperatures and ii) degradation in surface corrosion performance in association with chromium nitride formation at elevated treatment temperatures (and/or longer treatment times). In this study, a nitrogen-containing high-manganese ASS and a high-nickel ASS (i.e. Fe-17Cr-20Mn-0.5 N and Fe-19Cr-35Ni, in wt.%) were triode-plasma nitrided under a high nitrogen gas volume fraction and low (and close to monoenergetic) ion energy of ∼200 eV at 400 °C, 425 °C and 450 °C for 4hrs and 20hrs, respectively. Auxiliary radiant heating was used to facilitate different treatment temperatures at a deliberately controlled and constant substrate current density of ∼0.13 mA/cm2, under which material surface crystallographic structure was mainly influenced by the different treatment temperatures and times applied during nitriding. With respect to stacking fault energy (SFE), we illustrate and discuss i) the analogy of composition-induced plastic deformation phenomena to mechanical deformation processes, ii) two possible types of dislocation-mediated plasticity mechanism in γN, iii) two possible types of diffusional decomposition mechanism for γN, and iv) the lattice structures formed at low to moderate nitriding temperatures.