Austenitic stainless steels (ASSs) demonstrate excellent corrosion resistance, making them suitable for maritime, chemical, and nuclear applications, but suffer from poor hardness and tribological properties. Mechanical performance can be improved through the formation of a hardened surface layer, in this case through the plasma-assisted thermochemical diffusion treatments by nitrogen (N), carbon (C), or N+C into a near-surface region. Under suitable treatment conditions – namely low-temperate and/or short-duration treatment to suppress the formation of chromium nitrides and/or carbides – the N and C atoms will occupy interstitial sites in the austenite matrix, distorting it into so-called expanded austenite, i.e. γ_X where x: C, N or both. Therefore, expanded austenite may appear as a single layer as a result of relatively pure N or C diffusion (γ_N and γ_C), or it may exhibit a compositionally distinct duplex layer structure (γ_N/γ_C).

In this work AISI 316L stainless steel with two different surface finish conditions (ground or polished) is subjected to plasma-assisted nitriding, carburizing, and nitrocarburizing treatments.

Of primary interest is the characterization of anisotropic cross-sectional lattice curvature within the expanded austenite region. Curvature develops as a result of slip generated by high compressive residual stresses from the lattice expansion induced in the expanded austenite due to the uptake of the interstitials. Electron backscatter diffraction (EBSD) data is used to evaluate local curvature of the expanded austenite layer and determine a valid model of anisotropic evolution under strain, e.g. Sachs- or Taylor-derived. This evaluation is complemented with an analysis of the lattice expansion (strain) via Co-Kα₁ x-ray diffraction (XRD). Furthermore, the variations in curvature are correlated with composition-depth profiles in the near-surface region determined from electron probe microanalysis (EPMA) and glow-discharge optical emission spectroscopy (GDOES). All evaluations are compared with the visually observed expanded austenite layer through backscattered electron (BSE) imaging and chemically etched light optical microscope (LOM) imaging. Upon the correlative investigations of the crystallographic and compositional anisotropies, a robust definition of the chemical-crystallographic profile of single layer and duplex layer expanded austenite structures is thereby obtained.