Is "expanded austenite" really a solid solution? Mössbauer observation of an annealed AISI 316L nitrided sample

Expanded austenite, also known as S phase, m phase or γ_N phase forms during low temperature ( < 420 °C) nitriding of austenitic materials containing chromium. In this communication, annealing of a nitrided sample is used as a tool to understand the nature of expanded austenite. Starting from an AISI 316LN sample plasma nitrided for 4 h at 410 °C, the decomposition of expanded austenite is studied by conversion electron Mössbauer spectroscopy (CEMS) and X ray diffraction (XRD) for different annealing times at 400 °C. For the as nitrided sample, the CEMS spectra are analysed using two hyperfine field distributions (HFD). HFD A is attributed to a nitrogen enriched expanded austenite, HFD B is attributed to a bcc environment without or with very low nitrogen content. Both the local deformation of the bcc lattice and the presence of other elements as iron neighbouring atoms explain the broad range of hyperfine field in HFD B. The vanishing of HFD A, after 2 h of annealing, is explained by the formation of short-range order (SRO) between chromium and nitrogen and by the redistribution of nitrogen in excess. For the 3 h of annealing, a bcc phase is observed by XRD. We propose to describe this phase as martensite. An interplay between local internal stress and local nitrogen content is proposed to explain the unexpected formation of such a martensitic phase in a strongly gamma stabilizing environment. From the microscopic point of view expanded austenite can be considered to be constituted of at least two different environments: a gamma environment supersaturated in nitrogen and a martensitic environment without nitrogen.