In-situ observation on the isothermal decomposition process of γ-Fe[N] by X-ray diffraction

The isothermal decomposition process of γ-Fe[N] (9.32 at.% N) prepared by a two-step gas nitriding method was investigated in-situ by X-ray diffraction (XRD). The XRD spectra showed that the α-Fe phase precipitated first from the parent austenite when the sample was aged at 225 °C, while the γ′-Fe4N phase appeared half an hour later as extra fine particles. The quantity of these two phases increased quickly with the ageing time until the original austenite of γ-Fe[N] was completely consumed. Further investigation by scanning electronic microscopy (SEM) revealed that the α-Fe nucleated first at the primary grain boundaries and grew towards the grain interior with a lamellar bainitic-like structure, which was different to the homogeneous precipitates of γ′-Fe4N and α-Fe inside the grains. A new transformation mechanism has thus been proposed for the isothermal decomposition of high nitrogen austenite on the grain boundaries (GBs).