Structure of freshly formed and low-temperature aged Fe–N austenites

A transmission electron microscopy (TEM) study has been carried out to investigate the structural state of freshly formed and aged Fe–8.6 at.% N austenites. The experimental alloys were prepared by gas nitriding of iron foils under NH3/H2 mixture, followed by direct quenching into iced-brine. The observation has shown that the freshly formed Fe–8.6 at.% N alloy is essentially composed of a single austenite which does not contain the long-range atomic order of the interstitial nitrogen atoms. The results rather indicated that the as-nitrided Fe–N austenite would contain local atomic order to form isolated Fe4N-type and/or N–Fe–N 100γdumbbell-type atomic configuration in a short-range, as suggested by previous Mössbauer spectroscopy studies. Aging at low temperatures, i.e. 373 and 473 K, allowed further segregation of nitrogen atoms to result in the formation of the fine structure, resembling the {100} modulated structure by the spinodal decomposition. The resulting microstructure consisted of the N-enriched regions and the N-depleted regions, where the former further contained Fe4N-type long-range atomic order.