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

A transmission electron microscopy (TEM) study has been carried out to inve stigate the structural state of freshly formed and aged Fe-8.6 at.% N auste nites. The experimental alloys were prepared by gas nitriding of iron foils under NH3/H-2 mixture, followed by direct quenching into iced-brine. The o bservation has shown that the freshly formed Fe-8.6 at.% N alloy is essenti ally composed of a single austenite which does not contain the long-range a tomic order of the interstitial nitrogen atoms. The results rather indicate d that the as-nitrided Fe-N austenite would contain local atomic order to f orm isolated Fe4N-type and/or N-Fe-N (100)gamma dumbbell-type atomic config uration in a short-range, as suggested by previous Mossbauer spectroscopy s tudies. Aging at low temperatures, i.e. 373 and 473 K, allowed further segr egation of nitrogen atoms to result in the formation of the fine structure, resembling the (100) modulated structure by the spinodal decomposition. Th e resulting microstructure consisted of the N-enriched regions and the N-de pleted regions, where the former further contained Fe4N-type long-range ato mic order. (C) 1999 Elsevier Science S.A. All rights reserved.