Arrangements of interstitial atoms in fcc Fe-C and Fe-N solid solutions

The distribution of C and N atoms in the octahedral interstitial sites of the face-centred-cubic austenite phase of the Fe-C and the Fe-N alloys is controversial. In this work, Mossbauer experiments, the quasichemical approximation, the hard-blocking excluded-sites model, the chemical activity data, electron charge calculations and Monte Carlo simulations have been combined to advance in its understanding. A database is developed, with analyses of Mossbauer spectra using models assuming either ordered or random distributions of the interstitial atoms in the interstices around an Fe atom. The data are compared as a function the fraction of occupied sites, and various striking differences between Fe-N and Fe-C alloys are discussed. The experimental trends are confronted with predictions of combined theoretical approaches.