Distribution of interstitial atoms in FCC Fe-N alloys: A Mössbauer, thermodynamic and Monte Carlo approach

A study is presented of the distribution of interstitial atoms in the octahedral sites of the face-centered-cubic (FCC) austenite phase in Fe-N system. The approach is based on the interplay between an analysis of Mössbauer data, Monte Carlo (MC) simulations, analytical calculations based on the quasichemical approximation (QCA) to the statistical mechanics of interstitial solutions, and thermodynamic activity data. A database is developed, with analyses of Mössbauer spectra using models assuming either ordered or random distributions of the N atoms in the interstices around an Fe atom. The experimental trends are confronted with predictions of a combined theoretical approach. Thermodynamic activity data are analyzed, and the energies of the N-N pairs thus obtained are used in both MC simulations and QCA calculations, which are adopted as a suitable frame of reference for the critical comparison between the Fe-N and Fe-C systems. Monte Carlo predictions of the relative fractions of the Fe-N environment reveal that the consideration of only N-N interactions, as in the case of the Fe-C system, is not enough to describe the Fe-N austenite. Various implications of the present results are discussed.