Distribution of interstitials in fcc iron-carbon austenite: Monte Carlo simulations versus Mössbauer analysis

Abstract

A study of the distribution of carbon atoms in the octahedral interstitial sites of the face-centered-cubic (fcc) austenite phase in iron-carbon alloys combines an analysis of the chemical potential of C, based on the quasichemical approximation to the statistical mechanics of interstitial solutions, with three-dimensional Monte Carlo simulations and M\"ossbauer results. The simulations are performed using a C-C interaction energy extracted from available activity data by assuming a gas like mixture of C atoms and vacancies (V) in the octahedral interstitial sites. The number of C-C atom pairs, as well as C-V and $V\ensuremath{-}V$ pairs, are calculated and compared with those given by the quasichemical model. Furthermore, the relative fraction of the various Fe environments are calculated and compared with those extracted from the M\"ossbauer spectra. The simulations reproduce well the relative fractions obtained from M\"ossbauer spectra assuming the ${\mathrm{Fe}}_{8}{\mathrm{C}}_{1\ensuremath{-}x}$ model, which includes some blocking of the nearest neighbor interstitial sites by a C atom. With the information obtained in the present study, a critical discussion is reported of the extent to which such blocking effect is accounted for in the current thermodynamic models of the Fe-C fcc phase.