N-N interaction and nitrogen activity in the iron base austenite

The Monte Carlo computer simulation of the f.c.c. Fe-N alloy is performed u sing the data of abundances of different iron sites in the austenite lattic e as obtained by means of Mossbauer spectroscopy. The validity of the inter pretations of available Mossbauer spectra was tested based on the data of c alculation of N-N interaction energies in the two first coordination sphere s on the interstitial sublattice, which could satisfy the experimental data of nitrogen distribution in austenite derived from Mossbauer spectra. It i s shown that no values of N-N interaction energies exist that could be cons istent with the data of conversion electron Mossbauer spectroscopy (CEMS), where a thin surface layer only contributes to the Mossbauer spectra. A str ong N-N repulsion (>0.14 eV) in the first coordination sphere and a soft N- N repulsion (< 0.07 eV) in the second one was found to be consistent with t he studies performed by means of transmission Mossbauer spectroscopy (TMS), according to which single nitrogen atoms and 180 degrees N-N pairs exist i n the nitrogen austenite. A possibility of the long-range order-like Fe4N i s shown at the energy values determined for two coordination spheres, provi ded a small N-N repulsion in the third coordination sphere occurs. It is al so observed that the available data of nitrogen activity in Fe-N austenite are not consistent with the values of N-N interaction energies determined f rom the analysis of Mossbauer spectra, i.e. the available activity data do not correspond to the nitrogen distribution in the iron austenite as it fol lows from Mossbauer data. The concentration dependence of nitrogen activity and an effect of the N-2 gas pressure on the nitrogen solubility in Fe-N a ustenite are calculated using the values of N-N interaction energies in two coordination spheres. (C) 1999 Acta Metallurgica Inc. Published by Elsevie r Science Ltd. All rights reserved.