Statistical thermodynamic approach to austenitic Fe 1-y Nb y N x system

Abstract

Available equilibrium P-T-C (pressure-temperature-composition) relationships reported for fcc (face centred cubic) Fe_1-yNb_yN_x alloys with Nb content y up to 0.00561 were analysed on the basis of statistical thermodynamics. The reported P-T-C relationships showed a trend of rising N level x with increasing Nb content y under comparable conditions of temperature T and nitrogen partial pressure p(N₂). Statistical models assuming several types of atom clustering modes were compared. A model in which interstitial N atoms were preferentially consumed first to form Nb-N dipoles in the Fe_1-yNb_y lattice, and then the remaining N atoms were distributed over O sites (octahedral interstitial sites) surrounded by six Fe atoms without Nb, was concluded to be the most realistic one.