MONTE-CARLO ANALYSIS OF SHORT-RANGE ORDER IN NITROGEN-STRENGTHENED FE-NI-CR-N AUSTENITIC ALLOYS

A Monte Carlo simulation model which uses thermochemical experimental data to predict short-range atomic order in face-centered-cubic (f.c.c.), solid solution alloys of Fe containing both substitutional and interstitial solute elements is described by reference to the ordering of an Fe-Ni-Cr-N alloy. The interaction energies between pairs of atoms and between N and an interstitial vacancy are evaluated by matching the molar Gibbs free energy, predicted by a quasi-chemical model, to that deduced from thermochemical data. These are then used in a Monte Carlo simulation to determine the equilibrium short-range order, defined by the distribution of Ni, Cr and N atoms among the octahedral cells of the metal lattice. Applying the model to two Fe-40Ni-15Cr alloys-one with zero and the other with 0.25 wt.% N-at 1273 and 298 K revealed that, at the higher temperature, the distribution of the Ni and Cr atoms in the N-free alloy is nearly random but, in the alloy containing 0.25 wt.% N, there is appreciable short-range order involving both the metal and N atoms. Decreasing the temperature to 298 K increases the order in both alloys. The implication of the short-range order and N apportionment for the deformation mechanism in Fe-Ni-Cr-N f.c.c. alloys is discussed.