MODELS OF SHORT-RANGE ORDER IN A FACE-CENTERED-CUBIC FE-NI-CR ALLOY WITH A HIGH-CONCENTRATION OF NITROGEN

The calculation results of a cluster variation model and two different Monte Carlo models that have been used to deduce the atomic arrangement in a concentrated solid solution of nitrogen in an austenitic alloy (Fe-39.9Ni-14.96Cr-0.25N wt%) are compared. Quantitatively, the predictions of all three models are in good agreement. In the absence of nitrogen, the arrangement of the metal atoms at 1273 K was found to be close to random. Addition of nitrogen markedly increases the number of octahedral lattice clusters containing four or more chromium atoms, with the result that a large fraction of the nitrogen atoms occupy interstitial sites in these high-chromium clusters. The association of nitrogen and chromium atoms predicted at 298 K was found to be even more pronounced, the nitrogen being exclusively in clusters with six chromium atoms. The validity of the assumptions made in each of the models and the significance of the results are discussed.