Abstract

The cluster variation method has been applied to establish effective interaction potentials that describe both n '-Fe 4 N 1 m x - k -Fe 2 N 1 m z miscibility gaps in the Fe-N phase diagram. The calculated nitrogen distributions show that long-range order occurs in the n :'-Fe 4 N 1 m x phase and that short-range ordering as well as long-range ordering occurs in the k -Fe 2 N 1 m z phase. The calculated nitrogen distributions for the k -Fe 2 N 1 m z , pertaining to temperatures and concentrations at the n '- k -phase boundaries, are compared with available data obtained by Mössbauer spectrometry. Preferential occupation of specific interstitial sites occurs from about 16 at.% N onwards; at the highest concentration considered, about 25 at.% N, the occupation is that of Fe 3 N as proposed in the literature on the basis of diffraction data