Application of the cluster variation method to an interstitial solid solution: Calculation of the γ ′-Fe 4 N 1-x -ε-Fe 2 N 1-z equilibrium

The cluster variation method has been applied to establish effective interaction potentials that describe both n'-Fe 4 N 1 mx - k-Fe 2 N 1 mz 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 mx phase and that short-range ordering as well as long-range ordering occurs in the k-Fe 2 N 1 mz phase. The calculated nitrogen distributions for the k-Fe 2 N 1 mz , 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.