A combined cluster variation method and ab initio approach to the γ-Fe[N]/γ′-Fe4N1 − x phase equilibrium Original Research Article

A combination of ab initio and statistical thermodynamics, i.e., the cluster variation method (CVM), is applied to describe the FCC-based γ-Fe[N]/γ′-Fe4N1 − x phase equilibrium. The ab initio calculated internal energies of ordered compounds, using the full-potential (linearized) augmented plane-wave plus the local orbital method including the magnetic contributions, are used to obtain a set of volume-dependent effective cluster interactions which parametrize the internal energy of FCC-based Fe–N alloys. The γ-Fe[N]/γ′-Fe4N1 − x phase boundaries, the lattice parameters and the distribution of N atoms over the octahedral interstitial sites are calculated by applying the tetrahedron approximation. The vibrational contributions are also considered by using the Debye–Grüneisen model. The current calculations of the phase boundaries, the lattice parameters and the interstitial nitrogen distribution agree well with experimental data available in the literature.