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

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.