Thermodynamic assessment of the Ni–Si system by incorporating ab initio energetic calculations into the CALPHAD approach

Abstract

A thermodynamic assessment was conducted using experimental data on the phase boundaries of the Ni–Si system and other related thermochemical properties. In addition to the experimental data, the Gibbs energy of formation and the enthalpy of formation for both the ordered and the disordered fcc phases obtained from ab initio energetic calculations were incorporated as hypothetical experimental data. There was good agreement between the calculated and the experimental phase boundaries, and the thermodynamic properties were reproduced satisfactorily. In particular, the phase equilibria between the disordered fcc phase and the ordered fcc phase, Ni 3Si( β 1), which have not been reproduced by previously reported assessments, were consistent with the available experimental data. Furthermore, it should be noted that the evaluated enthalpy of formation for the ordered fcc phase was −47.1 kJ mol −1 of atoms, which agrees well with the value obtained by ab initio energetic calculations of −49.2 kJ (mole of atoms) −1. These values contrast with the experimental value reported by Oelson and Samson-Himmelstjerna of −37.2 kJ (mole of atoms) −1. Therefore, it is considered that the experimental data on the enthalpy of formation of the ordered fcc phase are less accurate, and that the incorporation of ab initio energetic calculations into the CALPHAD approach is valid.