Thermodynamic assessment of the Ga–Gd, Ga–Nb and Gd–Nb binary phase diagrams towards the Ga–Gd–Nb system

The present work gives a thermodynamic optimization of the Ga–Gd, Ga–Nb and Gd–Nb binary systems based on the CALPHAD method. Redlich-Kister polynomials are used to describe the excess Gibbs energy of the liquid phase and the solid solutions. The Ga2Gd (ϵ) and GaNb3 (A15) phases, which present a large homogeneity range, were described using a two-sublattice model with the formula (Ga)2(Ga,Gd)1 and (Nb)3(Ga,Nb)1 respectively. The remaining intermediate phases, ht_Ga4Nb, Ga3Nb, Ga13Nb5, Ga5 Nb4, Ga4 Nb5, Ga3Nb5, Ga6Gd, GaGd, Ga2Gd3 and Ga3Gd5, were treated as stoichiometric compounds. The generated thermodynamic parameters relative to the involved phases were critically discussed and used to build the Ga-Gd-Nb ternary system.