Thermodynamic study and re-assessment of the Ge-Ni system

The enthalpies of formation of the intermetallic compounds in the Ge-Ni binary system have been determined by calorimetric measurement and first-principles calculations. Based on the results obtained and information available in the literature, the phase diagram and thermodynamic properties of the Ge-Ni system have been re-assessed using the CALPHAD approach [L. Kaufman, H. Bernstein, Computer Calculation of Phase Diagrams, Academic Press, New York (1970)], applying appropriate thermodynamic models for the phases. The liquid phase and the Ni-based solid solution (Ni) were modeled as substitutional solutions using the Redlich-Kister equation to represent the excess Gibbs energy. The B8-type intermediate phases εNi5Ge3, Ni19Ge12 and Ni3Ge2 were treated as a single phase, designated as Ni5Ge3. A three-sublattice model with stoichiometry (Ge)(Ni)(Va,Ni) was used to describe the B8-type Ni5Ge3-phase based on its crystal structure. The order-disorder transformation between disordered FCC_A1 and the ordered L12-type phase, βNi3Ge, was treated using a two-sublattice model. The other five intermetallic compounds were treated as stoichiometric compounds. The phase diagram and the thermodynamic properties calculated from the optimized model parameters are in good agreement with most of the experimental data.