Thermodynamic modeling of the Co–Hf system supported by key experiments and first-principles calculations

Highlights: • Heat contents of Co₂Hf and CoHf₂ were measured by drop calorimetry. • Enthalpy of formation for Co₂₃Hf₆ was computed via first-principles calculations. • Co–Hf system was assessed by means of CALPHAD approach. • Order–disorder model is used to describe B2 (CoHf) and A2 (βHf). • Glass forming range of the Co–Hf amorphous alloys was predicted. - Abstract: Phase equilibria and thermodynamic properties of the Co–Hf system were investigated via calorimetric measurements, first-principles calculations and thermodynamic modeling. Heat contents of Co₂Hf and CoHf₂ were measured by drop calorimetry from 300 to 1200 °C. The enthalpy of formation for Co₂₃Hf₆ at 0 K was computed via first-principles calculations. Based on the experimental measurements and first-principles calculations from the present work and the literature, the Co–Hf system was assessed by means of CALPHAD (CALculation of PHAse Diagram) approach. The excess Gibbs energy of solution phases was modeled with Redlich–Kister polynomial. Sublattice models were employed to describe the homogeneity ranges of Co₂Hf, CoHf and CoHf₂. The order–disorder transition between B2 (CoHf) and A2 (βHf) phases was taken into account in the current optimization. Using the optimized parameters, glass forming range (GFR) of the Co–Hf amorphous alloys was predicted to be 15–75 at.% Hf, which is in satisfactory agreement with the experimental observation