Thermodynamic Properties of Alloys and Phase Equilibria in the Cu–Yb System

The partial and integral mixing enthalpies of Cu-Yb melts were first determined at 1453 K in the composition range 0 < x_Cu <0.7 by isoperibolic calorimetry. The Cu–Yb melts were established to form with the release of a small amount of heat: minimum mixing enthalpy ∆H = –9.7 ± 0.8 (at x_Cu = 0.7), which agrees with the published data for these melts in the composition range 0 <x_Yb < 0.3 at 1453 K and for other Cu–Ln systems. The model of ideal associated solutions was used to optimize and calculate all thermodynamic properties (Gibbs energies, enthalpies, and entropies of formation) for melts, intermetallic compounds, and associates in the Cu–Yb system. The calculated activities of components in the melts of this system exhibited moderate negative deviations from ideal solutions. The calculations with the ideal associated solution model also showed that $\varDelta {\overline{H}}_{\textrm{Yb}}^{\infty }$ increased insignificantly and $\varDelta {\overline{H}}_{\textrm{Cu}}^{\infty }$ more substantially in the Cu–Yb system with higher temperatures. The ideal associated solution model was applied to calculate temperature–composition dependences of the Gibbs energies, enthalpies, and entropies of formation for the melts and intermetallics to determine the coordinates of the liquidus curve in the studied phase diagram. The calculated and experimental data were in good agreement. Full information on the thermodynamic properties of all phases and phase equilibria in the Cu–Yb alloys was obtained.