Thermodynamic Properties and Phase Equilibria in Gd-Sn Alloys

The calorimetry method was employed to determine the mixing enthalpies of Gd-Sn melts and the ideal associated solution (IAS) model to calculate and optimize the thermodynamic properties of Gd-Sn alloys at 1510, 1640, and 1873 K in the composition range 0. xSn. 1.0. The minimum mixing enthalpies were -69.7 +/- 0.6 kJ/mole (1873 K) and -77.9 +/- 0.7 (1510 K) kJ/ mole at xSn = = 0.45. Using our own and published data on the thermochemical properties of melts and compounds and assuming the formation of two associates in the melt, Gd2Sn and GdSn, we calculated the activities of components, enthalpies, Gibbs energies, and entropies of formation for the liquid alloys and intermediate phases within the IAS model. The thermodynamic activities of components in the studied melts showed very large negative deviations from the ideal solutions. The mixing enthalpies of Sn-Gd melts optimized within the IAS model agreed well with the experimental values. The HGd.. temperature dependence agrees only qualitatively with other experimental data because of great errors in the published data. The excess Gibbs energy and mixing enthalpy of GdSn melts calculated with the IAS model at 1873 K greatly differed in magnitude, being indicative of a significant contribution of the entropy component to the excess molar Gibbs energy. According to the calculations, the minimum excess mixing entropy for Gd-Sn melts at 1873 K was -20.3 J/(mole center dot K) at x(Sn) = 0.45. The calculated and optimized enthalpies and entropies of formation for intermetallic phases in the Gd-Sn system, along with the IAS model parameters for melts, were used to calculate the liquidus and solidus curves of the phase diagram. Good agreement with most experimental data on the phase equilibria involving liquid and crystalline phases was shown.