THERMODYNAMIC PROPERTIES AND PHASE EQUILIBRIA IN Ba–Sn ALLOYS

Abstract

The enthalpies of mixing the liqiud of Ba-Sn alloys were determined by isoperibolic calorimetry at 1300 K over the entire range of compositions. Large exothermic mixing effects (ΔН_min = –54.8 ±1.8 kJ/mole at х_Ва = 0.56) were found; they are indicative of strong interaction between different components and of short ordering in the liquid alloys. The data on the thermochemical properties of melts and barium stannides and the phase equilibria in the Ba–Sn system were used to calculate the activities of components, molar fractions of associates, and formation enthalpies and entropies for liquid Ba_xSn_1–x alloys with the ideal associated solution (IAS) model. Two associates, BaSn and Ba₂Sn, were selected for this purpose. The calculated activities of components in the Ba–Sn melts show very large negative deviations from the ideal solution, which is consistent with their thermochemical properties. The calculated enthalpies of Ba_mSn_n intermetallic formation are rather large exothermic quantities, which are consistent only with the qualitative results. The maximum concentration of each associate is approximately 0.65 for respective compositions. The calculated formation enthalpies for BamSnn intermetallics are quite high exothermic values that agree well with the published data for BaSn₃ and only qualitatively for Ba₂Sn. The formation enthalpy for the associate agrees with that for corresponding BaSn intermetallic and is slightly less exothermic for Ba₂Sn. This all indicates that the bonding energies between different atoms in the corresponding intermetallics and melts are close to each other and quite large. The formation entropy for the Ba₂Sn associate is lower and that for BaSn is higher in magnitude. This demonstrates different degrees of ordering in BaSn and Ba₂Sn associates and intermetallics. This may be due to different changes in the oscillation frequencies of the atoms. The calculated liquidus curve of the Ba–Sn phase diagram agrees with data for the Sn–Sr system.