THERMODYNAMIC INVESTIGATION OF THE PD-PB BINARY-LIQUID ALLOYS

The enthalpy of formation, h(f), of the Pd-Pb alloys was measured by d irect reaction calorimetry (drop method) at 840 K and 1344 K. At 840 K the results are in good agreement with those obtained previously betw een 922 and 1154K (no temperature dependence). At 1344 K, they confirm that the enthalpy of formation of the liquid phase increases with inc reasing temperature above 1154 K (as previously observed at 1240 K). T he crystallization of Pd3Pb was observed and we can deduce its enthalp y of formation. The strong negative departures from ideality of the th ermodynamic behaviour of the melt (h(min)(f) = - 40 kJ mol(-1), C-p(XS ) > 0 at high temperature) correspond undoubtedly to the presence of c hemical short-range order. A ''short-range order-disorder transition'' begins between 1154 and 1240 K. In order to apply the regular associa ted solution model we calculated the free enthalpy of formation of the liquid phase from the phase diagram and the thermodynamic functions o f formation of the solid intermediate compounds. The application of th e model allowed us to explain the behaviour of the Pd-Pb melts by the existence of two kinds of associate i.e. Pd2Pb and PdPb.