Thin-film reactions of Pb with AgAu and AgPd alloys

Stability of AgAu and AgPd alloys with respect to their reactions with Pb have been studied by using thin-film samples and by free-energy calculations. Thin-film reactions of Pb with 80 at.% Ag-20 at.% Au and 80 at.% Ag-20 at.% Pd alloys were examined by x-ray diffraction. X-ray results show that the AgAu alloy is stable with Pb up to 200 °C and the AgPd alloy is stable with Pb at room temperature but unstable at 200 and 250 °C resulting in the formation of Pb₂Pd along with a dilute phase of Ag which is almost free of Pd. Free-energy calculations show that Pb cannot deplete Au from any AgAu alloy which contains less than 29.5 at.% Au, and yet Pb can deplete Pd down below 1 at.% from any AgPd alloys provided that the kinetics is fast enough.