RETARDATION OF GRAIN-BOUNDARY REACTIONS IN AG-PD-CU ALLOYS BY ADDITIONS OF SMALL AMOUNTS OF OTHER ELEMENTS

The grain-boundary reactions in Ag-25 mass % Pd-10 mass % Cu alloys with 1 mass % aluminum, cobalt, tin, chromium or indium, were investigated by optical microscopy, electrical resistivity, and hardness tests. Addition of tin and chromium retarded grain-boundary reactions. The growth rate of nodules with tin addition was 1/18 of the alloy without additons. Grain-interior reactions are accelerated with chromium, tin and indium addition, with chromium the most effective. The activation energies for the grain-interior and grain-boundary reactions are 192 to 196 and 144 to 208 kJ mol-1, respectively. Additions retard the growth but not the nucleation at the grain boundary. It is concluded that elements which form stable precipitates in the grains retard the grain-boundary reaction.