Precipitation and irradiation damage in proton-irradiated palladium-chromium alloys

Irradiation damage of Pd-Cr alloys containing 15, 20 and 25 at % Cr was studied over the temperature range 100 to 550° C, primarily in samples irradiated to a dose of 0.7 d.p.a. The solubility limit in this range of temperatures varies from 22 to 38% Cr, and precipitation of a phase having the L1₂ crystal structure was observed in unirradiated samples of the 25% Cr alloy aged at temperatures as low as 100° C. Octahedrally shaped voids, with faces parallel to {1 1 1}, were found only in the 25% Cr alloy irradiated from 350 to 550° C, but not in the other alloys at any temperature. The undersized chromium atoms migrate to point-defect sinks during irradiation, resulting in solute segregation and, eventually, precipitation under certain conditions. The precipitation of the L1₂ phase, which is the thermodynamically stable phase at higher chromium contents, was irradiation-induced at dislocation loops, voids and grain boundaries in the more concentrated undersaturated alloys. This irradiation-induced precipitation was observed in the mid-range and surface regions of the samples containing 20 and 25% Cr, but not in those containing 15% Cr. The behaviour of these alloys is compared with Pd-Fe and Ni-Si alloys, which have also been proton-irradiated in this laboratory, and the similarities and differences are noted and discussed.