Hydrogen solubility in Pd/Al2O3 composites prepared by internal oxidation of Pd-Al alloys

Internally oxidized Pd-Al alloys have been studied at 323 K using p(H2)-composition isotherms. Internal oxidation of Pd0.97Al0.03 alloys results in alumina precipitates within an essentially Pd matrix, i.e. internal oxidation results in a Pd/Al2O3 composite. The dilute hydrogen solubilities (323 K) in the composite are enhanced compared to annealed, pure Pd. Solubility enhancements have been measured after oxidations at 983 K, 1073 K, 1173 K and 1273 K for different times. Thermal residual stresses develop when the internally oxidized alloys are cooled from the high oxidation temperatures. The solubility enhancements are attributed to the presence of these residual stresses about the ceramic particles. The solubility enhancements for completely internally oxidized alloys are greater after lower, rather than higher, oxidation temperatures. Transmission electron microscopy indicates that alumina precipitates are nanometer-sized after oxidation al the lower temperatures and coherent with the Pd matrix. After oxidation at 1273 K, they precipitate as triangular prisms with their lengths being much greater than their sides while after oxidation at 1073 K, their sizes are smaller and their lengths are similar to their sides. Theoretical analysis of solubility enhancements about spherical and cylindrical precipitates reveal that the thermal stresses about the former are greater. The results of the analysis are consistent with the thermal residual stresses as the principal source of the experimental solubility enhancements. (C) 1997 Acta Metallurgica Inc.