Formation and recovery of irradiation and mechanical damage in stabilized δ-plutonium alloys

Based on the results of thermal expansion measurements of aged δ-phase Pu–Ga alloys and comparisons with historical bulk density, thermal expansion, and X-ray diffraction data of δ-phase Pu–Al and Pu–Ga alloys, it is being proposed that Pu-solute atom mixed 〈1 0 0〉 dumbbell interstitials, generated by damage processes, induce volumetric swelling and impede δ → α′ Martensitic transformation in these alloys. Damage induced volumetric swelling in δ-phase Pu–Al and Pu–Ga alloys is distinct from swelling induced by He bubble formation in that it appears to saturate as a function of damage eventually reaching a constant value which is linearly dependent on solute element atomic concentration. Based on arguments offered in this work, each Pu-solute atom mixed 〈1 0 0〉 dumbbell interstitial creates a strain field of approximately 4.8 times the Pu atomic volume over some 20 Å in extent. As the interstitial concentration increases with damage, so does the volumetric swelling up to the point of saturation with a concentration of 1% of the solute atomic concentration. These interstitials appear to anneal out at temperatures of 150 to 200 °C. This relatively high thermal stability is possibly due to an altered local bonding environment for the Pu-solute atom interstitial pair and first-nearest neighbors in response to reduced coordination number and lattice strain field.