Development of vacancy clusters in neutron-irradiated copper at high temperature

Pure copper was fission-neutron-irradiated at 473 and 573 K from 0.0003 to 0.14 dpa. In copper, which was irradiated at 573 K to 0.0003 dpa, the number of vacancies which were accumulated in the largest stacking fault tetrahedron (SFT) was 276, while it was 470 in the smallest voids. This is explained by a model in which at 573 K a SFT converts to a void when the number of vacancies exceeds about 400. In 573 K irradiated copper, the number of vacancies in a SFT and a void of average size increases with the neutron fluence. The number of vacancies in a void increases more rapidly than that in a SFT. The reason appears to be that small vacancy clusters relax at 573 K to a string-like cluster, move as a cluster and coalesce. Experimental results are presented which show the movement of voids. In copper which was neutron-irradiated at 573 K, the number density of SFTs and voids peaked at 0.0003 dpa and decreased with fluence. The reason appears to be a low sink efficiency of dislocations for point defect absorption at 0.0003 dpa. Due to a low sink efficiency straight extended dislocations were decorated with many interstitial clusters. After jogs are formed on dislocations by joining with grown interstitial loops, the absorption efficiency of point defects increases significantly which lowers the density of SFTs and voids with increasing of dpa.