THERMAL EVOLUTION OF ALPHA-INDUCED DEFECTS AND HELIUM BUBBLES IN TANTALUM STUDIED BY POSITRON-ANNIHILATION

Helium-implanted tantalum, subjected to isochronal annealing treatment , has been studied using positron annihilation technique. The defect e volution prior to the bubble nucleation stage consists of the processe s of coarsening of vacancy clusters to form voids and stabilization of helium-filled voids up to about 900 degrees C. The helium decorating the loops gets dissociated above 800 degrees C and the defects anneal out upon further annealing up to 1000 degrees C. Between 800 degrees C and 1000 degrees C, most of the helium-free voids anneal out but the rest, mainly helium-decorated voids, transform into densely filled hel ium bubbles above 950 degrees C. The properties of these helium bubble s have been studied quantitatively from an analysis based on the posit ron surface state model, which indicated the presence of over-pressuri sed helium bubbles. In the post-nucleation stage the bubble radius inc reases while the bubble concentration decreases with temperature, whic h indicates the bubble growth.