NUCLEATION AND GROWTH OF HELIUM BUBBLES IN NICKEL STUDIED BY POSITRON-ANNIHILATION SPECTROSCOPY

Positron-annihilation spectroscopy (PAS) has been used for the study of the behavior of helium in nickel. Helium has been homogeneously implanted in pure nickel by alpha-particle irradiation with use of a cyclotron. Post-implantation positron-lifetime and Doppler-broadened line-shape measurements have been carried out as a function of the isochronal annealing temperature on the samples containing 100 and 400 at. ppm of helium. The marked changes in the annihilation parameters, observed around 750 K in the helium-implanted Ni (100 at. ppm He), absent in the recovery of helium-free irradiated Ni, have been identified with bubble nucleation. In order to understand the effect of helium decoration of vacancies on the positron lifetime, ab initio theoretical calculations have been carried out as a function of the vacancy-cluster size and helium-to-vacancy ratio in the clusters. The computed positron lifetimes have been used to interpret the experimental PAS results in the bubble-nucleation stage. In the post-nucleation growth stage above 750 K, the bubble parameters, viz., bubble size and bubble concentration, have been extracted from an analysis of PAS data. An analysis of bubble growth in Ni indicates the existence of overpressurized bubbles and the implication of this result on bubble-growth mechanisms is discussed. The helium-dose dependence of the bubble parameters is also evaluated from these experimental PAS results.