Effect of purity on the vacancy defects induced in self–irradiated tungsten: A combination of PAS and TEM

Vacancy defects in tungsten induced by self-ion irradiation were characterized by positron annihilation spectroscopy (PAS) and transmission electron microscopy (TEM). Taking advantage of their complementarity, defects ranging from single vacancies up to vacancy clusters were detected, and the effects of sample purity (99.95 wt.% and 99.9999 wt.%) on their formation and evolution under low damage dose (0.01 to 0.02 dpa) irradiation with 20 and 1.2 MeV-W ions were studied at room temperature (RT), 500 and 700 °C respectively. When irradiation was performed at RT, PAS probed mainly single vacancies. At high irradiation temperatures, larger vacancy clusters were detected by both techniques. Both PAS and TEM revealed larger vacancy clusters in the purest samples after irradiation at 500 and 700 °C. A significant difference in the evolution of vacancy-type defects related to sample purity was observed. In view of the properties (migration, trapping) of some light-element impurities (LEs), present at high concentrations, determined by first-principles calculation, it is reasonable to assign the effect of purity during the irradiation response to the formation of vacancy-impurity complexes, which is critical for the understanding of the behavior of tungsten in the future fusion reactor.