Microstructure evolution in helium implanted self-irradiated tungsten annealed at 1700 K studied by TEM

Abstract Tungsten targets have been self-damaged with 20 MeV W6+ ions followed by decoration of defects with deuterium and 500 keV helium implantation. Such treatment creates a significant amount of irradiation-induced defects which depend on the distance from the surface. As a final step, selected samples were annealed at 1700 K for 30 min to study defect evolution at high temperature. Detailed TEM analysis, supported with quantification of microstructure, provided an insight into the effect of helium implantation on the previously established dislocation structure and its thermal stability. It was shown that helium is implanted into the material down to approximately 900 nm from the surface and creates a locally enhanced dislocation density which remained stable even after high temperature annealing. This phenomenon is opposite to annihilation of dislocations observed in the sample without helium treatment. At the same time, helium implanted zone is the source of internal stress fields which cover the whole irradiated depth. Thermal treatment releases lattice from this stress. The possible reasons of these occurrences are discussed in the light of helium interactions with vacancies and self-interstitials in the presence of dislocations which directly impacts hydrogen isotopes retention.