Thermal evolution of irradiation defects in ferritic/martensitic steel during isochronal annealing

The aim of investigating the annealing effect induced by helium/hydrogen ion irradiation on the micro-defects of ferritic/martensitic (FM) steel is to gain a basic understanding on the development of fusion reactor components. In this study, 250 keV He2+ and 130 keV H+ were used for irradiation. Then, micro-defects were annealed isochronally between 423 K and 673 K, and were monitored by the slow positron beam Doppler broadening technique. The results revealed that the He ions implanted into the steels combined with vacancies to form HenVm clusters, and were likely to grow and form overpressured HenVm clusters by thermal activation. The S-W line of the irradiated specimen deviated the line segment, which indicates the formation of overpressured HenVm clusters in the He-irradiated steel. Moreover, the ΔS-E curves indicate residual vacancies that were induced by He-irradiation at 723 K and later migrated toward the bulk of the sample at 423 K. Subsequently, the vacancies tended to migrate to the sample surface as the annealing temperature increased. The S parameter of the He+H-irradiated sample was much larger than that of all other samples, owing to the synergy effect of H and He. After annealing at 423 K, the H-V clusters decomposed and the hydrogen atoms in the He+H-irradiated sample escaped from the surface leaving a large number of vacancies.