Helium-induced defect clustering and thermal evolution in V-4Cr-4Ti: an alloy for fusion reactor applications

The V-4Cr-4Ti alloy is a promising candidate for the first-wall and blanket structural materials in future fusion reactors. In this study, a V-4Cr-4Ti alloy sample was irradiated at room temperature with 130 keV He⁺ ions to a dose of 1 × 10¹⁷ ions/cm² and depth-resolved defect studies under isochronal annealing were conducted using variable low-energy positron beam-based Doppler broadening spectroscopy. Density functional theory calculations were used to identify the energetically favourable configurations of helium-vacancy (He_mV_n) clusters in vanadium. Helium was found to prefer tetrahedral interstitial sites in the absence of vacancies, and octahedral sites when vacancies are present. Positron annihilation parameters were computed for these defect configurations and compared to experimental results. The irradiation produced vacancy-rich He_mV_n clusters near the surface, and helium-rich clusters deeper within the material. The He/V ratio of the clusters increases with annealing up to 773 K, beyond which the growth of larger He_mV_n clusters began through the absorption of thermal vacancies, which continued until 1273 K. These larger clusters began to dissociate by emitting vacancies at 1373 K. Notably, the onset of He_mV_n cluster growth in V-4Cr-4Ti occurs at 873 K, about 100 K higher than in Reduced Activation Ferritic/Martensitic (RAFM) steel, another fusion reactor material candidate.