Positron annihilation studies of the interaction between oxygen impurities and nanovoids in neutron-irradiated vanadium

Vanadium samples with controlled oxygen impurity of up to 2460 at. ppm were irradiated with fast neutrons to a fluency of 8.3 × 10¹⁸ n cm^{- 12} at 150 °C. The irradiation-induced nanovoids and the interaction between the defects and the oxygen impurities are studied systematically by combining the positron lifetime technique, the coincidence Doppler broadening technique and the first-principles calculations. The nanovoids are seen to be decorated with the oxygen impurities, which play a significant role in the dynamic processes of the formation of the nanovoids by irradiation and their subsequent recovery by post-irradiation annealing. The decoration with oxygen impurities not only enhances the nanovoid formation but also stabilizes them during low-temperature annealing (<400 °C). The oxygen impurities dissociate from the nanovoids by annealing above 400 °C and the irradiation-induced nanovoids recover on the annealing at 650 °C.