The effect of carbon on the evolution of vacancy defects in electron-irradiated nickel studied by positron annihilation

The effect of carbon atoms on the evolution of vacancy defects in nickel doped with controlled amounts of carbon impurities under electron irradiation at 300 and 423 K was investigated using positron annihilation spectroscopy. It has been found that migrating on stage III of annealing (around 350 K) vacancies are captured by immobile carbon atoms resulting in vacancy–carbon (V–C) pair formation. It was shown that the V–C pair is highly symmetric, i.e. C atom being located near the vacancy center. This pair cannot trap a positron. The dissociation of the V–C pairs was observed after the annealing above 450 K. Realized vacancies form vacancy clusters, which become unstable at 500 K and complete anneal out to 600 K. Under irradiation at elevated temperature (423 K), at which vacancies are mobile, the variation of annihilation parameters from the bulk values are not observed in Ni–C alloy. This phenomenon was connected with the formation during irradiation of the V–C pairs, which do not bind positrons.