Positron annihilation lifetime study of irradiated and deformed Ni

Isochronal annealing behaviors have been investigated for electron irradiated high-purity Ni specimens, and also for deformed Ni specimens by positron annihilation lifetime measurement, and it was found that rather short lifetimes around 130 psec, i.e., intermediate positron lifetime between lifetime at bulk and that at a single vacancy appeared at the later stage of recovering. These correspond to vacancy clusters of collapsed type structure, e.g., stacking fault tetrahedra (SFT) , vacancy type dislocation loops. Positron lifetime calculations were made for these defects and it was found that beyond a certain number of vacancies in a cluster collapsed type structures appeared and give rather short positron lifetimes which were observed in the experiments. Calculations were also made for the case of clusters on a partial dislocation line to explain the results in deformed Ni.