Radiation damage in nanocrystalline Ni under irradiation studied using positron annihilation spectroscopy

We studied the grain size dependence of the defect evolution that occurred in a nanocrystalline (NC) Ni and its coarse-grained counterpart under irradiation. The vacancy defects produced during irradiation were investigated via in situ observations of the positron annihilation Doppler broadening spectra. The annihilation line-shape parameter S was measured under beam-on (during irradiation) and beam-off (after irradiation) conditions. We found that different variations in S were observed depending on the grain size; for the coarse-grained specimen, the S measured under beam-on conditions was larger than that observed under beam-off conditions, while for the NC specimen, the S value remained unchanged under beam-on and beam-off conditions. The former result indicated that the vacancy concentration was enhanced during irradiation, due to transient vacancy production. The latter result may imply that defect accumulation was strongly suppressed under irradiation. We also studied the flux dependence of the radiation damage effects for the NC specimen, using high-flux He ions in the flux range of 1.2 × 1013–6 × 1014 ions/(cm2 s). A slight flux dependence was observed, which was due to microstructural changes at the intersections of the crystallite interfaces under the highest flux irradiation studied.