Dynamical process of defect clustering in Ni under the irradiation with low energy helium ions

Dynamical process of defects clustering in pure Ni under the irradiation with low energy (typically 5 keV) He ⁺ ions has been studied by in situ electron microscopy. Effects of the ion energy (0.5-20 keV), fluence, flux, depth and irradiation temperature on the formation of interstitial type dislocation loops (I-Loops) and bubbles were examined. The density of I-Loops sharply increased with the fluence, but was slightly dependent on the ion flux. It is demonstrated that the formation of I-Loops at room temperature is promoted 3-4 orders magnitude higher than that expected in the case of no assistance of helium atoms on the nucleation. These results suggest a possible nucleation mechanism where helium-vacancy complexes trap the self-interstitial atoms and act as nucleation sites of I-Loops. By irradiation with 20 keV He ⁺ ions, SFT were formed even at room temperature, coexisting with I-Loops. Bubbles are formed preferentially inside of I-Loops, and by more heavy irradiation, they coalesced or interconnected, leading to a characteristic channel structure.