Stability and mobility of defect clusters in copper under displacement cascade conditions

The recent molecular dynamics (MD) simulations have provided insights into the nature of displacement cascades, revealing defect cluster formation and their stability. In order to get direct experimental insights into the defect accumulation processes, in situ TEM observations in copper under irradiations with 100 keV C+ and 240 keV Cu+ ions at temperatures from 573 to 823 K were performed. Defect clusters produced by cascades were observed to be unstable with lifetimes of seconds, which depend on temperature, ion species and fluence. Multiple (2 or 3) defect clusters showing up their contrast in the same video frames, having a time resolution of 1/30 s, were concluded to be features related to subcascades and fast diffusion of defect clusters when located within 30 nm and from 30 to 140 nm, respectively. The detailed analysis of the defect cluster distribution shows that the direction of the fast diffusion is strongly related to crowdion directions, suggesting that the mechanism is based on motion of crowdion bundles. Instability and diffusion of defect clusters detected under ion irradiation are interpreted in terms of transformation into crowdion bundles, which is well described by MD simulations of dislocation loop stability under a compressive stress.