Investigation of nano-bubbles formation as a function of helium implantation rate in molybdenum

Understanding the influence of helium (He) implantation rate on defect evolution in structural materials is critical for advancing radiation-resistant materials for nuclear applications. In this study, we systematically investigate the effect of He implantation rate on the formation and evolution of nano-bubbles, subsurface fractures, and surface blistering in polycrystalline molybdenum (Mo) at room temperature. Using a combination of Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD), we analyze Mo samples implanted with He at two distinct rates across a range of doses. Our results reveal that higher implantation rates lead to lower densities of He nano-bubbles, wider and deeper projection ranges, and the development of macro-strains, while lower rates result in higher bubble densities and micro-strain formation. These effects are explained by increased diffusion of He atoms, through RED – Radiation Enhanced Diffusion - during the higher rate implantations. Furthermore, we propose a novel experimental approach to estimate the annealing time following a collision cascade event, by tracking the effect of implantations at different rates on the target. Through this approach we estimate that the recovery/annealing duration after He collision exceeds 45 ns, which is significantly longer than molecular dynamics simulations predictions. • Higher rates lead to wider projection zone and macrostrain formation. • Lower rates result in denser bubbles and microstrain development. • Blistering threshold in Mo lies between 5×10¹⁷ and 7.5×10¹⁷ He/cm². • The effects are explained by increased He diffusion at higher implantation rate. • Estimated annealing time post-cascade exceeds 45 nanoseconds.