The Impact of Cascade Overlap on the Long-Term Evolution of Helium Bubbles in Ferritic/Martensitic Steels

A comprehensive investigation into the evolution mechanisms of helium bubbles under irradiation is critical for understanding the swelling and embrittlement effects induced by helium bubbles in reduced activation ferritic/ martensitic (RAFM) steels. New findings have been made regarding the role of cascade overlap in the long-term evolution of helium bubbles in RAFM steels under irradiation, by combining Cluster Dynamics (CD) and Molecular Dynamics (MD) statistical trends. The Transmission Electron Microscopy (TEM) observations have been compared with the CD simulations, with the simulations including cascade overlap showing better consistency with the TEM observations. A comparative analysis of the simulation results from the CD model with helium bubble cascade overlap and the CD model without cascade overlap revealed that the cascade overlap significantly changes the size-density distribution of helium bubbles, the helium-to-vacancy (He/V) ratio, and helium bubble pressure. The atomically small helium bubbles initiating nucleation undergo complete dissolution following cascade overlap, which is a critical factor in reducing helium bubbles density. The current simulation results suggest that helium bubbles undergoing cascade overlap are more likely to stably nucleate through a stable configuration where the He/V ratio remains unchanged after multiple cascade overlaps and helium bubbles with a high He/V ratio in their vicinity, which alter the He/V ratio and pressure state of the helium bubbles. Cascade overlap leads to an elevation in vacancy and interstitial helium concentrations, and the formation of high He/V ratio helium bubbles, which together contribute to the expansion of helium bubble size.