A comprehensive atomistic investigation on the cascade induced helium bubble motion in bcc iron for neutron irradiated RAFM steels

Cascade induced by the primary knock-on atom could significantly influence the evolution of helium bubbles in RAFM steels. In this work, the helium bubble migration and helium bubbles coalescence under cascade overlapping, i.e., when the helium bubble was overlapped by the thermal spike of one cascade, in bcc Fe was investigated by molecular dynamics simulation. The influence of He-to-vacancy (He/V) ratio as well as cascade energy, temperature, bubble size and bubbles distance were investigated based on the statistical results of 4700 cases. A method which could quantitatively estimate the overlapping extent between the cascade and helium bubbles was put forward to confirm that the cascade did overlap with the helium bubble in all cases. It was found that under cascade overlapping, the helium bubble tends to migrate toward the thermal-vacancy-rich center. The mechanisms of migration change from vacancy diffusion to He atoms migration with the increase of the He/V ratio, and the corresponding migration distance could decrease one order of magnitude. The cascade induced coalescence of two close helium bubbles is associated with the synergy effect of migration, dissociation and joint strain field of the two bubbles. Especially, it was found that the joint strain field of two close helium bubbles could significantly promote the bubbles’ coalescence by fostering the two bubbles’ directional migration towards each other under cascade overlapping. It was also found that the stable He/V ratio of helium bubble under cascade overlapping is around 0.5.