A molecular dynamics study of a cascade induced irradiation creep mechanism in pure copper

Recently, in-situ TEM straining experiments on pure copper have unraveled a high stress irradiation creep mechanism. Irradiation induced unpinning of dislocations from defects has been observed and the mean pinning lifetime has been determined. In the present study, molecular dynamics simulations are performed on pure copper to investigate the impact of collision cascades on screw dislocations pinned on Frank loops under high-applied stresses at 300 K in order to further quantitatively elucidate this mechanism. The simulations indicate two possible dislocation unpinning mechanisms. Unpinning can occur through loop destruction when the cascade is generated on the pinning points of the dislocation (type 1 unpinning). Unpinning can also be triggered by the shear stresses building up around a cascade generated in front of the dislocation in the glide plane (type 2 unpinning). Type 2 unpinning generally leads to dislocation repinning on cascade residues, so that it should only marginally contribute to irradiation creep. The mean pinning lifetime due to type 1 unpinning in the conditions of the in-situ TEM experiments is derived from a simple model previously developed for zirconium, and is found in the same orders of magnitude as in the experiments.