Conversion of stacking fault tetrahedra to bubbles in dual (Kr, He)-beam irradiated copper

Stacking fault tetrahedrons (SFTs) are commonly observed in irradiated face-centered-cubic metals with low-to-medium stacking fault energies. Several mechanisms were previously proposed for the removal of SFTs in irradiated solids, including high temperature annealing, interactions with interstitial atoms, dislocations, and twin boundaries, and transformation to dislocation loop under compressive stress. We propose a previously unreported mechanism for the removal of SFT in irradiated copper, supported by experiments and atomistic simulations. In situ experiments showed that helium bubble density increased at the expense of SFT density following the initial phase of dual 1 MeV Kr/12 keV He ion irradiation, suggesting a possible conversion of SFTs to helium bubbles. Atomistic simulations of the interactions of helium atoms with SFTs confirmed this possibility and revealed the collective effects of helium-induced shear stress that deformed the atomic planes of Cu leading to the destruction of the SFT and leaving behind helium atoms in vacancy clusters (bubbles).