Bubbles formation in helium ion irradiated Cu/W multilayer nanocomposites: Effects on structure and mechanical properties

This study investigates the effects of He bubbles on structural and mechanical properties of sputter-deposited Cu/W multilayers. A multilayer with a periodicity of 10 nm was deposited and subjected to helium ion irradiation with two different fluences. He bubbles formed mostly in Cu layers and their distribution was affected by He concentration and radiation damage. According to SRIM calculations, in low He concentration regions bubbles formed mostly along interfaces, while more homogeneously distributed bubbles were found in Cu layers and along columnar grain boundaries in higher He concentration regions. We suggest that the capability of interfaces to annihilate point defects is weakened by the He bubbles shielding effect. Nanoindentation tests revealed a hardness decrease amounting to ∼0.5 and ∼1 GPa for low and high fluences, respectively. The observed softening effect is attributed to He storage-induced changes in residual stresses and columnar grain boundary/interfacial sliding facilitated by He bubbles.