The effect of grain size on bubble formation and evolution in helium-irradiated Cu-Fe-Ag

Nanostructured metals are a promising candidate for future applications in irradiative environments, such as nuclear energy facilities, due to a conceivable tolerance against radiation damage. As the presence of helium irradiation is frequently unavoidable, e.g. in nuclear fusion facilities, the effect of helium on the properties of nanostructured materials is of immanent interest. In this work, ultra-fine grained (UFG; 100 nm grain size) and nanocrystalline (NC; 20 nm grain size) Cu-Fe-Ag samples have been implanted with various fluences of helium and were investigated regarding helium-induced modifications using atomic force microscopy, nanoindentation and transmission electron microscopy. While for these nanostructured materials a tolerance against radiation damage has been reported earlier, we find that the influence of helium on swelling and mechanical properties is not negligible. The increased amount of closely spaced interfaces in the NC material provides swift diffusion paths of helium, thereby facilitating bubble nucleation in the early stages of irradiation. For high fluences of helium, however, the smaller grain size and larger amount of nucleation sites in the NC composite restrict the growth of individual bubbles, which has a positive effect on swelling and counteracts mechanical property degradation compared to UFG and conventional coarse-grained materials. As such, our investigations on immiscible Cu-Fe-Ag nanocomposites pave a promising strategy for designing novel highly radiation enduring materials for irradiative environments.