Unusual size-dependent strengthening mechanisms in helium ion-irradiated immiscible coherent Cu/Co nanolayers

Prior studies on He ion irradiation-induced damage in several immiscible metallic nanolayer systems with incoherent interfaces show a prominent size effect on mitigation of radiation damage frequently. In general, the magnitude of radiation hardening and defect cluster density are both lower in smaller individual layer thickness (h) than in larger h, as interfaces can effectively reduce the density of radiation-induced defect clusters. This research shows, however, an opposite size-dependent strengthening behavior in He ion-irradiated immiscible coherent Cu/Co multilayers, i.e. films with smaller h have greater radiation hardening. Such unusual size-dependent strengthening could be explained via a transition of strengthening mechanisms from partial dislocation (before radiation) to full dislocation transmission (after radiation) across layer interfaces as a result of the formation of He bubbles at the layer interfaces. Furthermore, it is shown that, similarly to incoherent immiscible systems, a coherent interface in the immiscible system can also effectively reduce the population of radiation-induced defect clusters.