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

Abstract 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.