Grain and interface boundaries governed strengthening mechanisms in metallic multilayers

To tailor the mechanical properties of an engineering material, manipulating its internal boundaries such as grain/interface boundaries (GBs/IBs) is a standard practice. Upon reevaluating the hardness of Cu/Al, Cu/W and other bimetallic multilayer systems, we demonstrated that apart from IBs, GBs also contribute to the strengthening process of polycrystalline multilayers. By analyzing in detail the coupling and competing effects between IBs and GBs, we revealed experimentally that the strength contributed from incoherent IBs in face-centered cubic/body-centered cubic (fcc/bcc) multilayer systems is lower than that from bcc GBs and fcc/fcc IBs. In light of existing data and theory, interface shear was proposed as the main mechanism, providing reasonable explanation of the “weakening effects” of incoherent IBs.