On the strengthening effects of interfaces in multilayer fcc metallic composites

The slip behaviour in coherent and semicoherent metallic bilayer composites is examined by atomic simulation in the Cu/Ni and Cu/Ag systems. The coherent interface in Cu/Ni, although energetically unfavourable relative to the semicoherent interface in thick layers, reveals several interesting phenomena. Linear elastic predictions of lattice strains to achieve coherency (removing the 2.7% lattice mismatch) are found not to satisfy equilibrium. The cause is nonlinearity in the elastic response, The application of stresses needed for glide dislocations to cross the interface or to escape from the interface exacerbates the nonlinearities in the elastic response of the system. Koehler forces, arising from elastic mismatch. are in some cases observed to have the wrong sign relative to linear elastic predictions. Core structures of misfit dislocations in semicoherent interfaces are observed to be quite different in the cube-on-cube oriented Cu/Ni and Cu/Ag systems with interfaces parallel to (010). In the former case, the (a/2)(110) misfit dislocations have very narrow cores in the plane of the interface but dissociate into Lomer-Cottrell locks out of the interface towards the Cu side. The dissociation is enhanced by the application of tensile stresses and can lead to reactions that form continuous stacking-fault structures. Such structures are shown to be potent barriers to slip. The stability of such structures are analysed and, within the approximations used, we find that such structures may be more stable than the usual two-dimensional flat grid of misfit dislocations. The misfit dislocations at Cu-Ag interfaces, on the other hand. are wide and so fairly mobile in the interface plane. Reactions between misfit dislocations and glide dislocations are discussed.