It was recalled that experimental results had shown that a nanolayered composite structure which was made of 2 types of metal was markedly strengthened as the layer-thickness was reduced. In epitaxial systems, this strengthening was attributed to modulus, lattice-parameter, γ-surface and slip-plane mismatches between the adjacent layers. The modulus mismatch (Koehler barrier) introduced a force between a dislocation, and its image in the interface. The lattice-parameter mismatch generated oscillating coherency stresses and van der Merwe misfit dislocations at, or near to, the interfaces. These then interacted with mobile dislocations. The γ-surface (chemical) mismatch produced a localized force on gliding dislocations due to core-energy changes at, or near to, the interfaces. Slip-plane misorientations across the interface required the cross-slip of mobile screw dislocations, for slip transmission, and the presence of other dislocations to leave a difference dislocation at the interface. Atomistic simulations, using the embedded-atom method, were used here to study systematically the 4 components of dislocation/interface interaction in epitaxial Cu-Ni multi-layers. The interaction of misfit dislocations with mobile dislocations was modelled by using continuum theory. In the case of thick Cu-Ni bi-layers, the Koehler barrier was almost independent of the interface orientation and dislocation type, and was equal to 0.01 and 0.015μ. But when the layer-thickness was comparable to the core width of a dislocation, the Koehler barrier fell rapidly; from 0.01μ at a wavelength of 10nm to 0.004μ at 1.75nm. This behaviour was in accord with available experimental data on the yield point of epitaxial Cu-Ni multi-layered systems. The γ-surface mismatch, or chemical strengthening component, of the blocking strength of Cu-Ni interfaces to (a/2)<110> screw dislocations was equal to 0.003μ. This was a factor of 3 lower than the Koehler stress. Coherency stresses, as well as exerting direct forces upon dislocations, altered the barrier strengths via 3 mechanisms. Firstly, they reduced the density of van der Merwe misfit dislocations. Secondly, they raised the Koehler barrier by altering the elastic constants of both Cu and Ni. Thirdly, non-glide stress components changed the core structure of the gliding dislocations and thus altered the Koehler barrier. Overall, the barrier strength of (111) interfaces was independent of the wavelength of the multi-layer, and was equal to about 0.02μ up to the coherence wavelength limit. In the case of Cu(001)/Ni(001) interfaces, the total barrier strength decreased from a value of 0.02μ at long wavelengths, to about 0.01μ at the coherence wavelength limit. It was noted that slip-plane misorientations were strong barriers to slip transmission.

Atomistic Simulations of Dislocation-Interface Interactions in the Cu-Ni Multilayer System. S.I.Rao, P.M.Hazzledine: Philosophical Magazine A, 2000, 80[9], 2011-40