Atomistic simulations of dislocation-interface interactions in the Cu-Ni multilayer system

Experimental results show that a nanolayered composite structure made of two kinds of metals strengthens dramatically as the layer thickness is reduced. In epitaxial systems, this strengthening has been attributed to the modulus, lattice parameter, gamma surface and slip-plane mismatches between adjacent layers. The modulus mismatch (the Koehler barrier) introduces a force between a dislocation and its image in the interface. The lattice parameter mismatch generates oscillating coherency stresses and van der Merwe misfit dislocations at or near the interfaces, which interact with mobile dislocations. The gamma surface (chemical) mismatch introduces a localized force on gliding dislocations due to cure energy changes at or near the interfaces. Slip-plane misorientations across the interfaces require mobile screw dislocations to cross-slip for slip transmission and other dislocations to leave a difference dislocation at the interface. In this paper, atomistic simulations using the embedded-atom method are used to study the four components of dislocation-interface interactions in epitaxial Cu-Ni multilayers in a systematic fashion. The interaction of misfit dislocations with mobile dislocations is modelled using continuum theory. In thick Cu-Ni bilayers, the Koehler barrier is almost independent of interface orientation and dislocation character and is equal to 0.01 mu-0.015 mu but, when the layer thickness is comparable with the core width of a dislocation, the Koehler barrier falls rapidly (from 0.01 mu at a wavelength of 10 nm to 0.004 mu at 1.75 nm). This behaviour is in accordance with available experimental observations in the literature on the yield of epitaxial Cu-Ni multilayered systems. The gamma surface mismatch or chemical strengthening component of the blocking strength of Cu-Ni interfaces to (a/2)[110] screw dislocations is 0.003 mu, a factor of three lower than the Koehler stress. Coherency stresses, apart from exerting direct forces on dislocations, alter the barrier strengths by three mechanisms: firstly, they reduce the density of van der Merwe misfit dislocations, secondly, they enhance the Koehler barrier by altering the elastic constants of both Cu and Ni and, thirdly, non-glide stress components change the core structure of gliding dislocations, thereby altering the Koehler barrier. Overall, the barrier strength of (111) interfaces is independent of the wavelength of the multilayer and about 0.02 mu up to the wavelength of lambda(c), the coherence wavelength limit. At Cu(001)-Ni(001) interfaces the total barrier strength decreases from a value of 0.02 mu at long wavelengths (lambda approximate to infinity) to about 0.01 mu at lambda = lambda(c), as considered by Rao et al. in 1995 in their yield stress model for Cu-Ni multilayered structures. Slip-plane misorientations provide powerful barriers to slip transmission. Even at a (111) twinned interface in a coherent Cu-Ni multilayer, screw dislocations cross-slip on to the interface rather than into Ni because the stacking-fault energy at the interface is lower than in Ni. The blocking strength of the same interface to 60 degrees dislocations (which must leave a step and a residual dislocation in the boundary) is very large, 0.03-0.04 mu.