Single-dislocation-based strengthening mechanisms in nanoscale metallic multilayers

A breakdown from the dislocation-pile-up-based Hall-Petch model is typically observed in metallic multilayers when the layer thickness (one half of the bilayer period) is of the order of a few tens of nanometres. The multilayer strength, however, continues to increase with decreasing layer thickness to a few nanometres. A model based on the glide of single dislocations is developed to interpret the increasing strength of multilayered metals with decreasing layer thickness when the Hall Petch model is no longer applicable. The model is built on the hypothesis that plastic flow is initially confined to one layer and occurs by the motion of single 'hairpin' dislocation loops that deposit misfit-type dislocations at the interface and transfer load to the other, elastically deforming layer. The composite yield occurs when slip is eventually transmitted across the interface, overcoming an additional resistance from the interface dislocation arrays. In a lower-bound estimate, the stress for the initial glide of 'hairpin' dislocation loops, confined to one layer, is similar to the classical Orowan stress. In the upper-bound estimate, the interaction of the glide loop with the existing misfit dislocation arrays at the interface is also considered in deriving the Orowan stress. The effect of in-plane residual stresses in the layers on the Orowan stress calculation is also considered. The model predictions compare favourably with experimentally measured strengths on Cu-based multilayers. When the layer thickness is decreased to a couple of nanometres, the strength reaches a plateau and, in some cases, drops with decreasing layer thickness. The single-dislocation model developed here predicts strengthening with decreasing layer thickness and, therefore, does not explain the deformation behaviour in this regime. In the regime of several nanometres, the deformation behaviour can be explained by dislocation transmission across the interface followed by glide of loops that span several layer thicknesses.