Influence of image forces on the size dependence of multilayer shear strength

The dependence of the multilayer shear strength on layer thickness is investigated using a discrete dislocation pileup model accounting for image force effects. A three-layer anisotropic model, in contrast to the bilayer model used in most previous works, allows for the explicit consideration of the layer thickness and three interfaces. The image forces are calculated using an image decomposition technique. The results show that attractive image forces are sufficiently large to cause a deviation from the Hall–Petch relation as well as a peak in the relation as the thickness decreases below ∼40 nm. Furthermore, faceting of the predicted strength–thickness relation can be traced to differences between nucleation-controlled and load-controlled yielding. The predicted results agree broadly with experimental data for metallic multilayers over three length scales from 102 nm down to 100 nm.