Planar-to-wavy transition of Cu-Ag nanolayered metals: a precursor mechanism to twinning

The interface-mediated plastic deformation mechanisms of a semi-coherent Cu-Ag bimetal nanolayered structure subjected to out-of-plane tension are characterized by molecular dynamics simulations. Results show that the initially planar Cu-Ag nanolayers abruptly become wavy at a critical tensile strain. This planar-to-wavy interlayer transition is facilitated by the low shear resistance of the Cu-Ag interlayer interface, which slides to accommodate the out-of-plane deformation. The process redistributes misfit dislocations along the interface to reduce the bending energy of the wavy structure. High stress concentrations subsequently develop at the summits and valleys of the wavy Cu-Ag interlayer interfaces, from which micro-twinning partials are emitted. These results demonstrate that the wavelength of the wavy Cu-Ag nanolayer structure forms a critical length scale for the localization of spatially periodic defect sources for twin nucleation. This planar-to-wavy interlayer transition mechanism is only activated in nanolayered metals with interfaces that are amenable to sliding prior to twin or dislocation emissions.