  We used nonequilibrium molecular dynamics simulations to study the shear
deformation of metallic composites composed of alternating layers of Cu and Au.
Our simulations reveal the formation of "vortices" or "swirls" if the
bimaterial interfaces are atomically rough and if none of the {111} planes that
accommodate slip in fcc materials is exactly parallel to this interface. We
trace the formation of these patterns back to grain rotation, induced by
hindering dislocations from crossing the bimaterial interface. The instability
is accompanied by shear-softening of the material. These calculations shed new
light on recent observations of pattern formation in plastic flow, mechanical
mixing of materials and the common formation of a tribomutation layer in
tribologically loaded systems.
