Interface structure and the inception of plasticity in Nb/NbC nanolayered composites

Molecular dynamics (MD) simulations were performed to explore the effect of interface structure on the inception of plastic deformation in Nb/NbC nanolayered composites. Using the atomistically informed Frank-Bilby method and disregistry analysis, we characterized the structure of the Nb/NbC interface, including misfit dislocations, dislocation nodes and three coherent interface structures. According to the crystallographic analysis of the interface, four possible coherent interface structures were identified. However, study of the interface energy showed that only three of these are energetically stable. After the relaxation of the interface, the unstable coherent region, which features Nb atoms in the Nb layer on the top of the Nb atoms in the NbC layer, evolves into a condensed interface dislocation node. Three stable coherent interface regions are retained in association with the formation, glide and reaction of interface misfit dislocation loops. Disregistry analysis of the Nb/NbC interface revealed that (i) all misfit dislocations are edge type, and (ii) misfit dislocations enclosing the coherent extended nodes have Burgers vectors along 〈1 1 0〉Nb. The role of the interface structure in the plastic deformation of Nb/NbC nanolayered composites was studied under two loading conditions, i.e. uniform compression and nanoindentation. Under uniform compression, lattice dislocations primarily nucleate from the condensed nodal regions where the local strains are the highest. Dislocations propagate in two {1 1 0} slip planes with the same Schmid factors. Under nanoindentation, by proper positioning of the indenter, lattice dislocations nucleate from the segments of the misfit dislocations and propagate in {1 1 2} slip planes. MD simulations also show cross-slip of lattice dislocations from {1 1 2} planes into {1 1 0} planes and the formation of vacancies as a result of climb of dislocation jogs.