High-temperature tensile and creep behavior of Cu–Nb composites: A discrete dislocation plasticity investigation

Abstract

Tensile and creep responses of Cu–Nb composites are predicted at 400°C using a high-temperature discrete dislocation plasticity (DDP) framework where the multilayer composites are idealized as two-dimensional unit cells with periodic boundary conditions. Considering the fact that the interfaces in multilayer composites dominate their plastic response, bi-material interfaces are treated here so as to store lattice dislocations via formation of steps at the interfaces. While dislocations in the Nb layer are assumed to move by glide-only at 400°C, they move by a combination of glide and climb in the Cu layer at this temperature. Material parameters for the layers are obtained at 400°C by fitting the DDP predictions of the tensile response of Cu–Nb composites to the experimental data corresponding to the composites with the layer thickness in the range 65nm≤h≤2μm. Using these material parameters, the framework enables quantitative predictions of the creep responses of these composites at 400°C, which are in very good agreement with the available experimental data. Investigations of the results demonstrate that: (i) the creep response of Cu–Nb composites, in line with observations, is dependent on the laminate length scale h as well as on the stress indicating that the creep behavior is dislocation dominated; (ii) the average stresses within the layers remain almost unchanged (with some fluctuations) during the time history indicating that there is a limited load transfer between the Nb and Cu layers during the creep process; and (iii) the dislocation and step densities in the Cu layers are higher than those in the Nb layers implying that the interfaces interact with a high flux of climbing dislocations from the Cu layers. Predictions of the deformed shapes of the Cu–Nb composites including the deformed configurations of the interfaces imply that annihilation of climbing Cu dislocations at the interfaces is the dominant interaction mechanism that occurs at the interfaces. This resembles a recovery mechanism in the bi-material interfaces that facilitates dislocation generation and movement in the Cu layers and thus enhances the creep strains.