Effects of Location of Twin Boundaries and Grain Size on Plastic Deformation of Nanocrystalline Copper

Nanocrystalline copper is considered to be a candidate for electrical contacts for dynamic systems. Both experiments and simulations show that introducing nanoscale twins into nanocrystalline copper is an effective approach to improve strength while maintaining high electrical conductivity. The present study investigated the influence of twin boundary (TB) and grain size refinement on plastic deformation in polycrystalline copper. Molecular dynamics (MD) simulation with embedded-atom method potential for copper was performed to simulate a cell of [011] textured microstructure embedded with four hexagonal grains. Simulation results showed that strength and toughness (i.e., energy per volume absorbed by system up to a certain strain in this study) of copper could be enhanced by introducing TBs within nanocrystalline grains. Nanotwins act as obstacles to dislocation motion that leads to strengthening, as well as sources of dislocation nucleation contributing to the toughness of the materials. The enhancement of the properties is apparently sensitive to the distance between TB and grain boundary (GB); i.e., it exhibits a maximum at an intermediate distance, while it decreases when TBs are far away or very close to GBs. This implies that the volume between TB and GB plays an important role on the plasticity of nanocrystalline copper. Moreover, the deformation behavior in different grains depends on their orientation with respect to the loading direction. The study has also confirmed that grain-size refinement in nanotwinned models would lead to an inverse Hall-Petch relationship.