Evolution of spherical nanovoids within copper polycrystals during plastic straining: Atomistic investigation

The evolution, growth and coalescence, of nanovoids play a significant role in the plasticity of nanocrystalline metallic materials. Previous atomistic studies of void growth in face centered copper, and other metallic materials, have considered voids in single crystals, and those located at grain boundaries in polycrystals. The evolution of a spherical nanovoid in copper polycrystals under uniaxial tension is the focus of this atomistic investigation. In this paper, a critical stress based criterion is proposed for emission of dislocations from spherical void surfaces. The preference of dislocation source, GBs or spherical void surfaces, is determined by the relative magnitude of the critical stresses to emit dislocations from GBs or spherical void surfaces. The criterion reveals that there exists a grain size dependent critical void diameter at which the dislocation emission transits from GBs to spherical void surfaces as the spherical void grows. Simulation results show that, in intragranular voided models, the critical void diameter is 13 nm for simulation models with a 16.32 nm grain size and is 5.5 nm for simulation models with a 6.92 nm grain size. The critical void diameters revealed by simulations are in agreement with that obtained by the criterion. In addition, the simulation results indicate that the Gurson's model may be extended to predict the global yield conditions for simulation models with intragranular spherical voids. In the simulation models with intergranular voids, the spherical void continuously grows due to the initial dislocation emission from the intersections of GBs and spherical void surfaces.