Density of grain boundaries and plasticity size effects: A discrete dislocation dynamics study

Discrete dislocation dynamics simulations are carried out to systematically investigate the microstructural and geometrical size dependence of films under tension that have a varying number of grains through their thickness. By varying film thickness, grain size and aspect ratio, more insight is gained into the competition between grain boundary hardening and film thickness effects. This provides a seamless link between previous dislocation plasticity studies and qualitative agreement with experimental data. In the simulations, plasticity arises from the collective motion of discrete dislocations of edge character. Their dynamics is incorporated through constitutive rules for nucleation, glide, pinning and annihilation. Grain boundaries are treated as impenetrable to dislocation motion. The numerical results show that the grain size dependence of yield in thin films as well as in bulk polycrystals is controlled by the density of grain boundaries.