Grain-size dependence of the relationship between intergranular and intragranular deformation of nanocrystalline Al by molecular dynamics simulations

The strength of nanocrystalline aluminum has been studied using molecular dynamics simulation. Nanocrystalline models consisting of hexagonal grains with grain size $d$ between 5 nm and 80 nm are deformed by the application of tension. A transition from grain-size hardening to grain-size softening can be observed in the region where $d\ensuremath{\approx}30\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, which is the optimum grain size for strength. In the grain-size hardening region, nanocrystalline models primarily deform by intragranular deformation. Consequently, a pile-up of dislocations can be observed. When the grain size becomes less than 30 nm, where the thickness of the grain boundaries cannot be neglected in comparison to the grain sizes, the dominant deformation mechanism of nanocrystalline metals is intergranular deformation by grain boundary sliding. Further, geometrical misfits by grain boundary sliding are accommodated by the grain rotation mechanism. Moreover, cooperative grain boundary sliding occurs in the 5 nm model. The optimum grain size is controlled by the relationship between resistance to intergranular deformation by grain boundary processes and intragranular deformation resisted by the grain boundary. Therefore, the primary role of the grain boundary changes in the region where the optimum grain size is observed.