Effect of grain boundary deformation on mechanical properties in nanocrystalline Cu film investigated by using phase field and molecular dynamics simulation methods

Molecular dynamics simulations are performed to study the mechanical behaviors and microstructural evolution in nanocrystalline Cu films created by the phase field model under different strain rates and temperatures. The results indicate that grain boundaries' (GBs) migration caused by shear stress difference of GBs is found in the initial deformation stage. The migration on the site with a small curvature radius of curved GBs is large due to the high stress difference. The migration process of curved GBs in the initial stage is that atoms migrate from FCC structures to GBs along the (111) surface, which is different from the mechanism of atomic shuffling for the flat GBs. Meanwhile, the initial GBs migration can make curved GBs become flat. In addition to temperature and stress difference, the hexagonal-close-packed (HCP) structures including stacking faults and twin boundaries can accelerate GBs' migration. The influence of initial GB migration on mechanical properties is achieved by changing the fraction and distribution of HCP structures. Larger initial GB migration at a higher temperature significantly reduces stress concentration on GBs, which leads to the distribution of HCP changing from the grains with large initial GB migration to other grains. Therefore, the sites of crack nucleation at conditions of low and high temperatures are different due to different magnitudes of initial GB migration.