Collective grain deformation of nanocrystalline metals by molecular dynamics simulations

The collective grain movement of nanocrystalline metals and its temperature dependence are studied by using molecular dynamics simulations. First, a unit structure that consists of eight aluminium grains in the regular hexagonal shape with 5 nm grain size is prepared, and then an analysis model is made by arranging the same 144 unit structures in the two-dimensional periodicity. Thus the total number of grains is 1152. Various collective grain deformations occur at different temperatures under tensile loading. In the case of 100 K, shear bands formed by the collective grain deformation can be observed remarkably. On the other hand, in the case of 300 or 500 K, no remarkable inhomogeneous deformation such as shear bands occurs. This might be due to the different accommodation mechanism for geometrical misfits by local shear deformation at each different temperature. In order to investigate the effect of the collective grain deformation on the macro-scale mechanical properties, the stress - strain curve for the model with 144 unit structures and an averaged strain - stress curve for the 60 cases of a model with one unit structure are compared. Consequently, it is found that the inhomogeneous plastic deformation mode such as a shear band can influence the strength of nanocrystalline metals.