Size effects in Cu50Zr50 metallic glass films revealed by molecular dynamics simulations

摘要

The mechanical behavior of metallic glasses shows similar size dependences as fine grained crystals, yet the strengthening mechanism is quite different. Tensile deformations of Cu50Zr50 metallic glass thin films by molecular dynamics simulation reveal that surface effects play an important role in affecting the deformation mode and ductility of the films. The presence of surfaces promotes the activation of shear transformation zones upon deformation, facilitating homogeneous plastic flow. While surface relaxation helps to annihilate the excess free volume generated in the film, and in turn retards the nucleation and/or propagation of shear bands, and consequently enhances the ductility of the films. The surface area to volume ratio is larger for thinner films and therefore the annihilation effect overwhelms the generation effect due to deformation, enabling the "smaller is stronger" phenomenon. Meanwhile, a transition of deformation mode from shear fracture dominated to mixed mode and then to homogeneous deformation is observed with the decreasing of film thickness. The findings suggest that the ductility and strength of metallic glass samples could be improved effectively by reducing the sample size or increasing its surface area to volume ratio. (C) 2016 Elsevier B.V. All rights reserved.