Gradient structure regulated plastic deformation mechanisms in polycrystalline nanotwinned copper

Abstract

Molecular dynamics simulations are performed to investigate the effects of grain size gradient and twin thickness gradient on uniaxial tensile deformation behaviors of gradient-structured polycrystalline copper. Simulation results reveal that there exist an inverse Hall–Petch effect and gradient structure regulated plastic deformation mechanisms. That is, the average flow stress of the gradient nanograined (GNG) model or the gradient nanotwinned (GNT) model decreases with decreasing the grain size or twin boundary (TB) spacing, exhibiting the breakdown in the Hall-Petch relationship when the grain size or TB spacing is smaller than a critical size. The dominant plastic deformation mechanism is found to transform from the grain boundary (GB)-mediated one to the dislocation-based counterpart with the increase of the applied strain in the GNG model. The TB migration and annihilation dominate the plastic deformation in the grains with small TB spacing; while in the grains with large TB spacing, the dislocation multiplication and cutting across TB are mainly responsible for accommodating the deformation in the GNT model. Furthermore, the gradient distribution of strain and incompatibility of deformation induced by GB or TB gradient distribution are observed by monitoring the microstructural evolution.