Understanding extra strengthening in gradient nanotwinned Cu using crystal plasticity model considering dislocation types and strain gradient effect

Gradient nanotwinned (GNT) Cu, composed of various homogeneous nanotwinned (HNT) components, successfully achieves the synergistic enhancement of strength and ductility due to the combined advantages of gradient and nanotwinned structures. However, the complicated dislocation-twin boundary (TB) interactions in each HNT component and extra strengthening mechanism in GNT Cu remain elusive. In this study, a crystal plasticity model, which takes into account the reaction characteristics of various dislocation types at the TB and the extra size dependent geometrically necessary dislocations (GNDs) induced by plastic strain gradients, is developed for understanding both HNT and GNT Cu. The developed model successfully captures the mechanical behavior and microstructure evolution of HNT Cu with different TB orientations and grain sizes (and twin thicknesses). The simulation results emphasize the importance of incorporating dislocation types when describing dislocation-TB interactions. Furthermore, the intrinsic mechanism for the extra strengthening in the GNT Cu is revealed through the analysis of deformation contours and microstructural evolutions. The results show that the HNT components with the lower original strength have the higher extra back stress increment with the increase of structural gradient. This study provides valuable insights into predicting and further optimizing the strength of the GNT Cu through manipulating the gradient microstructure.