SUMMARY

Nanostructured metals usually exhibit ultra-high strength but low ductility. However, recent experiments revealed the extraordinary intrinsic tensile ductility in nano-grained copper with gradient grain sizes, so-called gradient nano-grained copper. To provide a fundamental understanding of the mechanics and physical mechanisms governing the strength and ductility in gradient nano-grained metals, we will conduct both atomistic and crystal plasticity modeling studies of gradient nano-grained copper in this thesis. We will develop a Voronoi tessellation-based geometrical model to build the nano-grained structures with a flexible control of gradient size distributions. Both molecular dynamics and crystal plasticity finite element simulations will be performed to investigate how the dislocation and grain boundary mediated deformation mechanisms affect the grain size and its gradient effects on strength and ductility. The uncertainties arising from grain size and orientation distributions will be studied and quantified.
Moreover, we will also study the unit processes of plastic deformation, including dislocation slip, deformation twinning and dislocation-defect interaction. Strength and energy barrier of these unit processes will be determined using molecular dynamics and nudged elastic band methods. The uncertainty analysis of the unit processes will be conducted with a parametric study. This thesis work will advance the fundamental understanding of strength and ductility as well as unit deformation mechanisms of nanostructured metals. Furthermore, our uncertainty study has important implications for the design and fabrication of nanostructured materials.