Shear Localization in the Metallic Nanolayered Composites

In this dissertation, the influences of layer thickness (h), interface orientation relationship (OR) and dislocation slip activities on shear band (SB) formation mechanisms was investigated by means of molecular dynamics (MD) simulations. The effect of h and dislocation slip systems on the shear localization in Cu-FCC/Nb-BCC MNCs was studied. The strain softening observed in those samples was triggered by the SB formation. The microstructure evolutions and dislocation slips revealed that the unsymmetrical dislocation transmission across the interface induces the shear localization and promotes the SB formation. The quantitative analysis of the change in the separation distance of initially nearest neighbor atoms suggested that the plastic strain mainly comes from the interface sliding within the SB in large strains. Plastic deformation and mechanisms of SB behaviors of Cu/Au, Cu/Ag, Cu/Al and Cu/Ni MNCs subjected to the uniaxial compression loading were studied by MD. Results indicated that SB behaviors are associated with the layer thickness, stacking fault energy and interface structures. The SBs were only observed in Cu/Au and Cu/Ag MNCs, and the onset strain of SB behaviors increased with the increasing layer thickness. In comparison, no SB was observed in the Cu/Al and Cu/Ni MNCs. For the Cu/Au and Cu/Ag MNCs, deformation twinning was observed due to Shockley partial dislocations intersections, which could create steps within interfaces, meanwhile the SB was formed along the deformation twin plane direction. For the Cu/Al and Cu/Ni MNCs, many different dislocations slip systems could be activated during plastic deformation, leading to the relatively uniform deformation. The influence of thick three-dimension (3D) interface on the SB behaviors of Cu/Nb under compressive loading was studied using MD simulations. The 3D interface in Cu/Nb could weaken slip localization and prevent shear banding to some extent by changing dislocation-interface interactions. We analyzed dislocation slip activities, suggesting that 3D Cu/Nb has a relatively uniform dislocation slips in all the (111) planes, whereas 2D Cu/Nb has more strain contribution by dislocation slips in one plane leading to shear banding. Deformation twinning dominates the plasticity of Nb layers in 3D Cu/Nb, which prevents shear banding formation