Microstructural evolution and phase stability of Cu/Nb nanolaminates subjected to severe plastic deformation by high pressure torsion

Nanostructured metals and alloys have become attractive candidates for advanced material applications due to their potential to exhibit superior material properties, honed from the existence of large density of interfaces and their interactions with dislocations and defects. Severe plastic deformation has become one of the most promising techniques for producing nanostructured materials in large amounts, and, in some cases, in the bulk shape. This dissertation focuses on providing a better understanding of deformation mechanisms imposed via severe shear deformation to Cu-Nb nanolaminates fabricated by accumulative roll bonding. Severe plastic deformation was applied in a controlled way using high pressure torsion, reaching very large strains, allowing to investigate microstructural evolutions and phase stability as a function to strain, and to reach true steady states. The results show that the two dimensional layered microstructure was progressively replaced by a three-dimensional Cu-Nb nanocomposite. This structure remained stable with respect to grain size, morphology and crystallographic texture, and was comprised of biconnected Cu-rich and Nb-rich regions, with a remarkably small coexistence scale of 5-10 nm. In a second step, the effect of the initial layer thickness on the microstructures observed in ARB composites after HPT deformation was investigated by comparing results obtained for ARB materials with nominal layer thickness of 18 nm, 200 nm, and 2 µm For the 200 nm ARB composites, strains exceeding 3,400 were required for destabilization of layers. At a strain of 10,000, the microstructure has fully transformed to 3-D microstructure, similar to the ones observed for 18 nm and 2 μm composites. The 2 μm ARB composites subjected to high pressure torsion showed the presence of microscopic features like folds and swirls, previously observed on the 18 nm ARB composites, and complete destabilization of the layered structure was observed at a shear strain of ~4,200, suggesting that in the presence of folds and swirls, the length scale dependence of the strain required to reach steady state is compatible with a super-diffusive mechanism