Processing, microstructure, mechanical behavior and deformation mechanisms of bulk nanostructured copper and copper alloys

Bulk nanostructured (NS) metals, with structural units falling in the nanometer range, have emerged as a new class of materials. However, fabrication of artifact-free bulk NS metals with relatively large sample size still poses a challenge, the mechanical behavior needs to be further improved to achieve better combination of strength and ductility, and the fundamental understanding requires to be enhanced of the relationship between processing, microstructure and mechanical behavior. This dissertation selected Cu and Cu alloys as model materials to study processing, microstructure, mechanical behavior and deformation mechanisms of bulk NS metals. Bulk NS Cu and Cu alloys were fabricated by equal channel angular pressing (ECAP), spark plasma sintering (SPS) of cryomilled powders, or by high-pressure torsion (HPT) of coarse-grained or cryomilled powders, in efforts to simultaneously achieve high strength and satisfactory ductility, and to deepen our fundamental understanding of the microstructure and deformation mechanisms that govern the mechanical behavior of bulk NS metals.;The temperature at which ECAP is conducted was found to have significant effect on microstructure evolution and mechanical behavior of ultrafine-grained Cu processed by ECAP. A superior combination of strength and ductility was achieved when ECAP was performed at 523 K where discontinuous dynamic recrystallization induces a high fraction of equiaxed grains with a very low dislocation density. During SPS of cryomilled nanocrystalline (nc) Cu powders, the presence of very small amounts of O and N in the powders, which were introduced during cryomilling and handling, and the change of these impurities during SPS were demonstrated to significantly influence the densification response. A bulk NS Cu-Zn-Al alloy with ultrahigh strength was fabricated via SPS of cryomilled powders. The microstructure of the alloy was thoroughly characterized, and various strengthening contributions were quantified, including grain boundary, twin boundary, second-phase particle, dislocation and solid-solution strengthening, and the calculated overall strength achieved a reasonable agreement with the experimental strength. The twins in the cryomilled Cu-Zn-Al powders and sintered bulk samples were carefully investigated, in an effort to provide insight into the mechanisms governing the evolution of twins during SPS, including grain boundary migration, twin boundary migration, recrystallization and detwinning. HPT was applied to coarse-grained (CG) and nc Cu powders, with identical processing parameters. HPT of CG Cu powders resulted in exceptional grain refinement and increase in dislocation density, whereas significant grain growth, via grain rotation-induced grain coalescence, and decrease in dislocation density occurred during HPT of cryomilled nc Cu powders. Equilibrium structures were achieved under both conditions, with very similar stable grain sizes and dislocation densities, suggesting dynamic balances between deformation-induced grain refinement and grain growth, and between dislocation accumulation and dislocation annihilation.