Processing and mechanical properties of nanostructured copper

The high strength and other unusual mechanical properties of nanostructured materials have stimulated widespread interest in recent years. In spite of the enormous efforts over the last decade, the deformation behavior—especially the tensile ductility—of these novel materials remains poorly understood. This dissertation is focused on the processing and understanding of the mechanical properties (especially tensile properties) of nanocrystalline (grain sizes <100 nm) and ultrafine-grained (grain sizes between 100–500 nm) copper.; A confined cryogenic-temperature (CT) cold rolling technique was developed in this work to prepare nanostructured Cu through severe plastic deformation at low temperatures. Both the high-strain-rate loading and prolonged rolling are found to have strong effects on the development of nanoscale grain sizes in CT-rolled Cu, and are closely tied with migration or progressive dynamic recrystallization mechanisms.; Microsample tensile testing on nanocrystalline (nc) Cu with average grain sizes of 30 nm revealed a yield strength of 760 MPa but a very low tensile elongation to failure (<3%). Such an early failure should come as no surprise, as nc materials do not work harden through the stages as in coarse-grained forms. Subsequent tensile testing on ultrafine-grained (100–300 nm) Cu indicates that these materials are intrinsically ductile, but suffer from early inhomogeneous deformation (necking) and thus a marked drop of tensile ductility compared with coarse-grained Cu. The unstable tensile deformation is due to the small strain hardening capacity as well as a low strain rate sensitivity only slightly above that of coarse-grained Cu, as uncovered in compression tests. A good combination of strength and ductility (in terms of post-uniform elongation) was achieved by refining the grain size within the UFG regime.; In an effort to improve the uniform tensile elongation in nanostructured Cu, a bimodal microstructure was developed through CT rolling followed by thermal annealing/recrystallization treatment. The new material has an excellent combination of high strength and high ductility, far beyond the expectations of the strength-ductility trade-off known for all previous Cu. Deformation twinning was observed for the first time in Cu under quasistatic loading at room temperature. These results indicate that, by manipulating the grain size distribution, or by exploiting cryogenic temperatures to suppress dynamic recovery during deformation, a strong strain hardening, and hence large uniform strains are achievable in nanostructured Cu. The new findings are projected to have universal importance in developing high strength and high ductility nanostructured materials for structural applications.