Controlling factors in tensile deformation of nanocrystalline cobalt and nickel

In an effort to understand and enhance the tensile ductility of truly nanocrystalline metals, we have investigated and compared the mechanical behavior, especially the tensile behavior, of hexagonal close-packed (hcp) nanocrystalline cobalt (similar to 20 nm) and face-centered cubic (fcc) nanocrystalline nickel (similar to 28 nm). Although both materials exhibit obvious plasticity in tension, their uniform tensile ductility, tensile elongation-to-failure, and fracture behavior are drastically different. In-situ synchrotron x-ray diffraction and ultra-small angle x-ray scattering reveal distinct deformation disparity in terms of residual strain development, texture evolution, nanovoid formation, and subsequent strain-hardening and strain-rate-hardening behavior. The dependence of tensile property on the strain rate and temperature is examined and discussed. Factors that influence the strength and ductility of nanocrystalline metals are considered and prioritized according to the current findings. A new Hall-petch relationship is proposed for nanocrystalline nickel.