MECHANICAL-BEHAVIOR OF NANOCRYSTALLINE CU AND PD

This report gives results of a study of the bulk mechanical properties of samples of nanocrystalline Cu and Pd consolidated from powders prepared by inert gas condensation. Fourier analysis x-ray diffraction techniques, used to determine average grain size and mean lattice strains of the as-consolidated samples, show grain sizes in the range of 3-50 nm and lattice strains ranging from 0.02-3%. Sample densities range from 97-72% of the density of a coarse-grained standard. Microhardness of the nanocrystalline samples exceeds that of annealed, coarse-grained samples by a factor of 2-5, despite indications that sample porosity reduces hardness values below the ultimate value. Uniaxial tensile strength of the nanocrystalline samples is similarly elevated above the value of the coarse-grained standard samples. Restrictions on dislocation generation and mobility imposed by ultrafine grain size are believed to be the dominant factor in raising strength. Residual stress may also play a role. Room temperature diffusional creep, predicted to be appreciable in nanocrystalline samples, was not found. Instead, samples appear to show logarithmic creep that is much smaller than the predicted Coble creep.