Comparative studies of yield strength and elastic compressibility between nanocrystalline and bulk cobalt

Comparative studies of yield strength and elastic compressibility between nanocrystalline and bulk cobalt were conducted using synchrotron x-ray diffraction under tri-axial pressure loading-unloading conditions. Relative to micron Co, nano Co exhibits higher flow stress (2.9 GPa compared with 2.1 GPa in micro Co), extra degree of strain-induced peak broadening during loading yet a better strain recoverability after unloading. These observations suggest different deformation mechanisms with intergranular strains dominated in nano Co and intragranular strains in micron Co. The determined bulk modulus for nano Co is 216 GPa, ∼17% higher than that of micron Co (185 GPa). This finding supports a generalized model of nanocrystals with pre-compressed surface layers and indicates that the grain-size induced elastic strengthening and weakening are primarily determined by the nature of internal stress (compressed vs. tensile) present in the surface layer of a nanocrystal.