Grain growth of nanocrystalline Ni powders prepared by cryomilling

Grain growth of nanocrystalline Ni powders with an average grain size of similar to 22 nm prepared by cryogenic mechanical milling (or cryomilling) was investigated by using X-ray diffraction (XRD) and transmission electron microscopy (TEM). A dispersion of NiO and Ni3N particles with a size less than 5 nm was formed in the cryomilled powders. The Ni3N particles decomposed at 773 K. It was found that at 0.56 homologous temperature (T/T-M), Ni grains were retained at similar to 150 nm even after long annealing times (e.g., 4 hours). For 0.45 to 0.62 T/T-M, the time exponent n deduced from D-1/n - D-0(1/n) = kt was 0.16 to 0.32, tending toward 0.5 as T/T-M increased. The activation energy for grain growth in the Ni sample was determined to be 113 kJ/mol, which is close to the activation energy for grain boundary self-diffusion in polycrystalline Ni. The observed high grain size stability was attributed primarily to a grain boundary pinning mechanism arising from the NiO particles as well as impurity segregation.