Grain growth of nanocrystalline Ni powders prepared by cryomilling

Abstract

Grain growth of nanocrystalline Ni powders with an average grain size of ∼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 ∼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 ^1/n₀ =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.