Grain Growth in Cryomilled Ni Powder during Degassing

Grain growth during the degassing of Ni cryomilled in liquid nitrogen and that cryomilled in liquid argon was investigated by X-ray diffraction (XRD) and confirmed by transmission electron microscopy (TEM) studies. The Ni cryomilled in liquid argon was found to have significantly more thermal stability than the Ni cryomilled in liquid nitrogen, with grain sizes of 28 and 62 nm, respectively, after 32 hours of degassing at 723 K. The Ni cryomilled in liquid argon was found to have significantly greater amounts of aluminum, iron, and chromium than did the Ni cryomilled in liquid nitrogen. The measured grain-growth data conformed very well to the predictions from two models that were derived for impure nanocrystalline materials. That fact, along with the data obtained from chemical analysis, supports the conclusion that the kinetics of grain growth in cryomilled Ni are dominantly controlled by the drag force of impurities on the grain boundaries, as the impurities segregate to the grain boundaries.