High grain size stability of nanocrystalline Al prepared by mechanical attrition

Grain growth in nanocrystalline (nc) Al with a grain size of 26 nm produced by cryogenic mechanical milling was studied through x-ray diffraction, transmission electron microscopy, and differential scanning calorimetry. Grain growth kinetics resembled those of ball-milled nc Fe. For homologous temperatures (T/Tm) of 0.51-0.83, the time exponent n from D-1/n - D-0(1/n) = kt was 0.04-0.28, tending toward 0.5 as T/T-M increased. Two grain-growth regimes were distinguished: below T/T-M = 0.78 growth ceased at an approximate grain size of 50 nm while at higher temperatures, grain growth proceeded steadily to the submicrometer range. Grain growth over the range of temperatures studied cannot be explained in terms of a single thermally activated rate process. The observed high grain size stability was attributed primarily to impurity pinning drag associated with the grain growth process.