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, tra nsmission electron microscopy, and differential scanning calorimetry. Grain growth kinetics resembled those of ball-milled nc Fe. For homologous tempe ratures (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 reg imes 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 s teadily to the submicrometer range. Grain growth over the range of temperat ures studied cannot be explained in terms of a single thermally activated r ate process. The observed high grain size stability was attributed primaril y to impurity pinning drag associated with the grain growth process.