Formation of nanostructure in Al produced by a low-energy ball milling at cryogenic temperature

Nanocrystalline (nc) Al powders produced by ball milling at a low-energy level and cryogenic temperature (i.e., cryomilling) were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The average grain size (d) in the 8 h milled Al was 26 nm. The development of the nanostructure appears to be dominated by the total level of the microstrain generated by the deformation process. Two types of distinctive nanostructures were identified: primarily a random dispersion of equiaxed grains with typical diameters of 10–30 nm, and a less frequently observed lamellar structure with a length range of 100–300 nm and an average width close to d. The morphology of the mixed nanostructures in the cryomilled sample shows a large number of high angle grain boundaries (HAGBs) formed by a grain subdivision mechanism. Microstructural evidence for the formation of nanostructure by recrystallization mechanism is also discussed.