Nanostructure in an Al-Mg-Sc alloy processed by low-energy ball milling at cryogenic temperature

Spray-atomized Al-7.5Mg-0.3Sc (in wt pct) alloy powders were mechanically milled at a low-energy level and at cryogenic temperature (cryomilling). The low-energy milling effectively generated a nanoscale microstructure of a supersaturated face-centered cubic (fcc) solid solution with an average grain size of similar to26 nm. The nanoscale microstructure was fully characterized and the associated formation mechanisms were investigated. Two distinct nanostructures were identified by transmission electron microscopy (TEM) observations. Most frequently, the structure was comprised of randomly oriented equiaxed grains, typically 10 to 30 nm in diameter. Occasionally, a lamellar structure was observed in which the lamellas were 100 to 200 nm in length and similar to24 nm wide. The morphology of the mixed nanostructures in the cryomilled samples indicated that high-angle grain boundaries (HAGBs) formed by a grain subdivision mechanism, a process similar to which occurs in heavily cold-rolled materials. The microstructural evidence suggests that the subdivision mechanism observed here governs the development of fine-grain microstructures during low-energy milling.