Formation of Nanostructures and Solid Solubility Extension in Cryomilled Al-Cu and Al-Si Powders

Abstract

Nanostructured Al-2024 alloy and Al-12%Si were successfully synthesized via a cryogenic milling process. Equiaxed nanograins of 46 nm and a lattice strain of 0.2% were obtained after 18 hours of milling for the Al-Cu system and grain sizes of 46 nm with a lattice strain of 0.09% were obtained after only 4 hours milling time for the Al-Si system, as measured from the X-ray patterns. Transmission electron microscopy (TEM) characterization was also employed to evaluate the grain size in both systems. For the Al-Cu system, a bimodal grain size distribution (as measured from dark field (DF) TEM images) constituted of nanometric and ultrafine grains was obtained after 8 hours of milling. This bimodal behaviour changes with milling time and after 12 hours results in a unimodal distribution with more than 50% of the grains between 20-40 nm. Also, additional milling time (16 hours) narrows the distribution; and approximately 80% of the grains reach the 20-40 nm range. At this stage, the size of the grains was also verified via direct thickness measurements of selected area diffraction (SAD) rings. The solid solubility extension (SSE) of Cu in Al, as estimated from Vegard's law, in the final stage of milling (18 hours) reached a value of 0.41 atm%. For the Al-Si system the mechanism of particle evolution undergoes similar stages as in the Al-Cu system only in much shorter times such as after only 2 hours of milling the powders have completed the plastic deformation and reached the flattening phase. This was followed by fracturing, reaching an equiaxed morphology after only 4 hours of milling. At this stage the SSE of Si in the Al matrix was found to be 1.72 atm%.