X-ray diffraction investigation of the formation of nanostructured metastable phases during short-duration mechanical alloying of Cu-Al powder mixtures

Mechanical alloying is a powder processing technique used to process materials farther from equilibrium state. This technique is mainly used to process difficult to alloy materials in which the solid solubility is limited, and to process materials where non-equilibrium phases cannot be produced at room temperature through conventional processing techniques. In the present work, mechanical alloying/milling of selected compositions in the Al-Cu binary alloy system was carried out at a ball-to-powder weight ratio (BPR) of 2 : 1, to investigate alloying and subsequent heat treatment on microstructural changes as a result of short milling times. Copper-aluminum powder mixtures containing 5, 20, and 40 wt% Al (11, 37, and 61 at% Al, respectively) were subjected to mechanical alloying, and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC), after mechanical alloying and subsequent heat treatment. Nanometer-sized grains were observed in the as-milled crystalline powders in all compositions. Crystallite sizes were calculated using the Scherrer formula and found to be in the order of 10-20 nm after 360 minutes of milling time for all compositions. The XRD data show considerable solid solubility extension in these powders, and formation of intermetallic phases due to mechanical alloying and subsequent annealing. These changes are discussed in the context of the Al-Cu phase diagram.