Evolution of Intermetallic Cu₉Al₄ During the Mechanical Alloying of Cu-Al Mixtures in High-Energy Ball Milling

Cu-30 wt.% Al mixtures produced by mechanical alloying were studied using x-ray diffractometry (XRD), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). The results showed that the milling time affected the morphology and particle size, and the minimum particle size was 2.91 μm for a moderate milling time of 4 h. The XRD results showed that a Cu(Al) solid solution was not detected in the Cu-rich Cu-Al mechanically-alloyed mixtures, but intermetallic compounds were formed under the conditions of this experiment. The θ-CuAl₂ phase was formed during the early milling stage (2 h), but it disappeared, and only the γ₁-Cu₉Al₄ phase existed as the milling time continuously increased. In addition, the effect of thermally-assisted mechanisms on the formation of intermetallic compounds could be limited by controlling the temperature of the horizontal high-energy ball milling equipment. These observations indicate that the metastable γ₁-Cu₉Al₄ phase can exist stably due to non-thermally activated mechanisms. Moreover, the evolution of intermetallic compounds can be explained by the following process: as the ball milling progressed, intermetallics were formed in a layered structure and were then crushed to form fine particles that adhered to the large particles that were not completely broken.