The impact of the operating mode of a planetary mill and gas atmosphere on mechanical alloying of powders in the Ni – Cu system

This study experimentally investigates the formation dynamics of a Ni–Cu solid solution during high-energy ball milling (HEBM) of a Ni and Cu powder mixture in planetary mills. The influence of processing conditions and gas atmosphere composition is examined. It is shown that the formation of a solid solution critically depends on the milling speed: at 300 rpm, no mutual dissolution of metals was observed, whereas at 694 rpm, a solid solution was formed within a relatively short time – no more than 30 minutes. In an oxygen-free atmosphere, intense cold welding of particles occurred, forming large (up to 2–3 mm) disk-shaped and oval flakes, which later became rounded and acquired a spherical shape. In an air atmosphere, cold welding was slowed down, and the metals became less ductile due to oxygen absorption. HEBM in airresulted in the powder consisting of smaller particles with rounded and flake-like shapes (300–400 μm along the surface of flakes, less than 100 μm across). The most homogeneous solid solution structure was formed in an air atmosphere. Thus, the optimal conditions for the mechanosynthesis of metallic solid solutions are processing in a planetary mill at 500–700 rpm in an air atmosphere. The findings of this study can be used to optimize the mechanical synthesis of metallic solid solutions, including Cantor high-entropy alloys with a face-centered cubic structure.