The morphological evolution of hollow shells during the mechanical milling of ductile metals

Abstract

Mechanical alloying is a ball milling process during which powder particles are repeatedly fractured and welded to produce a homogeneous powder. If a ductile metal or alloy is milled at room temperature in a dry inert atmosphere, welding will predominate over fracture and the particle size will steadily increase with milling time. To inhibit welding and produce a fine powder, it is generally necessary to cool the vial or use process control agents (typically liquid or powdered organic compounds). Since large sized powders are less amenable to subsequent processing techniques, few investigations have been made into the morphology of ductile metals milled for extensive times without temperature control or a process control agent. In this paper the authors report on the morphological evolution of large hollow spheres which are formed when copper (a typical ductile metal) is milled without a process control agent. Milling under these conditions also produces secondary effects such as the formation of smaller spherical shells inside the primary spheres (the so-called Russian Doll effect), but these will not be discussed here. This work is part of a wide study on the morphological development of powders during mechanical alloying, part of which has been reported previously.