On the kinetics of mechanical alloying
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I.
INTRODUCTION
THE powder metallurgical technique of mechanical alloying is generally used to create materials with unique compositions and microstructures for specialized applications. Developed over 20 years ago to produce oxide dispersion-strengthened superalloys, m most subsequent work has been concerned with extending the range of materials which can be processed this way. Besides a characterization of the microstructural development during milling t2,31 there has been little attempt to understand the fundamental mechanisms involved in the mechanical alloying process, although there are a few notable exceptions, t4,51 These studies have generally been concemed with the physics of mechanical alloying in an effort to model such variables as impact velocities, collision energies, and temperature rises associated with ballpowder collision events. However, the effect of such variables on the kinetics of mechanical alloying does not appear to have been studied. It is only through an understanding of the factors affecting the reaction kinetics that it will become possible to optimize the long processing times. For whatever purpose mechanical alloying is used, the fundamental action is the occurrence of a solid-state reaction between the milled constituents. Most solid-state reactions involve the formation of one or more product phases between the reactants, and the reaction volume is continuously diminished as the reactants become spatially separated. Reaction rates are therefore influenced by initial contact areas, and hence, particle size, and by the diffusion of the reactant species through the product phases. Factors which influence diffusion rates, including defect structures and densities, local temperatures, and product morphology, clearly have an important effect on reaction kinetics.[6'7'8] It has been shown that mechanical alloying significantly increases solid-state reaction rates by dynamically maintaining high reaction interface areas t9] and by simultaneously providing the conditions for rapid diffusion, t~~ Mechanical alloying thereby minimizes the effect of product barriers on reaction kinetics and hence provides the conditions required for the promolgation of solid-state reactions at low temperatures. G.B. SCHAFFER, Lecturer, is in the Department of Mining and Metallurgical Engineering, The University of Queensland, Queensland 4072, Australia. P.G. McCORMICK, Professor, is with the Department of Mechanical Engineering, The University of Western Australia, Nedlands, Western Australia, 6009, Australia. Manuscript submitted September 25, 1991. METALLURGICAL TRANSACTIONS A
While mechanical alloying is often def'med as a "high ~ (collision) energy process, the magnitude of this energy requirement is seldom considered. During conventional ball milling, powder particles are simply mixed; particle size, shape, and chemistry are not altered. When ball milling is used as a communition process, the particle size is reduced by fracture during collisions; the particle chemistry is not altered. What distinguis
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