Displacement reactions during mechanical alloying
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I.
INTRODUCTION
MOST solid-state chemical reactions
involve the formation of one or more product phases between the reactants, which therefore become spacially separated. In general, solid-state reaction rates are controlled by diffusion of the reacting species through the product material. The reactivities of solids are therefore dependent on initial contact areas and hence particle size, on factors which influence diffusion rates, such as defect densities, and local temperatures, and on product morphology, tl,2,31 In conventional, thermally activated systems, the reactants remain substantially unchanged and spacially separated during the course of the reaction. A significant increase in reaction rate could result if a means were found to agitate the reaction mass and so continually bring unreacted material into contact. This would prevent the products from dividing the reactants. The powder processing method known as "mechanical alloying" provides such a potential. The mechanical alloying process is generally defined as the high-energy ball milling of powder material. Originally developed as a means to manufacture oxide dispersion-strengthened alloys,[4,5l the scope of the technique has subsequently been widened to include the formation of amorphous alloys, ttj superconducting materials, I7] rare earth permanent magnets, ts] superplastic alloys,tgl and intermetallic compounds. ~176The precise mechanism is unknown and appears to be dependent on the specific system under investigation. It is thought to involve the repeated fracture and welding of powder particles during ball-powder-ball and ball-powder-container collisions. Atomically clean surfaces are thus brought into intimate contact, and alloying takes place by interdiffusion across the welded interface. The high defect densities induced in the powder material by the mechanical attrition greatly enhance diffusion rates and will therefore have a significant effect on any mechanism. In initial investigations to test whether the mechanical
G.B. SCHAFFER, formerly University Research Fellow, now Lecturer, and P.G. McCORMICK, Associate Professor, are with the Mechanical Engineering Department, University of Western Australia, Nedlands, Western Australia 6009, Australia. Manuscript submitted February 20, 1990. METALLURGICAL TRANSACTIONS A
alloying technique could be used as a vehicle for solidstate reactions, m,12,131 displacement reactions of the type AxO + vB---* xA + BvO were caused to occur during the mechanical alloying of a simple metal oxide and a strong metallic reducing agent. Typically, copper metal formed when CuO was milled with calcium, and/3 brass formed when CuO and ZnO were simultaneously milled with calcium. When a chemically inert liquid was used as a process control agent, the reaction proceeded in a controlled manner, and the oxide was gradually reduced over the course of milling. When milled dry, however, an unstable combustion reaction occurred, after which the reduction reaction was virtually complete. This is similar in nature to the the
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