Cold spray deposition of nanocrystalline aluminum alloys
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I. INTRODUCTION
NANOCRYSTALLINE materials are characterized by a microstructural length scale in the 1- to 100-nm range.[1] A large fraction of the atoms in these materials are associated with grain boundaries or interfacial boundaries when grain size is small enough (50 pct for 10-nm grain size).[2] Thus, a significant amount of interfacial component between neighboring atoms associated with grain boundaries contributes to the physical properties of nanocrystalline materials.[2] A number of techniques are capable of producing nanocrystalline materials, including gas condensation, crystallization of amorphous alloys, chemical precipitation, spray conversion processing, vapor deposition, sputtering, electro-deposition, sol-gel processing, plasma synthesis, severe plastic deformation, and mechanical alloying/milling.[3] Mechanical alloying/milling techniques have been used to produce large quantities of nanocrystalline materials for possible commercial use.[3] These techniques use a high-energy ball milling process, in which elemental or prealloyed powders are used to produce metastable materials with controlled microstructures. Mechanical alloying has been widely used to synthesize amorphous alloys, intermetallic compounds, and nanocrystalline materials.[4,5,6] During mechanical milling, particle welding and fracturing result in severe plastic deformation. The continuous process produces micron size agglomerates with a nanocrystalline structure. Several factors can influence the process of mechanical alloying, including milling time, ball-to-powder charge ratio, milling environment, and the internal mechanics specific to each mill.[7] Over the past several years, the characteristics of crystal refinement and development of nanostructures during ball milling have been studied extensively. These studies have led to several important observations: (a) grain size decreases LEONARDO AJDELSZTAJN, Research Scientist, and JULIE M. SCHOENUNG, Associate Professor, are with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616-5254. Contact e-mail: lajd@ucdavis BERTRAND JODOIN, Assistant Professor, is with the Department of Mechanical Engineering, University of Ottawa, ON, Canada K1N 6N5. GEORGE E. KIM, President, is with the Perpetual Technologies, Montreal, PQ, Canada H3E 1T8. Manuscript submitted June 30, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
with milling time, reaching a minimum grain size, which is a characteristic of each metal;[8,9] (b) the minimum grain size obtainable by milling scales inversely with melting temperature[9,10,11] or the bulk modulus, B;[8] (c) for fcc nanocrystalline metals, there is a linear relationship between the minimum grain size and the critical equilibrium distance between two edge dislocations;[8] and (d) limited experimental evidence suggests that smaller nanograin sizes are obtained at lower milling temperatures.[10] A special type of milling is the cryogenic milling process, often referred to as “cryomilling.” During cryomilling
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