Modeling of the incorporation of ceramic participates in metallic droplets during spray atomization and coinjection
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INTRODUCTION
DISCONTINUOUSLY reinforced metal matrix composites (MMCs) are of interest for light-weight, highperformance applications due to their considerably low cost as compared to that of the continuously reinforced M M C s . [1-13] Compared to their monolithic matrix counterparts, MMCs offer competitively high specific modulus, good thermal stability, excellent wear resistance, and low coefficient of thermal expansion, tl~ Reinforcement materials that are being used to manufacture discontinuously reinforced MMCs include chopped fibers, whiskers, platelets, or particulates. For example, SiC is being used in aluminum and magnesium MMCs in all these f o r m s . [11A3] An important aspect of fabrication of discontinuously reinforced MMCs is the incorporation of ceramic reinforcements into metallic matrices. In particular, in the liquid and semisolid processing of particulate-reinforced MMCs, it is often required to incorporate the particulates into the matrix material through stir mixing, centrifugal mixing, or gas injection. These processes all involve the kinetic transfer of reinforcements from a gaseous phase to a liquid or semiliquid phase. The process of particulate incorporation and the subsequent particulatesolidifying matrix interactions have a significant influence on critical microstructural factors, such as reinforcement distribution and interfacial reactions. Spray atomization and codeposition processes, for example, involve the injection of particulates into a dispersion of matrix droplets under highly nonequilibrium thermal and solidification conditionsJ Ja'JS,16] Moreover, in spray atomization and coinjection, each particular droplet may be in the liquid, semiliquid, or solid state depending on factors such as droplet size and solidification kinetics. If
YUE WU and JINMIN Z H A N G , Graduate S t u d e n t s , and ENRIQUE J. LAVERNIA, Associate Professor, are with the Department of Materials Science and Engineering, University of California-Irvine, Irvine, CA 92717-3975. Manuscript submitted May 7, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS B
the kinetic energy of a particulate is not sufficiently high to overcome the alloy surface tension and/or viscosity, for example, the spray-deposited microstructure will consist of former droplet boundaries decorated with ceramic particulates. It has been reported that the incorporation behavior during the coinjection of ceramic particulates into atomized droplets depends on factors such as particulate size, solidification behavior of droplets, and the chemical compatibility between reinforcements and matrix. IIrA7ASI Variables such as surface tension and viscosity of the matrix alloy, fraction of solid in the matrix alloy, density of reinforcements, and reinforcement particulate morphology may critically affect the kinetics of gas-to-matrix transfer of particulates during reinforcement incorporation. Inspection of the available scientific literature, however, reveals that there are few systematic studies of the associated phenomena, t16'17'~81 In vi
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