Interaction mechanisms between ceramic particles and atomized metallic droplets
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INTRODUCTION
M E T A L matrix composites (MMCs) are attractive candidate materials for high-performance structural applications that require unusual combinations of physical and mechanical properties. [1'2m Among the various types of MMCs available, those that are reinforced with discontinuous fibers or particles have received considerable attention for three reasons. First, discontinuously reinforced MMCs can be formed utilizing conventional fabrication methods, such as forging, extrusion, and pressing, making them attractive candidate materials for cost-critical applications. Second, there are a wide variety of fibers and particles commercially available. Third, discontinuously reinforced MMCs offer essentially isotropic properties as a result of the small aspect ratio of the reinforcing phases. Discontinuously reinforced MMCs have
YUE WU, Graduate Student, and ENRIQUE J. LAVERNIA, Associate Professor, Materials Science and Engineering, are with the Department of Mechanical and Aerospace Engineering, The University of Califomia-Irvine, Irvine, CA 92717. Manuscript submitted October 15, 1991. METALLURGICAL TRANSACTIONS A
been successfully synthesized using a variety of solidification techniques, including stir casting of a mixture of liquid matrix with reinforcements,t4'51 rheocasting, [6,7,81 metal infiltration,t9-12[ and spray atomization and codeposition. D3-16[ A review of discontinuously reinforced MMCs is available elsewhere. [3[ The solidification conditions that are present during processing play an important role in dictating the microstructure and the mechanical and physical characteristics of MMCs. For example, the dendrite arm spacing, [17a81 solute segregation, i19,2~ and distribution of reinforcing phases I21'221 present in MMCs have all been shown to critically depend on the solidification conditions present during processing. A review of the available scientific literature shows that over the last three decades, various investigators have studied the fundamental problem posed by the interaction of a particle with a moving fluid front. [23-29] Although the results of these studies have enhanced our understanding of the kinetic and thermodynamic factors that govern the entrapment or rejection of particles by a moving front during solidification, it is often difficult to apply these models to the highly nonequilibrium conditions that exist during processing. For VOLUME 23A, OCTOBER 1992--2923
example, the coarsening of dendrite arms following solidstate cooling tends to smooth out concentration gradients resulting from the segregation of solute elements during solidification. The objective of the present work was to provide insight into the dynamic interactions that occur between ceramic particles and a metallic matrix undergoing solidification. Aluminum-silicon was selected as a matrix material for two reasons. First, A1-Si alloys have been reported to exhibit attractive combinations of wear and thermal properties. Second, this alloy system has been studied extensively, and as a result, the
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