Nucleation on ceramic particles in cast metal-matrix composites
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I. INTRODUCTION
THE observations of microstructures of solidificationprocessed hypoeutectic metal-matrix particle composites (MMCs) reveal that particulate reinforcements are generally segregated in the last-freezing-zone interdendritic regions of primary phases, resulting in a nonuniform distribution of the particles in the matrix and possibly leading to lower properties in cast MMCs. The reasons for the segregation of reinforcements into interdendritic regions can include (1) the absence of nucleation or remelting of primary a phases on the reinforcement particles and (2) the rejection of the particles by the solidifying interfaces; the latter phenomenon is also referred to as the particle-pushing phenomena.[1,2] According to several investigators,[2] there is a critical solid/ liquid interface velocity, above which the particles are engulfed by the solidifying interface; below the critical solidifying interface velocity, the particles are pushed by the interface. The critical interface velocities measured experimentally in rapidly solidified wedge castings were 13,100, 14,800, and 15,600 m/s for Al-12.5Si/5SiC, Al-7Si/5SiC, and Al-4Si/5SiC alloys, respectively.[3] However, interface velocities of this order of magnitude will be difficult to J.K. KIM, Research Associate, and P.K. ROHATGI, Wisconsin Distinguished Professor, are with the Materials Department, University of Wisconsin–Milwaukee, Milwaukee, WI 53211. Manuscript submitted April 2, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
achieve in conventional castings, and, therefore, the particles are likely to be pushed by the solidifying interfaces into last-freezing interdendritic regions under normal casting conditions, resulting in their segregation in the interdendritic regions. According to the thermodynamic criteria, the particles will have a tendency to be engulfed by the solidifying interface when the energy change after engulfment is negative. This can lead to the engulfment of the particles by the solidifying interfaces, resulting in the improvement of their distribution in the matrix.[4,5,6] The nucleation of primary phases on reinforcement particles is also likely to provide improved interfaces between the particles and the matrix, and this is also likely to enhance the properties of the composites, as discussed by Rohatgi et al.[7] Therefore, it is important to understand the mechanism of formation of primary phases on the particles present in the melt. To date, the formation of primary phases in composite melts in the presence of reinforcements has not been sufficiently understood to enable us to obtain desired microstructures in cast MMCs. Frequently, inoculating agents are added to the molten metals to produce the fine-grained materials. The inoculants or their products supposedly act as sites for heterogeneous nucleation. However, the mechanism of grain refinement through inoculants is far from understood. Heterogeneous nucleation on the surfaces of particles in alloy melts depends on many factors, including the difference in the surface
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