The influence of buoyant forces and volume fraction of particles on the particle pushing/entrapment transition during di
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I.
INTRODUCTION
THE distribution
of particles in metal matrix composites, manufactured by casting techniques, depends to a great extent on the nature of the interaction between the ceramic particles and the growing solidification front.ll,2~ Earlier, the authors examined the general behavior of the particles in front of a moving melt interface during directional solidification of some A1-Mg/SiC composites. t3,4,51 Several existing models were evaluated, and a new model was proposed. The latest model cal takes into account the role of drag forces on the particle, of surface energy, and of thermal conductivity of the particle and the liquid, when calculating the critical growth velocity at which the particles are engulfed* by a planar solid/ *The term engulfed is used when a particle is incorporated in a planar interface. The term entrapped is used when the particle is incorporated by a nonplanar (e.g., dendritic) interface which grows around it.14~
liquid interface. Most of the existing experimental work deals with particle engulfment in a planar interface. The influence of interface morphology, and the role of buoyant forces on the pushing/engulfment transition, is not yet well documented for metallic alloys. Thus, a number of experiments have been carried out to document the role of D O R U M. STEFANESCU, University Research Professor, and AVIJIT MOITRA and A. SEDAT K A C A R , Graduate Research Assistants, are with the Solidification Laboratory, Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487-0202. BRIJ K. D H I N D A W , Professor, permanently at I.I.T. Kharagpur, India, was a Visiting Scholar with the Solidification Laboratory at the time this work was performed. Manuscript submitted October 31, 1988. METALLURGICAL TRANSACTIONS A
various factors, such as buoyant forces, surface energy, volume fraction of particles, and interface morphology, affecting the behavior of particles in front of the melt interface, and, therefore, the final particle distribution in directionally solidified (DS) composites. An attempt was also made to develop a comprehensive model, incorporating the main processing and material variables. II.
EXPERIMENTAL
Four different metal/ceramic systems were studied. Three different types of alloys, i.e., A1-2 pct Mg, A117 pct Cu, A1-6 pct Ni, and two different types of particles, i.e., graphite (Gr) and silicon carbide (SIC), were combined to produce four types of metal matrix composites. The relation and selection of these systems were such as to allow for the study of particle interaction with dendritic interfaces (A1-2 pct Ms, AI-17 pct Cu) and eutectic interfaces (A1-6 pct Ni), as well as for positive and negative differences in density between particles and liquid. Details on the composition of these composites and the particle sizes used are given in Table I. One sample in the series A1-2Mg/Gr was produced with a 20 wt pct graphite rather than 6 wt pct (G6). It is interesting to note that this particular sample had a uniform struc
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