Behavior of ceramic particles at the solid- liquid metal interface in metal matrix composites
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INTRODUCTION AND BACKGROUND
THEproblem of insoluble particle behavior at the solidification interface studied by Uhlmann et al. [~]has been found to be quite relevant for systems of practical significance. For example, the distribution of particles in metal matrix composites (MMC) t2,3J manufactured by casting technique depends, to a great extent, on the nature of the interaction between ceramic particles and the growing solidification front. Also, distribution of inclusions in steel, structure formation in monotectic alloys, and incorporation of gaseous particles to form porosity are some of the other phenomena which are related to the concept of behavior of particles in front of a moving solid/liquid interface. Basically, when a moving solidification front intercepts an insoluble particle, it can either push it or engulf it.* En*The reader is cautioned that different authors are using these terms in slightly different ways. The present terminology is based upon Webster's Dictionary.
gulfment occurs through growth of the solid over the particle, followed by enclosure of the particle in the solid. If, for various reasons, the solidification front breaks down into cells, dendrites, or equiaxed grains, two or more solidification fronts can converge on the particle. In this case, if the particle is not engulfed by one of the fronts, it will be pushed in between two or more solidification fronts, and will finally be entrapped in the solid at the end of local solidification. There are two basic theoretical approaches to the study of particle behavior at a solid-liquid interface, i.e., the thermodynamic and the kinetic approaches. The thermodynamic model suggested by Omenyi and Neumann I41 predicts engulfment when the net change in free energy due to engulfment is negative. The kinetic approach is based on the simple idea that as long as a finite layer of liquid exists between the particle D.M. STEFANESCU, Professor, B.K. DHINDAW, Visiting Scholar (permanently a Professor at I.I.T. Kharagpur), and S. A. KACAR and A. MOITRA, Graduate Research Assistants, are with the Department of Metallurgical Engineering, The University of Alabama, Tuscaloosa, AL 35487. Manuscript submitted August 7, 1987. METALLURGICALTRANSACTIONS A
and the solid, the particle will not be engulfed. The concept of a critical interface velocity, Vcr, below which particles are pushed and above which particles are engulfed, was postulated. A number of models were proposed by Uhlmann et al.,[l] Boiling and Cissr, [5] and Chernov et al. [6] They all assumed a planar liquid-solid interface, only one spherical particle at the interface, and that the thermal conductivity of the particle, ke, is equal to that of the liquid, kr. Pushing was postulated to take place due to repulsive forces between the particle and the solid and engulfment due to drag forces. The nature of the repulsive forces considered differs among various investigators. The existence of a critical interface velocity, Vcr, has been documented experimentally by Uhlmann et al.,U] as well as
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