Solidification of binary hypoeutectic alloy matrix composite castings
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INTRODUCTION
M E T A L matrix composite solidification processes, in which a reinforcing phase and a metal are combined while the latter is fully or partly liquid, have gained in engineering importance over the past decade, owing to their low cost, their capacity for net-shape component fabrication, and the high microstructural integrity that can be produced in the resulting material. Several different classes of composite solidification processes exist, such as infiltration or spray casting; however, an important step in all of these composite production processes is that in which the liquid metal matrix solidifies and acquires a microstructure. It is now well known that rules developed for solidification of unreinforced metals can not be applied directly to metal matrix composites. In the matrix of a composite, the solid phase must grow within the confines of narrow interstices left between neighboring elements of the reinforcing phase, such as fibers, whiskers, or particles. The reinforcing phase thus places an upper limit on any microstructural element within the growing solid matrix while also affecting capillary equilibria, heat flow, diffusion, and fluid transport, all of which govern the progress of matrix solidification. Composite matrix microstructures can therefore be quite different from what would be expected in the same unreinforced alloy solidified under similar conditions.
ANDREAS MORTENSEN, Professor, and MERTON C. FLEMINGS, Professor, are with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. This article is based on a presentation made at the "Analysis and Modeling of Solidification" symposium as part of the 1994 Fall meeting of TMS in Rosemont, Illinois, October 2 6, 1994, under the auspices of the TMS Solidification Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
There has, for this reason, been considerable recent scientific interest in the solidification of metal matrix composites. Experimental and theoretical studies, which have been summarized in several reviews, IJ.2,31 have thus addressed various aspects of the solidification of reinforced metals, such as dendrite tip growth conditions, solidification morphology transitions, and alterations in matrix microsegregation. An important tool in the study of alloy solidification is the Bridgman furnace, which produces well-controlled steady-state directional solidification conditions. Steadystate directional solidification experiments consist of pulling a long sample of material at a constant speed through a region of steeply descending temperature. In the investigation of transparent metal analogue materials, this region is placed within an optical microscope for direct examination of the growing solid, while with metals, the solidifying sample is pulled rapidly into the heat sink of the apparatus to quench the solidification microstructure for subsequent metallographic characterization. These experiments, which have produced a wealth of experimental solidification data
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