In situ method of following the reactivity between SiC and liquid aluminum
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Communications
Alloys A356 F3A10S
In Situ Method of Following the
Reactivity between SiC and Liquid Aluminum S. GOWRI and M. BOUCHARD Metal matrix composites are gaining use in automotive and aircraft industries. They can be produced from either liquid or solid state. The liquid state, which involves mixing of the reinforcements into the melt and casting subsequently by various casting processes, is an extremely cost effective way of processing the composites. The reinforcements impart superior mechanical and physical properties and wear resistance to the base matrix alloy.V.2] However, the processing conditions of these cast composites are restricted by the reactivity of the reinforcement with the molten aluminum. For example, a SiC reinforcement is not thermodynamically stable in liquid aluminum and reacts with the matrix, forming aluminum carbide according to the following reaction:
4Al + 3SiC--->3Si + Al4C3
[1]
The degradation reaction of SiC proceeds towards right when the silicon level in the alloy is low with the formation of aluminum carbide. Once the carbide is formed, the material cannot be saved. The reaction (carbide formation) also increases the Si level of the alloy, thereby modifying the chemistry of the matrix alloy, increases the viscosity of the melt, and reduces the corrosive resistance of the casting. The viscosity of the composite is increased because the A14C3 formed has a lower density than SiC; the reaction increases the volume fraction of solids. The extent of this reaction depends on the temperature, time at temperature, and percentage of silicon in the matrix. For example, in a 7 wt pct Si with 10 or 20 pct SiC composite, an operating temperature of 760 ~ or below is recommended. Above 790 ~ aluminum carbide is observed to form instantaneously.m It is therefore important to follow the extent of this reaction. The extent of reaction (Eq. [1]) has been followed by various methods: measurement of aluminum carbide and silicon X-ray intensity peaks from the composite powder, chemical analysis,t41 and measurement of the thickness of the fiber matrix interaction layer.t5~ Lloyd and Jint61 used the method of following the changes in the liquidus temperature and relating the temperature to phase diagram and composition. From Eq. [1], it is seen that the amount of silicon increases in the matrix as SiC reacts with aluminum in a direct proportion. The increase in silicon exhibits a significant influence on the liquidus. As the silicon in the matrix increases, the liquidus temperature of an Al-binary or ternary alloy decreases. The
S. GOWRI, Rese~trch Associate, and M. BOUCHARD, Professor, are with the Department of Applied Sciences, University of Quebec at Chicoutimi, Chicoutimi, PQ, Canada G7H 2B 1. Manuscript submitted September 22, 1994. 1904---VOLUME 26A, JULY 1995
Chemical Composition of F3A10S Composite
Si 7.11 7.35
Fe 0.10 0.13
Elements (Wt Pct) Cu Mg Ti Sr 0.01 0 . 4 0 0.09 -0 . 0 2 0 . 3 5 0 . 1 0 0.014
SiC -10.2
binary and ternary phase diagrams of most of the commercial alumi
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