Study of the interface and its effect on mechanical properties of continuous graphite fiber-reinforced 201 aluminum
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INTRODUCTION
A D V A N C E D fiber composites consist of fibers, such as graphite, glass, kevlar, and boron, which are usually embedded in a polymeric matrix. However, polymeric matrixbased composites exhibit quasi-brittle behavior and are not suitable in most high temperature applications. For these reasons, there is considerable interest in developing metal-matrix composites, such as graphite fiber-reinforced aluminum. Due to a very high strength-to-weight ratio, graphite reinforced aluminum composites have been considered for aerospace applications. In addition to having good specific strengths, aluminum matrix reacts readily with many elements. Thus its strength can be mcreased significantly with relatively small alloy addition. Some disadvantages, however, do exist for aluminum as a matrix material. The basal planes of an oriented graphite fiber are oriented parallel to lateral surfaces, a factor which inhibits bonding to the matrix, t~l Thus, most metals, including aluminum, do not wet the surface of graphite fibers. At the melting point of aluminum, the contact angle between graphite and the liquid aluminum is greater than 90 deg and thus wetting is not spontaneous. ~zJ The aluminum, then, must be forced between the graphite fibers under pressure to assure intimate contact between fibers and metal, and thus provide adequate load transfer from matrix to fiber in the final composite. Perhaps the most serious disadvantage in graphitereinforced aluminum is the chemical reactivity of aluminum. The rhombohedral aluminum carbide AI,C3, which is formed at the interface between fiber and matrix during fabrication and service, may significantly affect the me-
H. NAYEB-HASHEMI. Associate Professor. is with the Department of Mechanical Engineering, Northeastern University, Boston. MA 02115. J. SEYYEDI is with Digital Equipment Corporation. Andover. MA 01810-2434. Manuscript submitted August 28. 1987.
METALLURGICAL TRANSACTIONS A
chanical properties. Although the development of interaction zones of some thickness (e.g.. a few angstroms) is desirable for establishing fiber/matrix bonding, excessive interaction may be detrimental to composite strength. Several studies have been performed to evaluate the effects of reaction products on Gr/A1 composite properties, although the conclusions are apparently contradictory. Blankenburgs p[ showed that aluminum carbide. A 1 4 C 3 , formed on the surface of carbon fibers at temperatures above 500 ~ The tensile strength of the composite improved considerably after development of small amounts of carbide. However, further growth of the carbide phase did not degrade the composite strength. Similarly, Harrigan reported that even long exposures at high temperatures neither significantly changed the interface nor degraded the strength of the composite.~4J In contrast, Jackson ~5[and Khan ~rJ showed that up to 500 ~ little degradation of strength occurred while for composites exposed to higher temperatures, the strength declined rather significantly. The formation of aluminum carbi
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