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The mechanical properties of aluminum-graphite composites were measured at room temperature in the as-received condition, after elevated temperature exposure and after thermal cycling. The composites were fabricated by solid-state diffusion bonding of liquid-phase Al-infiltrated Thornel 50 fibers. The results showed that the maximum longitudinal tensile strength of the a s - r e c e i v e d material was 80,000 psi (552 MN/m2), which corresponds well with the rule of mixture value. The composite strength was observed to vary widely, depending on the extent of wetting of the fibers by the aluminum. The strength of the composites in the t r a n s v e r s e direction was generally very low, due to poor interfacial bonding. Aluminum carbide (A14Cs) formed at the surface of the fibers at temperatures greater than 500~ (773 K). Development of the carbide was shown to be diffusion-controlled and was dependent on the time and temperature used. It was shown that the tensile strength was virtually unaffected by heat-treatment up to 500~ (773 K); beyond that temperature a drastic degradation of tensile strength occurred. The degradation could be correlated with the extent of carbide development at the interface. T h e r mal cycling of the composites below 500~ (773 K) resulted in an observable degradation of the composite strength. Scanning electron microscopy of fractured surfaces indicated that the relatively weak interface governs the mode of failure in tension. CONSIDERABLE interest has been shown recently in the development of aluminum-graphite composites for aerospace applications. This is because of both the high specific strength and modulus theoretically obtainable, and the potential low cost of the composites. However, major problems encountered in the fabrication of such composites include wetting, bonding, and interfacial reaction at the fiber-matrix interface. The reaction at the interface can degrade the mechanical properties so severely as to render the material useless for practical applications. Baker e t al, I have shown in compatibility tests that carbide growth occurs at temperatures above 600~ (873 K). J a c k s o n e t al e observed chemical interaction between A1 and the fibers during s t r e s s - r u p t u r e tests at 400~ (673 K), resulting in a significant lowering of the composite strength. However, Harrigan and French 3 have r e ported no degradation of the composites at 465~ (738 K). A better understanding and, hence, control of the interface reaction in the reactive composite system is, therefore, important if the full potential of aluminum-graphite as a structural material is to be realized. A number of methods have been reported for fabricating aluminum-graphite composites from multifiber yarns and tows. 4 Among them, liquid-phase hot pressing and solid-state diffusion bonding are now commonly used. F r o m results reported so fax, s the solid-state diffusion bonding process appears to yield aluminum-graphite composites of the highest quality and with the most consistent mechanical properties.