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J. A. Graves Rockwell International Science Center, Thousand Oaks, California 91358 (Received 11 May 1992; accepted 24 November 1992)

The effects of fiber surface coatings on the mechanical behavior and damage mechanisms of SCS-6 fiber-reinforced titanium aluminide matrix composites have been studied. Two different coating layers are used as model material: a brittle TiB2 and a ductile Ag/Ta duplex layer. The role of the coating layer on the interfacial reaction, interfacial properties, and mechanical behavior of the composites was characterized. Results indicate that both TiE$2 and Ag/Ta are effective diffusion barriers in preventing fiber/matrix interfacial reactions during composite consolidation. However, the deformation mechanisms and crack propagation characteristics in these two coated composites are quite different. The criteria for selecting an improved interlayer to tailor a strong and tough fiber-reinforced titanium aluminide matrix composite are also discussed.

I. INTRODUCTION Continuous fiber-reinforced titanium aluminide intermetallic matrix composites are prime candidates for high-temperature aerospace applications.1'2 However, several key issues challenge the fabrication and application of these composites. A major factor is their complex interfacial phenomena. The high reactivity between the Ti3Al (a 2) matrix and reinforcing ceramic fibers leads to the formation of brittle reaction products at the fiber/matrix interface. When subjected to mechanical or thermomechanical loading, premature cracking of the brittle reaction layer causes severe degradation of the mechanical properties.3 Also, the high coefficient of thermal expansion (CTE) mismatch between the fiber and matrix leads to the development of complicated thermal residual stresses at the fiber/matrix interface.4 These residual stresses coupled with the low ductility of the titanium aluminide matrix can lead to premature cracking of the interfacial reaction layer and matrix during cool-down from high-temperature processing or application. When subjected to thermal cycling, such cracking can be exacerbated by oxidation along crack faces, thus limiting the useful life of the material. Furthermore, the interfacial reaction and residual stress also play an important role in affecting the interfacial shear strength and post-debonded frictional stress.5 The interfacial shear strength and frictional stress are the key parameters controlling the damage mechanisms of this composite under mechanical loading. Therefore, understanding and controlling the chemical, physical, and mechanical phenomena that occur at the fiber/matrix interface are required. J. Mater. Res., Vol. 8, No. 4, Apr 1993 http://journals.cambridge.org

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Recently, a significant amount of work has been aimed at developing a better understanding of the complex interfacial phenomena and their effects on the damage mechanisms. The detailed microstructure of the interfacial reaction zones and the kinetics of the interface reaction have been characterized.6"14 Micro