Structure and behavior of metal/ceramic interfaces in Ti alloy/SiC metal matrix composites
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The structure and mechanical behavior of the fiber/matrix interface in Ti alloy/SCS-6 SiC metal matrix composites were studied. In these composites the interface region consists of a fiber-coating region and a metal reaction zone between the SiC fiber body and the metal matrix. The fiber coating consists of a number of zones or layers which are comprised of cubic SiC particles in a turbostratic carbon matrix. Some ambiguity remains, concerning the number of distinct layers and the size, shape, and density of the SiC particles. The effect of composite fabrication and heat treatment on the coating structure is relatively small. Studies of the metal reaction zone adjacent to the fiber in Ti alloy/SCS-6 SiC MMC's have shown that a number of discrete zones or layers form. Nearest the fiber, a zone of cubic TiC occurs, with increasing grain size with distance from the fiber. Nearest the metal matrix, a zone of Ti5Si3 forms. In high Al content alloys, an intermediate zone forms that consists of Ti 2 AlC or Ti 3 AlC. The fiber/matrix interface plays an important role during transverse tensile loading of these composites. The tensile behavior is controlled by debonding at the interface, followed by deformation of the matrix ligaments. Replica observations show that the debonding initiates and propagates within the coating layers, but is not confined to a single layer interface.
I. INTRODUCTION Titanium and titanium aluminide/SiC fibrous composites are presently under investigation for possible use in aircraft engine and aerospace components, and their fabrication, microstructure, and properties are being studied in some detail.1"22 These composites can exhibit substantial strength increases relative to the unreinforced matrix alloys. However, ductility of the composites can be limited, especially at room temperature, and failure can be seen before reaching predicted values of ultimate tensile strength. The low ductility and premature failure may be related to phenomena that occur at the fiber/matrix interface.20"22 There have been a number of recent studies that have shown that two distinct regions need to be considered at that interface. The first of these is a reaction zone that occurs on the metal side of the original matrix/fiber interface and is formed by diffusion of Si and C from the fiber into the metal matrix. Studies have shown that brittle ceramic phases occur in this zone.8"16 The second important region of investigation is the fiber coating. One of the SiC fibers that is often used for Ti composites (SCS-6, manufactured by Textron Specialty Materials) is coated with a complex C/Si coating to protect the fiber during handling and processing.1'2'6'15"1823 The nature of this coating in the as-fabricated fiber, and changes caused by composite fabrication and heat treatment, are of great interest since 1158
http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 5, May 1993
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they may affect the strength of the fiber and composite or the formation of the metal reaction zones. Recently, a
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