A study of the reaction zone in an SiC fiber-reinforced titanium alloy composite
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I.
INTRODUCTION
S I L I C O N carbide fiber-reinforced titanium alloy composites hold immense potential for structural application in the aerospace industry as they can lead to a major weight savings due to higher specific mechanical properties, better dimensional stability, and strength retention to elevated temperatures compared to their monolithic counterparts. However, reactivity of titanium alloys with fibers at fabrication temperatures and subsequent use temperatures degrades the properties of metal matrix composites (MMC's) through formation of brittle reaction products, t~-41 This has adversely affected the development of these composites. Approaches, such as surface coatings of SiC fibers and alloy additions to matrices, I51 are designed to protect the fiber by reducing fiber/matrix interaction and reducing the activity of diffusing species through the reaction zone. An important consequence of diffusion of elements during fabrication and postfabrication thermal exposures of composites is the localized changes in hardness of the matrix around the reaction zone. This may influence the load transfer capabilities of the matrix to the fiber. Thus, to achieve the potential of these MMC's, not only an understanding and control of the reaction zone are necessary, but also information on modification of the matrix and fiber around the reaction zone is required. As a continuation of an earlier work, 16~the present paper describes results of microstructural characterization of the fiber, matrix, and composite with special emphasis on the reaction zone in both as-fabricated and thermally exposed conditions. In addition, determination of hardness variations in the matrix around the reaction zone is presented. These results are discussed in light of G. DAS, Senior Scientist, is with Metcut-Materials Research Group, Wright-Patterson Air Force Base, OH 45433-0511. This paper is based on a presentation made in the s y m p o s i u m "Interfaces and Surfaces of Titanium Materials" presented at the 1988 T M S / A I M E fall meeting in Chicago, IL, September 2 5 - 2 9 , 1988, under the auspices of the TMS Titanium Committee. METALLURGICAL TRANSACTIONS A
the present understanding of the reaction zone in other SiC/Ti alloy composites.
II.
MATERIALS AND PROCEDURES
IMI-829, a near a-titanium alloy, in the form of powder ( - 3 5 mesh) produced by the plasma rotating electrode process (PREP) I7~is used as the matrix. The chemical composition of IMI-829 powder is illustrated in Table I. SiC fibers (Avco SCS-6 single coated) are used as reinforcements. A schematic of the fiber cross section is shown in Figure l(a). It is produced by chemical vapor deposition (CVD) on an -37-/xm-diameter carbon filament. The -142-/xm-diameter fiber has two distinct zones of columnar/3-silicon carbide grains separated by a midradius (MR) boundary. An outer coating is applied to blunt the intrinsic surface flaws resulting from CVD and to promote abrasion resistance tSl and compatibility with the matrix. It contains an overlayer of carbon-rich nons
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