Characterization of Fiber/Matrix Interfaces by Transmission Electron Microscopy in Titanium Aluminide/Sic Composites
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CHARACTERIZATION OF FIBER/MATRIX INTERFACES BY TRANSMISSION ELECTRON MICROSCOPY IN TITANIUM ALUMINIDE/SIC COMPOSITES CECIL G. RHODES Rockwell International Science Center, 1049 Camino Dos Rios, Thousand Oaks, CA 91360
ABSTRACT This paper presents examples of the use of transmission electron microscopy to characterize matrix/reinforcement interaction in titanium aluminide matrix composites reinforced with continuous SCS-6 type SiC. As a result of the high temperature required for consolidating this type composite, reaction products form in the interface. Using diffraction and x-ray energy dispersive spectroscopy techniques, reaction products in Ti3AI and Ti2AINb alloy matrix composites have been identified. TiC1 -x and Ti5 Si 3 compounds are common in these composites, with AlTi 3 C also present depending on consolidation temperature and matrix composition. Residual stress calculations indicate that these reaction products may be subject to cracking during cooling from consolidation temperatures.
INTRODUCTION There are materials from which certain significant information can only be gained by the use of transmission electron microscopy (TEM). Among those special materials are composites, in which interphase interfaces contribute strongly to composite properties. In the case of composites in which the matrix and reinforcement react chemically during processing, TEM is a useful tool for characterizing reaction products. In those composites in which matrix and reinforcement do not react, the structure of the interface can be analyzed by TEM. For instance, interfacial dislocations and ledges, which can contribute to the strength of interfaces and thus to the 'composite', can only be characterized by TEM. This paper presents some examples of the use of TEM to characterize matrix/reinforcement interaction in titanium aluminide matrix composites reinforced with continuous SiC fiber. The significance of these results in the tailoring of titanium aluminide matrix composites is also discussed. INTERFACES IN TITANIUM ALUMINIDE MATRIX COMPOSITES The state-of-the-art reinforcement for titanium aluminides, whether Ti 3 AI, Ti 2 NbAl, or TiAl based, is SCS-6 SiC. SiC was selected for Mat. Res. Soc. Symp. Proc. Vol. 273. 01992 Materials Research Society
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its high strength and modulus and the carbon-rich coating (SCS) was developed for improved post-consolidation strength of the fiber[l,2]. The titanium aluminides, like all intermetallics, require very high temperature for solid state composite consolidation. As a result of the consolidation cycle, two conditions develop. First, there is diffusional reaction between the titanium aluminide and SiC at the consolidation temperature. Reaction products, on the order of 0.5 to 2.0 gm in breadth, form in the matrix/fiber interfaces[3-9]. Second, differences in coefficients of thermal expansion result in thermal mismatch stresses during cooling from the consolidation temperature. Both of these conditions lead to a complex fiber/matrix interfacial region. In order to control the inter
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