Effect of fiber coating on creep behavior of SiC fiber-reinforced titanium aluminide matrix composites
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J.A. Graves Rockwell International Science Center, Thousand Oaks, California 91358 (Received 14 June 1993; accepted 1 September 1993)
The effect of fiber coating on the creep behavior and damage mechanisms of unnotched SCS-6 fiber-reinforced Ti3Al matrix composites under longitudinal and transverse loading was investigated at 700 °C. Stresses ranging from 700 to 900 MPa and 200 to 400 MPa were used for longitudinal and transverse loading, respectively. An Ag/Ta duplex layer was coated onto the SCS-6 fiber prior to consolidation via physical vapor deposition. The microstructure of the crept composites was examined to determine the creep deformation mechanisms. The creep cracking behavior of the notched composites was also studied at initial stress intensity factors, Kh ranging from 15 to 20 MPa-m1/2. Microstructural observation revealed that multiple fiber fracture (at low to medium stress levels), microcracking along the reaction zone/matrix interface (at medium stress levels), and matrix cracking extending from the broken fiber ends (at high stress levels) were the major damage mechanisms during quasi-steady state creep under longitudinal loading. The results show that the Ag/Ta duplex coating significantly improved the creep resistance and flexural strength of the composite under transverse loading. The Ag/Ta duplex coating was also shown to significantly prolong the creep rupture life of SiC fiber-reinforced Ti3Al composites.
I. INTRODUCTION Titanium aluminide (Ti3Al) matrix composites reinforced with continuous SiC fibers have recently received considerable attention for high-temperature aerospace structural applications.1'2 Potential applications include hypersonic airframes and advanced turbine engines where high specific stiffness, high specific strength, and environmental resistance at elevated temperatures are critical. However, several issues associated with the complex physical, chemical, and mechanical interaction between the fiber and matrix must be solved. Excessive chemical reactions between the fiber and matrix lead to the formation of a brittle reaction zone at the interface. Thermal residual stresses induced by the thermal expansion mismatch between the constituents may lead to microcracking of the matrix near the interface during cooling. The processing-induced microcracking and premature failure of the brittle interfacial reaction layer will certainly affect the thermomechanical durability of the composite. Coating the SiC fiber to retard the interfacial reaction and to provide a compliant interlayer between the fiber and matrix is currently being viewed as the most effective approach to improve the interfacial compatibility of Ti-rich alloy matrix composites.3 Recently, Ti3Al-based composites reinforced with Ag/Ta-coated SCS-6 fibers have been successfully 198 http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 1, Jan 1994
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developed.4 Ta, a /8-Ti stabilizer, will diffuse and dissolve into the Ti3Al matrix and form a ductile /3-Ti layer around the fiber. The re
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