Laminate Orientation and Thickness Effects on An Scs-6/Ti-24ai-11 Nb Composite Under Thermal Fatigue
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LAMINATE ORIENTATION AND THICKNESS EFFECTS ON AN SCS-6/Ti-24AI11 Nb COMPOSITE UNDER THERMAL FATIGUE WILLIAM C. REVELOS* AND ITZHAK ROMAN** *Materials Directorate, Wright Laboratory, WL/MLLN Wright-Patterson AFB, OH 45433 "**The Hebrew University of Jerusalem, Jerusalem, Israel 91904 ABSTRACT A SiC/Ti-24AI-1 1Nb (at. %) composite (30-35 vol. %) was thermally cycled in air and an inert environment between 150 0C and 815 0 C for various cycle counts. Various hold times at maximum temperature were employed to determine timedependent effects on composite integrity. Laminate orientations investigated included: [014, [018, [90]4, [90]8 and [0/9012S. Acoustic emission produced during thermal fatigue of selected specimens was employed to monitor damage progression. Post-cycling room temperature tension tests as well as optical and scanning electron microscopy were used to document damage, which was particularly acute when hold times at temperature were employed on tests performed in air. The roles of the environment, composite thickness, and off-axis fibers during thermal fatigue on the composite strength and integrity are discussed. INTRODUCTION Continuously reinforced titanium aluminide metal matrix composites have potential for application in advanced engineering systems where specific strength and stiffness, environmental stability, and durability at elevated temperature are paramount. Specific applications under consideration include advanced gas turbine engines and hypersonic vehicles. Recent investigations of the thermal fatigue behavior of the a2-based class of titanium aluminide composites, specifically, SCS-6/Ti-24AI-11Nb (at. 1%) have demonstrated their susceptibility to acute damage when tested in air. . 2 This damage, measured by severe reductions in post-cycling room temperature tensile strength, is absent 2when these composites are thermally cycled in vacuum or an inert environment. , 3 It has been proposed that the sensitivity of the a2 matrix to interstitial embrittlement and oxidation is the 1main vehicle for damage accumulation when these composites are cycled in air. , 2 Residual stresses caused by the coefficient of thermal expansion (CTE) mismatch between fiber and matrix is thought to contribute as well; however, the environmental component must be present for significant losses in residual strength to be observed. For composites cycled in air, cracks tend to initiate at the composite surface and propagate in a direction perpendicular to the fiber direction, reach the fiber matrix interface, and allow environmental attack of the interface and subsequent damage to the silicon carbide (SiC) reinforcement. Degradation of the SiC fiber strength appears to be primarily responsible for the loss in residual composite strength after thermal 2 cycling in air. To develop a more complete picture of these phenomena, the present work investigated effects of laminate thickness, orientation, and hold times at maximum temperature during thermal cycling. MATERIAL AND EXPERIMENTAL PROCEDURE The composite used in this s
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