Thermal cycling of Ti-1421/SiC metal matrix composites

Composites consisting of a Ti-14Al-21Nb (wt-pct) aluminide matrix and SCS-6 SiC fibers were fabricated by plasma-spraying and hot-isostatic-pressing. The room temperature longitudinal tensile properties were evaluated after processing and after isothermal and cyclic exposure in air and in argon. The isothermal ages were done at 760 C, and the thermal cycle was from room temperature to 760 C, which is the projected service temperature for Ti-aluminide-based composites. Some loss of tensile strength occurred after isothermal and cyclic exposure for long times (8 to 12 weeks) in argon, with no apparent difference in behavior between the isothermal and cyclic samples. Substantial degradation in tensile properties was seen after isothermal exposure in air, while a much more severe loss in properties occurred after cycling in air. In the isothermal samples exposed in air, degradation in properties was associated with surface embrittlement of the matrix, while during air-cycling this effect was enhanced by a rapid decrease in the strength of the SCS-6 fiber, and an increase in the amount of fiber breakage. 19 refs