The effect of service-temperature exposure on the tensile properties of Ti-1421/SiC metal matrix composites

Composites consisting of a Ti-14Al-21Nb (wt%) aluminide matrix and SCS-6 SiC fibres 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 performed 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 behaviour 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, with the longitudinal room temperature tensile strength decreasing linearly with increasing thickness of the embrittled zone. During air-cycling, this effect was enhanced by a rapid decrease in the strength of the SCS-6 fibre, and an increase in the amount of fibre breakage. Isolating the fibres from direct contact with the environment resulted in a decrease in the amount of composite strength degradation after air-cycling. This revised version was published online in November 2006 with corrections to the Cover Date.