Thermal fatigue of a SiC/Ti-15Mo-2.7Nb-3Al-0.2Si composite

The influence of thermal cycling and isothermal exposures in air on the residual ambient temperature strength of SCS-6/Ti-15Mo-2.7Nb-3Al-0.2Si (weight percent) metal-matrix composites comprised of [0]4 and [0/90]s laminates has been determined. A maximum temperature of 815 °C was used in thermal cycling and isothermal exposure. Temperature range, cycle count, maximum/minimum temperature, environment, and hold time at temperature were systematically varied. Postexposure ambient-temperature tension testing, scanning electron and optical microscopy, and fractography were performed on selected specimens to determine the degree of damage. A reduced residual strength was noted in thermal fatigue with increasing cycle count, maximum temperature, and hold time for all specimens tested in air. Isothermal exposures at 815 °C also substantially reduced residual ambient-temperature strength. Considerably less reduction in strength occurred in inert environment than in air. Damage processes included matrix cracking, fiber/matrix interface damage, matrix embrittlement by interstitials, and oxide scale formation at specimen surfaces and, in some cases, at matrix/fiber interfaces. Fiber orientations which allowed rapid ingress of oxygen lead to greater matrix embrittlement and resulted in more pronounced reductions in strength.