Effect of environment on the thermal fatigue response of an SCS-6/Ti-24Al-11Nb composite

A study has been conducted examining the thermal fatigue characteristics of an α₂/SiC composite; in particular, SCS-6 reinforced Ti-24Al-11Nb (at. pct). The effort included the investigation of the effect of the environment by cycling coated and uncoated specimens in air and in an inert environment. Damage assessment was determined by postcycling room-temperature tension testing as well as by microstructural examination, including both optical microscopy and scanning electron microscopy (SEM). Significant reductions in postcycling tensile strength were observed for coated and uncoated specimens thermally cycled in air from 150 °C to 815 °C for 500 cycles, while no measurable loss of strength was found for specimens cycled in a low-pressure inert environment under otherwise identical conditions. The synergistic effect of residual stresses due to a coefficient of thermal expansion (CTE) mismatch and environment on the degradation of tensile properties of the thermally cycled composite is found to be the critical damage evolution mechanism for both coated and uncoated composites cycled in air. Residual stresses alone were found not to be critical in creating damage that could be tracked by a loss in residual strength.