FAILURE MECHANISMS DURING ISOTHERMAL FATIGUE OF SIC/TI-24AL-11NB COMPOSITES

Failure mechanisms during isothermal fatigue of unidirectional SiC/Ti-24Al-11Nb (at.%) composites have been determined by microstructural analysis of samples from tests interrupted prior to the end of life and from tests conducted to failure. Specimens from three regions of life were examined based on the maximum strain from a fatigue life diagram: Region I (high strain). Region II (mid-strain) and Region III (low strain). Crack lengths were also measured from interrupted samples and compared based on temperature (23-815 degrees C), region of life and numbers of cycles. Region I was controlled by fibre-dominated failure. A transition zone was observed between Regions I and II due to competition between failure mechanisms. Failure in Region II was generally described as surface-initiated cracking with varying amounts of fibre bridging. However, the specific descriptions of crack propagation through the fibres and matrix varied with strain and temperature over this broad region. Region III exhibited endurance behaviour at 23 degrees C with no cracking after 10(6) cycles. However at 425 degrees C, surface-initialed cracking was observed after 10(6) cycles with fractured fibres in the crack wake. If endurance behaviour exists for conditions of isothermal fatigue in air at temperatures of greater than or equal to 425 degrees C, it may only be found at very low strains and at greater than 10(6) cycles.