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 an alysis 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 st rain). Crack lengths were also measured from interrupted samples and c ompared based on temperature (23-815 degrees C), region of life and nu mbers of cycles. Region I was controlled by fibre-dominated failure. A transition zone was observed between Regions I and II due to competit ion between failure mechanisms. Failure in Region II was generally des cribed as surface-initiated cracking with varying amounts of fibre bri dging. However, the specific descriptions of crack propagation through the fibres and matrix varied with strain and temperature over this br oad region. Region III exhibited endurance behaviour at 23 degrees C w ith no cracking after 10(6) cycles. However at 425 degrees C, surface- initialed cracking was observed after 10(6) cycles with fractured fibr es 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.