EFFECTS OF MICROSTRUCTURE ON THE STRENGTH AND FATIGUE BEHAVIOR OF A SILICON-CARBIDE FIBER-REINFORCED TITANIUM MATRIX COMPOSITE AND ITS CONSTITUENTS

The results of a systematic study of the effects of microstructure on the strength and fatigue behavior of a symmetric [0/90](2s) Ti-15Al-3C r-3Al-3Sn/SiC (SCS-6) composite are presented along with relevant info rmation on failure mechanisms in the composite constituents, i.e., the interface, fiber, and matrix materials. Damage micromechanisms are el ucidated via optical microscopy, scanning electron microscopy (SEM), a nd nondestructive acoustic emission (AE) and ultrasonic techniques. Co mposite damage is shown to initiate early under eyelid loading conditi ons and is dominated by longitudinal and transverse interfacial cracki ng. Subsequent fatigue-damage occurs by matrix slip band formation, ma trix and fiber cracking, and crack coalescence, prior to the onset of catastrophic failure. However, the sequence of the damage is different in material annealed above or below the beta solvus of the Ti-15-3 ma trix material. Mechanistically based micromechanics models are applied to the prediction of the changes in modulus induced by fatigue damage . Idealized fracture mechanics models are also employed in the predict ion of the fatigue lives of smooth specimens deformed to failure at ro om temperature. The article highlights the potential to develop mechan istically based predictive models based on simplified mechanics ideali zations of experimental observations.