FATIGUE-CRACK GROWTH THROUGH THE LAMELLAR MICROSTRUCTURE OF AN ALLOY BASED ON TIAL AT 25-DEGREES-C AND 800-DEGREES-C

Fatigue cracks were grown at 25-degrees-C and 800-degrees-C in a titanium aluminide alloy heat-treated to give a gamma + alpha2 lamellar microstructure. These lamellae, having widths of almost-equal-to 0.5 to 2 mum, were in colonies approximately 1.2 mm across. Crack growth was observed and photographed under high resolution conditions using a loading and heating cyclic stage for the scanning electron microscope. Stereoimaging was used to measure displacements around crack tips, from which crack opening displacements and strains were derived. Cracks were found to grow about 10 times faster at 25-degrees-C than at 800-degrees-C, and the threshold stress intensity for fatigue crack growth was lower at 25-degrees-C. Strain to fracture the lamellae was determined as almost-equal-to 0.08, while fatigue crack tips could sustain up to 0.3 strain at 25-degrees-C and 0.5 strain at 800-degrees-C. The lamellar microstructure was found to have a strong influence on crack tip behavior.