Fatigue cracking characteristics of β-annealed large colony Ti-11 alloy

In order to better understand the large scatter in the fatigue results associated with β-annealed microstructures of α+ β titanium alloys, the fatigue crack initiation and propagation behavior of thin center notched Ti-11 specimens with a large colony α platelet microstructure was investigated. Colonies with a mean intercept diam of greater than 1 mm were grown in 2 mm thick specimens by means of a vacuum β-annealing process. This enabled crack path morphologies and crack propagation rates to be determined within single colonies by means of optical microscopy on the polished and etched surfaces. The results showed that the fracture is related to a shear mechanism across the colonies from the initiation stage through the overload fracture. Intense shear bands were observed ahead of and in the same direction as the propagating cracks. The density of the shear bands increased with increasing stress intensity. Since the colonies are randomly oriented, the fatigue cracks propagated at various angles with respect to the tensile axis. The crack propagation rate across a single colony is no faster than the propagation rate in the equiaxed α+ β microstructure of the same material. However, cracks were halted at the colony boundaries and forced to reinitiate through a cycle consuming process into the next colony. It is mainly this reinitiation process and microstructurally dependent growth which are responsible for the slower crack growth rates and large scatter band obtained for the β-annealed microstructures when compared to the α+ β microstructures. It is suggested that by reducing the colony size the crack growth rates will be reduced and the fatigue scatter band will be narrowed.