The fracture resistance of a binary TiAl alloy

The fracture resistance of a binary Ti-47Al (in at, pet) alloy has been investigated. The binary alloy was cast, forged, and heat treated to a fully lamellar microstructure with a colony size of either 640 or 1425 mu m. Fracture toughness tests were performed in a scanning electron microscope (SEM) equipped with a loading stage. Direct observations of,the fracture process indicated that crack extension commenced at a stress intensity level of 1.2 to 4 MPa root m. The crack path was primarily interlamellar and crack extension across an individual colony or,across similarly oriented colonies was relatively easy. In contrast, crack arrest was prevalent when the crack encountered the boundaries of unfavorably oriented colonies. To extend into an unfavorably oriented neighboring colony, the K level of the approaching crack had to be increased significantly to renucleate a microcrack at a location away from the crack tip, resulting in the formation of an interconnecting ligament that must be fractured to further crack growth. This interaction between the crack and the microstructure led to a large variation in the slope of the K-R curves. Comparison of the K-R curves for the binary Ti-47Al alloy against published data for quinary Ti-47Al-xNb-yCr-zV alloys indicates that the initiation toughness of the quinary alloys is higher by a factor of 5 to 10, Implying the existence of a significant beneficial effect of alloying additions on the initiation toughness.