Effect of notches and microstructure on the fracture toughness of TiAl-based alloys

The effects of notches and microstructure on the fracture behavior of TiAl-based alloys are systematically investigated. It has been determined that the apparent fracture toughness, KA, for four typical microstructures—which include fully lamellar, nearly lamellar, duplex, and nearly gamma microstructures—exhibits similar variational trends with the notch root radius. KA is independent of the notch root radius ρ when ρ is smaller than the critical radius ρ0, and increases linearly with ρ12 when ρ is larger than ρ0. The value of ρ0 is of the same order of magnitude as the grain size, and the slope of the KA-ρ12 curve is dependent on the microstructure type. The good agreement between the statistical data of the crack initiation site in the blunt bent bar and the peak stress location indicates that the fracture of TiAl-based alloys is controlled by the maximum tensile stress rather than the strain or strain gradients as in Ti3Al-Nb-based alloys. By using the Tetelman model of stress-controlled fracture, the effect of the notch root radius on the apparent fracture toughness can be explained very well.