Fatigue-crack growth and fracture resistance of a two-phase (γ + α₂ ) TiAl alloy in duplex and lamellar microstructures

The fatigue-crack propagation and fracture-toughness behavior of a two-phase (γ + α₂ ) Ti-47.3Al-2.3Nb-1.5Cr-0.4V (in at.%) alloy in fine-equiaxed (duplex) and fully-lamellar microstructural conditions were examined at room temperature. It is observed that the lamellar microstructure displays superior fracture toughness and fatigue-crack growth resistance compared with the duplex microstructure, although the extent of improvement is significantly greater during quasi-static fracture. Crack extension under monotonic loading is characterized by resistance-curve behavior with plateau (steady-state) toughnesses of about 30 MPa m^½ in the fully-lamellar condition compared to a crack-initiation toughness of about 10 MPa m^½ for the duplex. Corresponding thresholds for cyclic fatigue-crack propagation are of the order of 10 and about 6.5 MPa m^½ in the lamellar and duplex structures, respectively. Such improved properties of the lamellar alloy are attributed principally to extrinsic shielding effects from tortuous crack paths, microcracking and formation of shear ligaments resulting from the microlaminated, composite-like features of the alloy.