Intrinsic and extrinsic fracture resistance in lamellar TiAl alloys

The roles of colony boundary and crack orientations in the fracture resistance of two-phase lamellar TiAt alloys (Ti-46.5Al and Ti-47Al-2Nb-1.6Cr-1V, all in at.%) were investigated. In situ fracture testing of single-colony thick compact-tension specimens was performed at ambient temperature in a scanning electron microscope equipped with a loading stage. Near-tip micrographs of kink or twist cracks approaching a colony boundary were obtained as a function of applied loads and subsequently analyzed using a machine-vision-based stereoimaging technique to determine the displacement and strain fields. Metallographic and fractographic techniques were utilized to measure the kink and twist angles of the crack as well as the orientation of the colony boundary. The stress intensity factors of the kink and twist cracks were computed using a 2D boundary-integral-equation method and a 3D finite-element method. Comparison of the computed stress intensity factors against those deduced from the near-tip strain field indicates that the intrinsic fracture resistance in the lamellar colonies of the TiAl alloys is relatively low (about 3 MPa rootm, but its apparent value can be increased as the result of shear ligament bridging. The magnitude of shear ligament toughening is strongly influenced by the kink and twist angles of the crack, as well as the orientation of the colony boundary. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.