Effect of microstructure on the fracture toughness of multi-phase high Nb-containing TiAl alloys

A series of fracture toughness tests was conducted on specimens of multi-phase high Nb-containing TiAl alloys with three different microstructures. The fracture path profiles and microscopic toughening mechanisms were analyzed to investigate the effect of constituent phases and microstructures on the fracture toughness. The results show that the fracture toughness of the multi-phase TiAl alloys mainly depends on the lamellar structure and the phase arrangement at colony boundaries. A combination of microscopic toughening mechanisms including crack deflection, shear ligaments, plasticity and microcracking contributes to the toughness of high Nb-containing TiAl alloys. Twinning and dislocation glide in γ phase as well as prismatic slip in α₂ phase promotes the plasticity toughening. B2 phase is normally detrimental to the crack resistance due to its brittleness. However, the B2 phase located at colony boundaries results in microcracking away from the main crack, which is regarded as a toughening way.