Improved Fracture Toughness of Polycrystalline γ-TiAl-Based Intermetallic Alloys with a Favorable Deformation Mechanism of Twinning

High-density deformation nanotwins markedly strengthen TiAl-based alloys; however, the improvement in strength generally leads to a reduction in fracture toughness for most structural materials. It is, therefore, necessary to investigate the benefits of high-density deformation nanotwins for the improvement in fracture toughness of TiAl-based alloys. Herein, the fracture toughness of two Ti-45.5Al-4Cr-2.5Nb alloys with a favorable deformation mechanism of twinning (prepared by annealing the continuous casting (C. C.) alloy at 1250 °C and 1270 °C for different durations, respectively) is investigated and compared with that of the unannealed continuous casting (C. C.) alloy, in terms of room-temperature (RT) tensile properties and microstructures. It is found that the two heat-treated Ti-45.5Al-4Cr-2.5Nb alloys exhibits a higher fracture toughness than the C. C. alloy. Shear ligaments and slip bands are the main toughening mechanisms for the two heat-treated alloys; their generation is closely related to the enhancement in the plastic deformability of lamellar structures. In addition, the increase in (B2 + γ)-coupled structures is found to impose a negative effect on the toughening of the investigated alloys. High-density deformation nanotwins favor the improvement in fracture toughness of TiAl-based alloys by improving their fracture strength and plastic deformability, while decreasing their work-hardening exponent.