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

Abstract

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.