Fatigue and fracture behavior of a fine-grained lamellar TiAl alloy

The fatigue and fracture resistance of a TiAl alloy, Ti-47Al-2Nb-2Cr, with 0.2 at. pet boron addition was studied by performing tensile, fracture toughness, and fatigue crack growth tests. The material was heat treated to exhibit a fine-grained, fully lamellar microstructure with approximately 150-mu m grain size and 1-mu m lamellae spacing. Conventional tensile tests were conducted as a function of temperature to define the brittle-to-ductile transition temperature (BDTT), while fracture and fatigue tests were performed at 25 degrees C and 815 degrees C. Fracture toughness tests were performed inside a scanning electron microscope (SEM) equipped with a high-temperature loading stage, as well as using ASTM standard techniques. Fatigue crack growth of large and small cracks was studied in air using conventional methods and by testing inside the SEM. Fatigue and fracture mechanisms in the fine-grained, fully lamellar microstructure were identified and correlated with the corresponding properties. The results showed that the lamellar TiAl alloy exhibited moderate fracture toughness and fatigue crack growth resistance, despite low tensile ductility. The sources of ductility, fracture toughness, and fatigue resistance were identified and related to pertinent microstructural variables.