Effects of thermal cycling and microstructure on the fatigue crack propagation in forged titanium-aluminide alloys under thermomechanical fatigue conditions

The mechanical properties and fatigue strength of titanium-aluminide (TiAl) alloys are sensitive to the environmental conditions, such as temperature, and their microstructures can be controlled by thermomechanical processing. In this study, two samples of a forged TiAl alloy were manufactured through high-temperature forging followed by different heat treatments to obtain a near-lamellar microstructure and a triplex microstructure, which contains lamellar and equiaxed gamma and beta grains. The fatigue crack propagation tests were conducted under isothermal low-cycle fatigue (LCF) and the out-of-phase type thermomechanical fatigue (OP-TMF) conditions. The experimental results indicated that the microstructure strongly affects the crack propagation behavior because the near-lamellar microstructure had a higher resistance to fatigue crack propagation compared to the triplex microstructure. This also revealed that the fatigue crack was remarkably accelerated by the OP-TMF conditions compared to the LCF conditions. The oxygen diffusion into the beta phase occurred at the crack tip and lead to the transformation of the beta phase into the brittle alpha phase. The results of the scanning electron microscope (SEM), energy dispersive X-ray (EDX), and electron backscatter diffraction (EBSD) analyses indicated that this transformation induced the acceleration of crack propagation under the OP-TMF loading conditions.