Ni-based superalloys are widely used in the turbine blades of aero-engines and land-based gas turbines as these applications require excellent high-temperature mechanical properties and corrosion resistance. However, longtime exposure of the Ni-based superalloy to a high-temperature environment leads to a change in its microstructure and a decrease in the high-temperature strength. Ni-based superalloys are widely used in the turbine blades of aero-engines and land-based gas turbines as these applications require excellent high-temperature mechanical properties and corrosion resistance. However, longtime exposure of the Ni-based superalloy to a high-temperature environment leads to a change in its microstructure and a decrease in the high-temperature strength. Therefore, the prediction of the fatigue life of service-exposed alloys is important for improving the efficiency of the targeted component.
In this study, the fatigue properties of service-exposed GTD-111 DS alloys and base metal were investigated. The usage history of the gas turbine blades is about 20,000 equivalent operating hours. Low-cycle fatigue tests were carried out at different temperatures (room temperature, 760, and 870 °C) and strain amplitudes. Prior to the fatigue test, the microstructures of the used airfoil and the base metal were compared to determine the extent of degradation during the service exposure. The microstructural examinations showed the presence of coarsening, MC carbide decomposition, and M23C6 carbide formation, indicating degradation of the microstructure of the airfoil, along with the formation of s phase, which is a topologically close-packed phase. The results of the low-cycle fatigue tests indicated that the fatigue life of the base metal is the longest at 760 °C and the shortest at 870 °C, whereas the fatigue life of the used airfoil decreased with increase in the test temperature. In addition, the fatigue lives of the base metal and the used airfoil decreased with increase in the total strain amplitude. A cyclic hardening response was observed at room temperature and 760 °C; however, the tests conducted at 870 °C showed a cyclic softening response. In addition, stress relaxation was observed at 870 °C because of the creep effects of the holding time. The fatigue life was evaluated using the Coffin-Manson equation, and the predicted fatigue lives were compared with the experimental results. Fractography carried out by scanning electron microscopy indicated that initial cracks originated from the carbides. The cracks propagated along the carbides, and secondary cracks were observed in the region of crack propagation.