Microstructural investigation and creep behavior of long-term exposed directionally solidified GTD-111 nickel based superalloy

In this research, the effect of the long-term exposure on the creep behavior and microstructure of a directionally solidified GTD-111 precipitation-hardened Ni-based superalloy extracted from a turbine blade, after 76,000 exposure hours, without any interval rejuvenations was investigated. The microstructure observation results demonstrated that there are considerable changes in the microstructure features such as the decomposition of MC carbide, coarsening and spheroidization of primary γ′ (increasing size from 600 to 750 nm and circularity factor from 0.6 to 0.78), dissolution of secondary γ′, and formation of huge deleterious topological closed packed (TCP) phases which profoundly impact its mechanical properties. Furthermore, the rafting phenomenon occurred during the long-term operation at the perpendicular to the loading direction. The formation of dislocation networks can be related to the dislocation cross-slipping in the narrow γ channels, and transition electron microscopic observations confirmed that limited dislocations can shear the γ′ phases. The results showed that the deformation mechanism of the exposed alloy during the steady state creep is dislocation cross slipping and climbing. Meanwhile, the creep behavior was investigated in the longitudinal direction by Larson-Miller, Monkman-grant, and modified Norton-Baily model at stress ranging from 150 to 310 MPa and temperature of 871–982 °C. Long-term exposure to DS GTD-111 significantly decreases its creep properties due to severe microstructural degradation. Based on the fracture analysis of the crept specimens, the main fracture mechanism was an interdendritic fracture, and also the decomposed MC and TCP phases were revealed on the fracture surface.