Oxidation and Microstructural Evolution of GTD-111 at 850 °C and 1000 °C

The oxidation behavior and microstructures of the GTD-111 Ni-based superalloy were investigated following heat treatment at 850 °C and 1000 °C for up to 5000 h, using Optical Microscopy (OM), X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), and Transmission Electron Microscopy (TEM). SEM/EDS analysis showed that the microstructure of the samples mainly consisted of γ′ precipitates in the matrix, eutectic phases, and several types of carbides. Cross-sectional analysis revealed that the oxidation region was composed of three layers: a top layer (NiO, TiO2, Cr2O3), a sublayer (Ta2O5, TiO2), and an inner layer (Al2O3), followed by a needle-like Ti-containing phase. The oxidation kinetics followed the parabolic law as a function of time at each temperature. After the heat treatments, the dendritic regions of all specimens consisted of cuboidal primary γ′ precipitates and spherical secondary γ′ precipitates. Chinese-script-like and blocky-shaped MC carbides, as well as three types of M23C6 carbides, were found in the interdendritic region. The fracture mode of the tensile specimens transformed from cleavage (brittle) fracture to ductile fracture as the temperature increased. Cracks were observed inside the MC carbides on the fracture surface, which may serve as significant crack initiation sites.