Microstructural Investigation and High-Temperature Oxidation Performance of K417G Alloy Prepared by Wide-Gap Brazing

K417G superalloy is widely applied in gas turbine components such as blades, vanes, and nozzles. In this work, the oxidation behavior and mechanism of K417G alloy prepared by wide-gap brazing were investigated in air at 800, 900, 1000, and 1100 °C. Microstructures of the bonded joints differ in the wide-gap braze region (WGBR) and base metal (BM). The surface and cross-sectional morphology, composition, and structure of specimens were analyzed by XRD, SEM, and EDS after oxidation tests. The experimental data demonstrate that the WGBR (wide-gap brazed region) exhibits markedly superior oxidation resistance compared to the BM (base material) under elevated-temperature conditions exceeding 1000 °C. This performance disparity is quantitatively validated by oxidation kinetics analysis, where the weight gain curve of the WGBR demonstrates parabolic oxidation kinetics, as evidenced by its significantly lower parabolic rate constant relative to the BM. The oxide layers of the BM and WGBR are similar after oxidation at high temperatures of 800–900 °C, and they consist of an outermost layer of NiO, a middle mixed layer of Cr2O3, and an innermost layer of dendritic Al2O3. However, when the temperature is between 1000 and 1100 °C, the NiO on the surface of the BM spalls of due to thermal expansion coefficient mismatch in coarse-grained regions, resulting in oxidation mainly divided into outer layer Cr2O3 and inner layer Al2O3 and TiO2. Under high-temperature oxidation conditions (1000–1100 °C), a structural transition occurs in the oxide scale of the BM, with the underlying mechanism attributable to grain-coarsening-induced oxide scale destabilization. Specifically, the coarse-grained structure of the BM (characteristic grain size exceeding 50 μm) is exhibited. Therefore, the WGBR demonstrates outstanding oxidation resistance, as evidenced by the formation of a continuous Al2O3 scale with parabolic rate constants of about 1.38 × 10−3 mg2·cm−4·min−1 at 1100 °C.