Evaluation of the Effect of δ-phase Precipitates on High-Temperature Creep Damage of GH4169 Superalloy Based on Microstructural Observations

抄録

The operating temperature of aircraft jet engines has been increased to improve thermal efficiency for addressing global warming by reducing greenhouse gas emissions. The Ni-based superalloy GH4169 (IN718) is used in aircraft components because of its high-temperature strength, fracture toughness, and oxidation resistance. In GH4169, which has a higher Nb content (about five mass%) than other nickel-base alloys, the γ"(Ni₃Nb) phase coarsens at temperatures above 650°C and precipitates as the more stable δ-phase in needle-like form near grain boundaries. Under high-temperature creep loading, fine voids accumulate near the interface between the δ-phase precipitated around grain boundaries and the matrix phase and accelerate intergranular cracking, indicating that the effect of δ-phase precipitates on creep damage evolution should be considered when this alloy is used in a high-temperature condition. To investigate the mechanism by which the δ-phase reduces grain boundary strength, intermittent creep tests were conducted at around 800°C on GH4169. Scanning electron microscopy (SEM) observations were performed on the interrupted creep specimens to quantitatively evaluate the δ-phase content and the temperature and stress dependence of the precipitation rate. The tensile strength decreased in specimens where the δ-phase precipitated compared to specimens without the δ-phase. After creep loading, an increase in the δ-phase content was observed, particularly at 825°C, where precipitation within the grains accelerates, leading to a significant increase in the δ-phase content. Additionally, the content of the δ-phase increased in the region of higher load at the same temperature, indicating that the synergistic effect of temperature and stress accelerates the precipitation of the δ-phase.