Mechanism of high-temperature oxidation effects in fatigue crack propagation and fracture mode for FGH97 superalloy

The high-temperature fatigue crack growth behaviors in powder metallurgy (P/M) Ni-based superalloy FGH97 for turbine disk application were investigated at different temperatures (650, 700 and 800 °C) in air using a combination a servohydraulic test system, fractographic and microanalytical investigations. It is found that there is a temperature-sensitive region in which the fatigue life of FGH97 alloy decreases sharply. To further evaluate the crack propagation mode and oxidation effects, interruption experiments were conducted at 700 and 300 °C, respectively. The results indicate that the reduction of the fatigue lifetime for FGH97 takes place when the fracture mechanism transforms from a predominantly transgranular mode to an intergranular one as the temperature increases. Although the microstructures and mechanical properties may vary with the temperature, they are not the dominating factors contributing to the temperature sensitivity of fatigue property for FGH97. It is the oxidation that governs the fatigue crack growth behaviors in air at elevated temperature. The enhanced thermal activity of oxygen and certain active metal elements result in accelerated oxidation reaction. The brittle oxide intrusions formed at the crack tip and grain boundaries of crack frontier lead to grain boundary weakness, which is responsible for the transformation of crack growth mode and degradation of the fatigue property of FGH97 alloy.