Microscopic damage mechanisms during fatigue crack propagation at high temperature in GH4169 superalloy

The air environment strongly influences the damage behaviors of superalloys at high temperatures. To investigate the microscopic damage mechanisms during high-temperature fatigue crack growth in standard heat-treated GH4169 superalloys, low-cycle-fatigue crack growth tests were conducted at 650℃ with initial stress intensity factor ΔK=30 MPa·m^1/2 and stress ratio R=0.05 under the air environment. The fracture surface, outside surface, and central sectioned surface of the specimen were observed and analyzed using scanning electron microscopy (SEM) and energy-dispersive spectrometry (EDS). The results show that the main fatigue crack initiates and propagates intergranularly, followed by the appearance of intergranular secondary cracks, whose quantity and length increase along the propagating direction of the main crack. In the rupture stage, a dimpled morphology appears on the fracture surface. Oxidation occurs at the interfaces between δ phases and the matrix during the fatigue crack propagation process, which leads to secondary cracks propagating along the interfaces. This leads to their inflection, which in turn retards their propagation. A grain-boundary oxidation damage zone exists at the outside surface of the specimen near the main crack. The size and degree of grain-boundary cracking increase along the propagating direction of the main crack.