High-temperature fatigue cracking mechanism and microstructure evolution of aero-engine K4169 superalloy in service process

By using fatigue crack propagation testing and microstructural characterization, the crack fracture and propagation mechanisms of K4169 superalloy under various loads were investigated. The results demonstrate that the grain sizes of K4169 superalloy significantly increase, and the precipitation of the needle-like δ phase and the Laves phase is observed. Voids and microcracks form at location of Laves phase enrichment, creating conditions for crack propagation. By the a−N (a is the crack length, and N is the number of cycles) relationship curve, the change in the fatigue crack growth rate with the increasing number of cycles progresses through three separate stages. The fracture process of K4169 superalloy under low-stress cyclic loading (3 kN) exhibits the ductile fracture. Subsequently, the fracture process starts to change from the ductile fracture to the brittle fracture as the stress increases to 4.5 kN. In the microstructures of fractures in both stress states, intergranular propagation is the mechanism responsible for crack propagation. Moreover, the Laves phase exists near the fracture crack, which is in line with the post-service structural phenomenon.