Ex-situ investigation of micro-pore evolution and crack initiation/propagation during fatigue in a single crystal Ni-based superalloy

Ex-situ fatigue tests combined with X-ray Computed Tomography (XCT) observation and detailed microstructure characterization were conducted under low and high stress amplitude at 760 °C to investigate pore evolution, crack initiation and propagation. It was found that, in the early stages of fatigue loading, the decrease in the quantity of solidification/homogenization-pore (S/H-pore) was attributed to the diffusion of vacancies to sample surface and the effect of local deformation. Some “disappeared” (below the resolution of XCT) S/H-pores re-appeared at the same sites as fatigue progressed. The formation of deformation-pores (D-pore) was observed directly, depending on the loading stress amplitude. Under low stress amplitude, the largest S-pore in fatigue sample was the preferred site for crack initiation. Severe local deformation near S-pores induced lattice rotation, the formation of slip band and resulted in micro-crack initiation. New mechanisms of micro-crack formation near the pore in the very early stages of fatigue, involving carbide oxidation and diffusion-induced cavity were also observed. In addition, ex-situ XCT observation revealed that large pores promoted crack propagation, whereas carbides played a lesser role in crack growth. However, at high stress amplitude, cracked carbide at the surface promoted crack initiation and propagation. 3D reconstruction of the crack by XCT indicated that pores had little effect on crack growth at high stress amplitude.