Fracture mechanism of a Ni-base alloy under high-temperature cyclic deformation: Experiments and microstructure characterization

The components of power generations are subjected to the repeated thermal strain, owing to the temperature fluctuations associated with start-up and shut-up. Therefore, fatigue properties of the candidate materials are essential to be understood. To fully elucidate the fracture mechanism and microstructure evolution of the Nickel-based alloy under high-temperature cyclic loading, low cycle fatigue tests were carried out under the strain amplitude of 0.4%, in which temperature was in the range of 700–800 °C and strain rates from 5 × 10 −4  s −1 to 5 × 10 −3  s −1. The fatigued specimens were characterized by transmission electron microscope (TEM), scanning electron microscope (SEM), and electron backscatter diffraction (EBSD) observations. Slip band deformations mainly dominated under the temperature of 700 °C and 720 °C, and paralleled dislocation transmissions around twin boundaries were dominant under 750 °C. While free dislocation lines were typical dislocation configurations in 800 °C. Due to stress concentration caused by the accommodated dislocations, some cracks preferred to propagate along the twin boundary. Molecular dynamics simulated the dynamic interaction between dislocation and twin boundary at 750 °C. Dislocation movement was constrained on the twin boundary, which was verified by molecular dynamics simulations. As the temperature increased, the strength of the grain boundary degraded, and the fracture pattern changed from totally transgranular to both transgranular and intergranular cracking.