Influence of Strain Amplitude on Low-Cycle Fatigue Behaviors of a Fourth-Generation Ni-Based Single-Crystal Superalloy at 980 °C

Total strain-control, low-cycle fatigue experiments of a fourth-generation Ni-based single-crystal superalloy were performed at 980 degrees C. Scanning electron microscopy and transmission electron microscopy are employed to determine fracture morphologies and dislocation characteristics of the samples. As the strain amplitude increased from 0.6 to 1.0%, the cyclic stress and plastic strain per cycle increased, the cyclic lifetime decreased, more interfacial dislocation networks were formed, and the formation rate accelerated. Cyclic hardening is associated with the reaction of accumulated dislocations and dislocation networks, which hinder the movement of dislocations. The presence of interfacial dislocations reduces the lattice mismatch between the gamma and gamma ' phases, and the presence of dislocation networks that absorb mobile dislocations results in cyclic softening. At a strain amplitude of 1.0%, the reaction of a high density of dislocations results in initial cyclic hardening, and the dislocation cutting into the gamma ' phase is one of the reasons for cyclic softening. The crack initiation site changed from a near-surface defect to a surface defect when the strain amplitude increased from 0.6 to 0.8 to 1.0%. The number of secondary cracks initiated from the micropores decreased during the growth stage as the strain amplitude increased.

Automated summary

Total strain-control, low-cycle fatigue experiments were conducted on a fourth-generation Ni-based single-crystal superalloy at 980 degrees C. Fracture morphologies and dislocation characteristics of the samples were determined using scanning electron microscopy and transmission electron microscopy. The increase in strain amplitude led to higher cyclic stress and plastic strain per cycle, as well as a decrease in cyclic lifetime. The presence of interfacial dislocation networks and accumulated dislocations contributed to cyclic hardening and softening, respectively. Furthermore, the strain amplitude affected the location and number of crack initiation sites and secondary cracks.