Low-Cycle Fatigue and Creep-Fatigue Behaviors of a Second-Generation Nickel-Based Single-Crystal Superalloy at 760 °C

Low-cycle fatigue (LCF) behaviors of a second-generation nickel-based single-crystal superalloys with [001] orientation at 760 degrees C have been investigated. Different strain amplitudes were introduced to investigate the creep-fatigue effects. The LCF life of none tensile holding (NTH) was higher than that of the 60-s tensile hold (TH) at any strain amplitude. As the strain amplitude was 0.7%, the stacking and cross-slip dislocations appeared together at the gamma/gamma' coherent microstructure in both TH and NTH specimens. At the strain amplitude of 0.9%, plenty of the cross-slip dislocations appeared in gamma channel and other dislocations were stacking at gamma/gamma' interfaces. However, the SFs still appeared in gamma' phase with 60-s TH which caused cyclic softening. As the strain amplitude increased up to 1.2%, the dislocations are piling up at the gamma/gamma' interfaces and cutting through the gamma' phase in both TH and NTH tests, which caused cyclic hardening. The influences of strain amplitude and holding time were complicated. Different stress response behaviors occurred in different loading conditions. The surface characteristic and fracture mechanism were observed by scanning electron microscopy. This result is helpful for building the relationship of various blade fatigue failure modes, cyclic stress response and microstructure deformation under different strain amplitudes.