Characterization of deformation mechanisms during low cycle fatigue of a single crystal nickel-based superalloy

The deformation and fracture mechanisms of a single crystal nickel-based superalloy CMSX-4 have been investigated during low cycle fatigue (LCF) tests at temperatures of 750, 850, and 950 °C under strain-controlled R = 0. It was found that LCF lives at 750 and 850 °C were similar and longer than those at 950 °C. The specimens tested at 750 and 850 °C showed fatigue crack initiation at internal pores, and their failure occurred by cracking at persistent {111} slip bands. On the other hand, at 950 °C the crack initiated at the oxide-layered surface and propagated along γ channel until fracture. At the two lower temperatures, a/2 dislocations with low density was rarely present within γ channels, and a/3 partial dislocations were occasionally seen to shear γ′ leaving superlattice stacking faults behind. At 950 °C, homogeneous deformation was produced by perfect dislocation movements of cross-slip and climb in the γ channel and a limited γ′ shearing by superdislocation was observed. At total strain range lower than 0.6%, well-developed polygonal dislocation network formed at rafted γ′ interface. Comparison of dislocation structures revealed that load-controlled LCF tests lead to more severe deformation to specimens than strain-controlled tests.