Low cycle fatigue mechanism of René 80 at high temperature–high strain

The low cycle fatigue René 80, a Ni-base superalloy, was studied at temperature of 871 °C, R = (_min/_max) = 0 and strain rate of about 2 × 10⁻³ s⁻¹. The dislocation structure and failure surface observations were evaluated through TEM and SEM. TEM studies showed that at Δ_t = 0.8% during the first cycle the dislocations formed a hexagonal network in the γ-phase matrix. When the number of cycles increased, the density of dislocations increased as well. At N = N_f and Δ_t = 0.8% the cutting of γ′ precipitates took place. SEM studies at Δ_t = 0.8% and N = N_f showed that fatigue crack initiation generally occurred at the surface, where it is depleted of the γ′ phase as a result of oxidation by the high-temperature exposure. In addition to depleted zones, the grain boundary oxidation and oxide spikes were also considered as further crack initiation sites.