Investigation on low-cycle fatigue behaviors of wrought superalloy GH4742 at room-temperature and 700 °C

Abstract Low-cycle fatigue (LCF) behavior of wrought superalloy GH4742 was studied at room-temperature (RT) and 700 °C. Scanning electron microscope (SEM) and Transmission electron microscope (TEM) were used to study the development of cracks and deformation mechanism. An obvious decrease in fatigue life occurred as the temperature was increased from RT to 700 °C and as the total strain amplitude was raised from 0.5% to 1.0%. The Halford-Marrow relationship based on plastic strain energy density was used to predict the fatigue life, which is in good accordance with experimental data. At RT, for all investigated samples, initial cycle hardening is followed by gradual cycle softening especially at higher strain amplitude ( ∆ e t ≥ 0.8 % ). Detailed TEM observation revealed that γ′ phase was sheared by dislocations. The shearing mechanism, reducing the ordering degree and diameter of γ′ phase, should be responsible for fatigue softening. Differently, at 700 °C, the tested samples exhibited continuous cycle hardening. A sharp drop in the number of stacking faults occurred. Lots of dislocation loops existed around tertiary γ′ phase and paired dislocations moved in secondary γ′ phase, indicating that Orowan and shearing mechanism have occurred. Fatigue cracks usually initiated from the surface. SEM investigation showed that the failure mode changed from transgranular to mixed transgranular and intergranular modes with increasing temperature.