Fatigue Failure Behavior of Inconel 718 Alloy under High Temperature Conditions

To investigate the high temperature low cycle fatigue and cyclic softening behaviour of Inconel 718 (IN718) alloy for aero engine hot section components under air environment, fully strain controlled fatigue tests were performed at 23 ℃, 650 ℃ and 700 ℃ with total strain amplitudes ranging from 0.5% to 0.7%. The average fatigue lives corresponding to strain amplitude of 0.5%, 0.6% and 0.7% at 23 ℃ are 8050, 4250 and 1970 cycles, respectively; those at 650 ℃ are 4566, 1928 and 765 cycles; and those at 700 ℃ are 1835, 905 and 235 cycles. Electron back scatter diffraction (EBSD) was employed to characterize the local orientation gradients of post fatigue specimens, and kernel average misorientation (KAM) was used for statistical evaluation. The average KAM values of the as heat treated specimen and fatigued specimens tested at 23 ℃, 650 ℃ and 700 ℃ are 0.20°, 0.75°, 0.58° and 0.45°, respectively. Specimens fatigued at room temperature exhibit higher local orientation gradients and more pronounced deformation inhomogeneity. As the test temperature rises, the post fatigue KAM decreases, indicating that high temperature promotes dynamic recovery and rearrangement of dislocation substructures. Based on Lemaitre's strain equivalence principle, the degradation of cyclic load bearing capacity is characterized by a dimensionless internal variable of equivalent softening, which is multiplicatively coupled with the Coffin Manson relation. A response relationship among total strain amplitude, temperature and equivalent softening rate is further established. Parameter identification was carried out using data from 27 specimens. The coefficient of determination R² between predicted and experimental lives in logarithmic domain reaches 0.982, with a mean absolute percentage error of 11.2%. All data points fall within the ±1.5 scatter band. The proposed model is dimensionally consistent. It can describe the nonlinear degradation of cyclic load bearing capacity before macroscopic crack propagation, and provides a reference for low cycle fatigue life evaluation of IN 718 alloy within 23 ℃-700 ℃.