Creep–Fatigue Behavior and Life Prediction of FGH95 PM Nickel-Based Superalloy Under Different Control Modes

FGH95 is a powder metallurgy nickel-based superalloy used for aero-engine turbine disks, which can experience combined creep and fatigue loading at elevated temperature. Its response was examined at 620 °C under asymmetric cycling (stress ratio R = 0) using maximum strain control from 1.0% to 2.0% and maximum stress control from 1200 to 1430 MPa, each with either 0 or 30 s tensile dwell. Under stress control, the dwell reduced the mean fatigue life by 96.3–98.4%. Under strain control, the response depended on the maximum strain: the measured life changed from 3343 to 3632 cycles at 1.0%, whereas reductions of 36.5–63.5% occurred at 1.2–2.0%. This difference is associated with continuous ratcheting strain accumulation under stress control and progressive stress relaxation under strain control. A unified strain-based life model was calibrated using creep strain rate and inelastic strain range to represent creep–fatigue interaction. For the calibration dataset, all calculated lives fall within a factor-of-two scatter band and show closer agreement than the conventional time fraction rule, whose maximum error factor is 3.69.