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GH4169 at 650 °C in atmosphere was investigated by using single edge notch tensile specimens. The number of main cracks and crack initiation mechanisms at the notch surface strongly depended on the grain size. The crack initiation life accounted for more percentages of the total fatigue life for the alloy with smaller grain size. The fatigue life generally increased with increasing crack initiation life. The small crack transited to long crack when its length reached ˜10 times the grain size.

The aim of the present work is to propose a generalized strain energy density exhaustion model to predict the creep-fatigue lives. The oxidation damage, which is described by a correction factor representing the non-linear oxidation damage mechanism, is considered in the proposed model. All the 77 experimental data sets of GH4169 superalloy at 650 °C, Inconel 738 superalloy at 850 °C and P91 steel at 550 °C in both tension-hold-only and compression-hold-only tests are used to validate the prediction capabilities of the model. The fatigue, creep and oxidation damages per cycle are separately calculated. Then a three-dimensional damage interaction diagram combined with a suitable enveloping surface is proposed.

The purposes of the present work are to extend the previous energy-based model on the basis of strain energy density exhaustion and to estimate creep-fatigue endurance and accumulated damage using cycle-by-cycle concept in tension-hold-only tests. The nickel-based GH4169 superalloy at 650 °C is employed to fit material constants and to verify the prediction capacity of the present model under various loading conditions. The present model exhibits a higher accuracy than some existing models, especially for certain loading waveforms, where the half-life cycle cannot be considered as a representative one. In addition, the trajectory for time-dependent accumulated damage during the operating period can be monitored in the creep-fatigue interaction diagram.

The effect of heat treatment on the strain-controlled fatigue behavior of Nickel-based GH4169 alloy at 650 °C was investigated. The volume fraction of δ phase increases as the longer solution annealing time. The maximum phase fraction reaches 13.79% after double-aging heat treatment. At the same time the yield strength and the tensile strength of GH4169 alloy decrease gradually. The heat treatment had almost no effect on fatigue life at strain amplitude higher than 0.6%. However, the fatigue life slightly increased with increasing the solution annealing time at strain amplitude lower than 0.5%. The alloy does not exhibit dual-slope Coffin-Manson relationships at 650 °C. The needle-like δ phase obstruct movement of dislocations. This makes the alloy exhibited the high-temperature fatigue life higher than the room-temperature fatigue life at low strain amplitude.