Abstract
This study reports on the creep–fatigue interaction of the oxide dispersion strengthened Ni-base superalloy PM 1000. Fully reversed symmetrical push–pull isothermal fatigue, thermomechanical fatigue (TMF), slow-fast and tensile hold time tests were conducted in the temperature range from 450 to 1050 °C. TMF tests resulted in unexpectedly low fatigue lives. Grain boundary cavitation observed in the in-phase TMF tests indicated that creep damage plays an important role under TMF loading conditions. Similarly, fatigue life dependence on wave-shape demonstrated that creep–fatigue tests exhibit the shortest cyclic life due to the additional creep damage in the tensile-going part of the cycle. Damage mechanisms were observed to vary substantially depending on the test mode. In the creep–fatigue tests, the grain boundaries of fine equiaxed grains triggered multiple internal crack initiation, thereby pointing to the adverse effects of these microstructural features. Cavitation occurs by a local stress directed diffusion and growth of voids, as evidenced by dispersoid free zones in areas near the transverse grain boundary cavities. Time-dependent damage occurring at and above 850 °C in the creep–fatigue tests is mostly due to creep whereas environmental attack plays only a minor role.
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