Low cycle fatigue in rene 88DT at 650 °C: Crack nucleation mechanisms and modeling

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Intermediate-temperature low-cycle-fatigue experiments were conducted on three microstructural conditions of the nickel-base superalloy Rene 88DT over several strain ranges. The most significant difference between the microstructural conditions was the grain size; two of the conditions had an average grain diameter of approximately 20 µm, while the third condition had an average grain diameter of 6 µm. Two dominant crack nucleation mechanisms were observed on the specimen fracture surfaces: crack nucleation from surface slip band damage accumulation and crack nucleation around subsurface inclusion clusters. The experimental results indicate that both the grain size and the applied strain range are contributing factors in the prevailing crack nucleation mechanism. The Fatemi-Socie parameter, a multiaxial fatigue damage parameter, was used to examine the ease and probability of surface slip band crack nucleation for these microstructural conditions. The parameter was modified to explicitly include grain size, fatigue R-ratio, and applied strain range, thus giving it a more physically meaningful basis. This modified Fatemi-Socie parameter was found to be a suitable parameter for characterizing the ease of surface slip band cracking for materials with different grain sizes and is able to explain the observed disparity in fatigue lives based on microstructural design.