Combined high and low cycle fatigue of DD6 single-crystal superalloy: experiment, modeling, and coupled damage quantification at 850 ℃

This study presents a systematic investigation into the combined high and low cycle fatigue (CCF) behavior under tension-bending loading of a [0 0 1]-oriented DD6 superalloy at its key service temperature of 850 °C. To simulate the multiaxial combined loading spectrum (centrifugal tension and vibrational bending) experienced by actual turbine blades, a novel orthogonal decoupling test platform and fixture were developed. This system independently applies axial low-cycle fatigue (LCF) loads and bending high-cycle fatigue (HCF) loads. Through detailed characterization of fracture morphology, crack growth paths, and microstructural evolution, the damage mechanisms of the DD6 alloy under combined fatigue conditions were revealed. The results indicate that under these multiaxial combined fatigue conditions, the crack initiation stage accounts for 80–90% of the total life, and with the increase in the frequency ratio, both the coupled damage and the HCF damage increase. Unique secondary crack initiation zones were observed on the fracture surfaces, the formation of which is closely related to the contribution of LCF loading. Based on the integrated experimental and finite element analysis, the life predictions made using the Fatemi-Socie critical plane model achieved an accuracy within the twofold scatter band from a damage analysis perspective. Furthermore, through coupled damage analysis, the contributions of HCF damage, LCF damage, and their coupling interaction were decomposed and quantified. This study elucidates the CCF damage mechanisms of the DD6 alloy under combined tension-bending loading, providing a foundation for a more physics-based life prediction methodology for critical turbine blade components.