Effects of Grain Boundary Misorientation on the High-Cycle Fatigue Behavior of Nickel-Based Superalloy Bicrystals

Nickel-based single-crystal superalloys are key materials for manufacturing aero-engine turbine blades. Different grain boundaries are inevitably formed during the production of superalloys and weaken the fatigue performance of the alloys. Systematic exploration of the effect of grain boundary misorientation (GBM) on the fatigue properties of superalloys is of great significance. Available research cannot fully illustrate the influence of GBM on the high-cycle fatigue (HCF) damage mechanism of superalloys, especially its coupling with inherent casting defects. In this study, bicrystal specimens with misorientations of 4°, 8° and 12° were fabricated from a second-generation nickel-based single-crystal superalloy. The influence mechanism of misorientation variation on HCF performance was systematically investigated. The test results show that the HCF life of the alloy decreases obviously as GBM rises from 4° to 8° and then declines slowly. Fracture analysis indicates that fatigue damage is closely associated with GBM and casting defects. A 4° grain boundary promotes coordinated deformation and inhibits cracking, whereas misorientations over 8° cause dislocation pile-up and speed up crack propagation. Based on the significant effects of GBM and casting defects on fatigue damage behavior, as well as the analysis of the two key parameters in the Basquin model, a linear correlation is established between the fatigue strength coefficient (σ′f) and misorientation; a coupling relationship is constructed among the fatigue strength exponent (b), misorientation, and defect size. Prediction results confirm that the model achieves higher accuracy by incorporating casting defect parameters.