Enhanced high-temperature fatigue crack growth resistance in a nickel-based superalloy with engineered irregular grains and fan-type γ′ structures

The fatigue crack propagation characteristics of the nickel-based superalloy GH4742 with two distinct microstructures, a fine-grained material and an irregular-grained material containing large fan-type γ′ structures, were systematically investigated over a temperature range from 20 °C to 750 °C. Fatigue crack growth rate tests show that the material with irregular grains and fan-type γ′ structures exhibits a pronounced reduction in crack growth rate at elevated temperature, with the performance difference between the two materials increasing markedly with temperature. Fractographic analysis reveals a transition from boundary-assisted or intergranular crack propagation in the fine-grained material to predominantly transgranular crack growth in the material containing fan-type γ′ structures. At elevated temperatures, fatigue cracks are repeatedly deflected and forced to cut across fan-type γ′ structures, leading to increased crack path tortuosity and crack front segmentation. Kernel average misorientation analysis near the fracture surface indicates enhanced deformation partitioning at elevated temperatures in the material with fan-type γ′ structures, whereas the fine-grained material maintains a more homogeneous deformation pattern. The combined evidence demonstrates that the superior high-temperature crack growth resistance of GH4742 with irregular grains and fan-type γ′ structures arises from the suppression of grain boundary-controlled cracking and the activation of γ′ mediated crack retarding mechanisms.