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The fatigue crack initiation process accounts for the main portion of the high-cycle fatigue (HCF) life. Although coated superalloys are well-known to be prone to surface microcracks, the influence of the coating-substrate microstructure on the damage mechanism during HCF of the coated single crystal (SX) superalloys remains unclear. In the current study, HCF failure and interrupted tests were conducted at 900 °C under a low applied maximum stress on a PtAl-coated third-generation SX superalloy using sheet specimens. The HCF crack initiation was investigated by analyzing the evolution of surface microcracks and coating-substrate microstructure. The results show that the surface microcracks nucleation and growth are controlled by the initial microstructure of the coating-substrate region and the oxidation process during the HCF crack initiation stage. The rapid nucleation and growth of the surface microcracks are promoted by grain boundaries (GBs) in the coating and the interdiffusion zone (IDZ). Comparatively, the surface microcracks are blunted by oxidation in the SX substrate due to the loss of GBs, leading to a slower microcrack growth. However, the surface microcracks can gradually grow further into the substrate by repeating the processes as follows: oxidation at the crack tip, formation of γ′-depleted region due to oxidation, recrystallization at the γ′-depleted region and cracking along the GBs in the recrystallized γ′-depleted region. Subsequently, the surface microcracks, which reach the critical size, propagate along the slip bands during the progressive propagation stage. This study will contribute to improve the HCF durability of the PtAl-coated SX superalloy.

An interesting phenomenon is found in Nickel-based single crystal superalloy thin-wall specimens which have the same [001] primary orientation and different secondary orientation with [100], [210], and [110] at 850 °C. Generally, the testing of cylindrical samples exhibits similar creep and tensile properties regardless of their vertical orientation distribution due to the circumferential symmetry of their geometric structure. However, the 850 °C experimental results of thin-wall specimens with the same [001] primary orientation show a significant difference in tensile ductility and creep life when the secondary orientation is different. The mechanism of the secondary orientation effect is completely discussed by combining experiment, digital image correlation (DIC), and finite element modelling (FEM), which result from the strengthening effect of Lomer-Cottrel dislocations and the ability to coordinate plastic deformation in tensile, the shrinkage rate of cross-section caused by the single slip system ({111} ) and deviation from [001] with a small angle in creep.

Hydrogen-fueled and hydrogen-hybridized aircraft engines are a new trend in the aviation industry for environmental reasons. Single crystalline Ni-based superalloys are the most commonly used engine materials and their hydrogen embrittlement properties need urgent investigation. In this study, the hydrogen embrittlement behavior and underlying fracture mechanism of a second-generation Ni-based single crystal superalloy with electrochemical hydrogen pre-charge were investigated. The superalloy showed tremendous susceptibility to hydrogen embrittlement with reduced strength and ductility. A large number of micropores and cracks on the fracture surface are found in hydrogen-charged specimens, leading to embrittlement and ultimate cracking. More dislocations, stacking faults and DSBs are observed in specimens with hydrogen uptake. Hydrogen-induced micropores first form at the γ/γ′ interface and then propagate into the γ′ phase, leading to cracking, which was analyzed using in situ environmental studies with a transmission electron microscope. Hydrogen reduces the cohesive strength between the γ- and γ′-phase and accelerates crack propagation along the voids. Hydrogen embrittlement fracture in Ni-based single crystal superalloys is due to synergistic hydrogen-enhanced local plasticity, strain-induced vacancies and decohesion in the hydrogen-induced cracking process.