Probing the failure mechanisms and microstructure evolution of a high-pressure turbine blade coated with AlSiY

Abstract A long-term test run followed by room-temperature fatigue test was performed for the first-stage high-pressure turbine blades of an aero-engine. The investigation showed the appearance of cracks at the trailing edge of the blade coated with AlSiY. The failure mechanism was investigated by macroscopic observation, fracture analysis, metallographic analysis, hardness testing and structural evolution analysis by electron backscatter diffraction(EBSD), kernel average misorientation (KAM) and strain analysis. The results showed that thermal fatigue cracks initiated and propagated on the trailing edge during the test run and cracks further propagated during the fatigue test. Brittle and hard phases including M23C6 carbides, μ phase and γ'(Ni3Al) phase precipitated in the inter diffusion zone with a high dislocation density and maximum hardness and strain, resulting in thermal fatigue cracks initiation during frequent start-stop of the aero-engine test run. The cracks propagated outward along the β-NiAl grain boundaries with interconnected γ'(Ni3Al) phases in coating, and propagated inward along the lathlike precipitates including M23C6 carbides, M6C carbides and μ phase in the secondary reaction zone to the grain boundaries of the substrate. Chainlike M6C carbides locally precipitated on the substrate grain boundary, causing grain boundary embrittlement and further promoting cracks propagation.