Pt-Al bond coat dependence on the high-cycle fatigue rupture and deformation mechanisms of a fourth-generation single crystal superalloy at various temperatures

The impact of a Pt-Al bond coat on the high-cycle fatigue (HCF) behaviour of fourth-generation single crystal superalloys at 760℃ and 900℃ was investigated. The Pt-Al bond coat was found to be almost negligible under low stresses at 760℃; however, the bond coat effect was detrimental at 900℃ or under high stresses at 760℃. At 760 °C with increasing high-amplitude stress, the tip cracks preferentially nucleated from defects within coat and propagated inwards, accelerating the fracture. The fatigue life of the coated alloys was considerably decreased at 900 °C, which was attributed to the damage accumulated in the bond coating via oxidation, crack-induced oxide cracking and interfacial microstructure degradation. An Elevated temperature led to the crack-induced oxides shifting from a small fan-shape to a large umbrella-shape, increasing the density of slip bands and quantity of secondary cracks in the substrate. Furthermore, more Ru diffused outwards at 900℃ than that at 760℃, which caused the disappearance of L-C dislocation locks and tertiary γ′ phases and aggravated TCP phases precipitation in the substrate at 900℃. Ultimately, to assess the degradation of the HCF life, an empirical life prediction method was developed, and the calculations results well matched the test results.