| Summary: | Single-crystal nickel-base superalloys are extensively used for turbine blades with thermal barrier coating (TBC) systems in gas turbine engines. In the event of the coating failure in service, the substrate is directly exposed to a corrosive gas environment and is damaged by oxidation. Understanding the oxidation behaviour of the bare metal is crucial to improve the performance of single-crystal nickel-base superalloys. This research focuses on elucidating the high temperature oxidation behaviour of the CMSX-4 alloy doped with lanthanum, including the transient oxidation behaviour during heating up with some partial pressure of oxygen. To do this, a two-stage oxidation experiment using isotopic oxygen was performed and various analyzing methods such as FEG-SEM, TEM with EDX, FIB-SEM, FIB-SIMS, and ToF-SIMS were applied. The transient oxidation behaviour of a nickel-base superalloy in different oxygen partial pressures showed that at high pO2 a triple-layered oxide developed: this consisted of an outer nickel oxide layer, then a complex aluminium-chromium oxide layer, and finally an inner alumina layer. A lower pO2 produced only alumina, as the reduced oxygen activity suppressed nickel oxide formation. An aluminium-depleted region in the alloy with a width proportional to pO2 was observed. The isothermal oxidation of a bare CMSX-4 alloy exhibited that the oxide was grown by the inward diffusion of oxygen via grain boundary of alumina oxide. In addition, the surface scale was spalled at the interface between the precipitation free zone (PFZ) formed beneath the alumina oxide layer and the fine precipitation zone due to the difference in mechanical properties during cooling down. In lanthanum-doped CMSX-4 alloy, lanthanum was detected throughout the surface scale including the top surface. The PFZ was not formed below the alumina oxide layer as a result of suppression of inward diffusion oxygen via grain boundary of alumina oxide due to the segregation of reactive elements to grain boundary. Consequently, the surface scale was well adhered to the substrate due to the absence of PFZ under the alumina oxide layer.
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