A study on the multiple stages of oxidation kinetics in a single crystal nickel-based superalloy

Highlights: • A single stage was observed in the intermediate layer, while two stages exhibited for the inner Al₂O₃ and outer NiO layers. • Cracks and voids were preferably initiated nearby the nano-grains of Ta₂O₅ phases. • A grain size gradient exists from the outer layer to the inner layer. • The growth of the intermediate layer with nano-grains determined the whole oxidation behavior. • The concentration gradient of various solutes in the γ'-free zone promoted the multiple stages of oxidation kinetics. Two stages of mass gain kinetics were observed in a single crystal nickel-based superalloy that corresponds to the growth of various layers. However, only one stage appeared in the intermediate layer, while the inner Al₂O₃ and outer NiO layers exhibited two stages. This abnormal kinetics behavior was attributed to the undermining effect of simple tetragonal structure Ta₂O₅ phases on the continuous hexagonal structure Al₂O₃. Furthermore, the Cr concentration gradient in the γ'-free zone transformed from depletion to enrichment with increase of exposure time, while Ni displayed a converse trend. This phenomenon further promoted the different growth kinetics of various layers.