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Different pre-annealing and pre-oxidation treatments were conducted on a dual phase γ + β Ni-21Co-18Cr-22Al-0.2Y (at.%) bond coating for 1 hr at 1373 K (i) with or without a native oxide upon heating, (ii) in two different atmospheres upon heating, and (iii) under various oxygen partial pressures (pO 2 ) in the range of 0.1-10 5 Pa during oxidation. The chemical composition, structure, morphology and phase constitution of the resulting oxide layers were investigated using a range of analytical techniques. It is found that the exclusive formation of a continuous α-Al 2 O 3 layer without the simultaneous formation of NiAl 2 O 4 spinel was promoted for oxidation at low pO 2 . The formation of metastable θ-Al 2 O 3 was suppressed for a low fraction of the β phase, coupled with a high fraction of segregated Y at the initial bond coat surface. Initial Y segregation and incorporation of Y 2 O 3 and Y 3 Al 5 O 12 within the developing oxide layer was promoted in the absence of a native oxide and for heating in an inert atmosphere. The development of protrusions (i.e. pegs) at the oxide/coating interface, as a result of the incorporation of internal Y 2 O 3 precipitates by the inward growing oxide layer, was most pronounced upon heating in an inert atmosphere, followed by oxidation at an intermediate pO 2 .

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The relation between the Y distribution in the alloy and the growth kinetics of the developing oxide scale was studied for the thermal oxidation of two .2Y (at.%) alloys at 1,373 K: (i) a coarse-grain cast alloy with large Ni 5 Y intermetallic precipitates, and (ii) a fine-grain freestanding coating with small Ni 5 Y precipitates. Using a combination of experiments and model calculations, it is shown that the average growth kinetics of a NiCoCrAlY alloy are dependent on the size and distribution of Y-rich oxide inclusions (pegs) in the a-Al 2 O 3 oxide layer. Alumina scales containing a high density of small Y-oxide inclusions grow faster than a-Al 2 O 3 scales containing only a few, large Y-oxide inclusions. Upon oxidation of the freestanding coating, the Y-oxide inclusions in the scale attain their maximum size after the Y in the coating is completely consumed. After this point, a decrease in the average oxidation kinetics occurs.

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