Effect of Y distribution on the oxidation kinetics of NiCoCrAlY bond coat alloys

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 Ni-20Co-19Cr-24Al-0.2Y (at.%) alloys at 1,373 K: (i) a coarse-grain cast alloy with large Ni5Y intermetallic precipitates, and (ii) a fine-grain freestanding coating with small Ni5Y 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 alpha-Al2O3 oxide layer. Alumina scales containing a high density of small Y-oxide inclusions grow faster than alpha-Al2O3 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.