Control of Polymorphism and Mass-Transfer in Al₂O₃ Scale on Alumina Forming Alloys

The transformation from metastable polymorphs to stable alpha-Al2O3 in the scale formed on a CoNiCrAlY alloy is accelerated under lower oxygen partial pressure (PO2), where both Al and Cr in the alloy are simultaneously oxidized, resulting in the formation of a dense and monolithic alpha-Al2O3 scale. Under higher PO2, where all components of the alloy are oxidized, the transformation is retarded and (Co,Ni)(Al,Cr)2O4 is also produced. The oxygen permeability in polycrystalline alpha-Al2O3 wafers exposed to steep oxygen potential gradients is evaluated at high temperatures to investigate the complicated mass-transfer phenomena through the scale formed on the alloy. The diffusion of Al and O species, which are responsible for the oxygen permeation along the grain boundaries of Al2O3, is dependent on the formation of an oxygen potential gradients. For Lu-doped Al2O3 polycrystals, it was found that Lu depressed the mobility of oxygen, but did not directly influence the migration of Al.