Effects of Minor Alloying Elements on Alumina Transformation During the Transient Oxidation of β-NiAl

The transient stage of alumina scale formation on alumina-forming alloys is often accompanied by the formation of metastable Al2O3 phases that may affect oxidation kinetics and scale microstructure. To clarify the role of dopants on the transient stage of NiAl oxidation, we characterized Al2O3 scales on a series of oxidized model β-NiAl samples that were un-doped or doped with additions of Ti, Y, Si, Ti + Y, and Si + Y. Our results confirm that different dopant elements (Ti, Si, and Y) have varied effects on the length of transient oxidation of alumina-forming alloys. Ti and Si accelerated the θ- to α-Al2O3 transformation via two different mechanisms. Ti promoted α-Al2O3 nucleation, while Si delayed nucleation but accelerated the lateral growth of the α-Al2O3 patches. In contrast, Y had little effect on alumina transformation, possibly due to the low Y concentration used in the current study.