Mechanism of Aluminum-Oxide Scale Formation on some High-Temperature Structural Alloys
The formation of Al2O3 scale on high-temperature structural alloys is a subject of immense technological importance, as well as of considerable scientific interest. Contrary to much current thinking in the field, the kinetics of scale growth appear to be controlled by the electrical conductivity of the scales, rather than solely by the diffusion of aluminum and oxygen at grain boundaries. Considerations of band structure thus become of major importance. The atomic structures and the electronic density of states were computed for a group of bi-crystal boundaries using density-functional theory. These were found to provide insights into the mechanism(s) involved in the so-called Reactive Element effect, which is the dramatically improved oxidation resistance of Fe-base and Ni-base structural alloys resulting from quite small additions (~0.1%) of a group of alloy dopants including Y, Zr, and Hf. Finally, oxygen and aluminum grain-boundary diffusivities have been measured in Al2O3 scales grown on a group of Ni75Al20Cr5 alloys containing 0.1% Y, Zr, or Hf, and these were compared with the computed results.