Oxygen and aluminum diffusion in α-Al2O3:: How much do we really understand?

The major diffusion processes in alpha-Al2O3 have been reviewed, including oxygen and aluminum lattice diffusion, D-oxy and D-Al, respectively; oxygen grain boundary diffusion, Db-oxy; and pipe diffusion, D-p. Their relevance to creep of Al2O3 and oxidation of Al2O3 scale-forming alloys is briefly considered. It is concluded that the answer to the question included in the title is "not a great deal". Eight major questions and issues have been identified that would constitute fitting subjects for future research: (i) What is the nature of the "buffering" that appears to control D-oxy? (ii) What process(es) are occurring during annealing that eliminate non-Fickian behavior when measuring D-oxy? (iii) How should the magnitude of the measured activation energies for D-oxy be interpreted? (iv) Have all the defect types involved in oxygen diffusion been identified? (v) Does oxygen diffusion occur by an interstitialcy mechanism? (vi) What is the order of magnitude of D-Al/D-oxy? (vii) What is the order of magnitude of Db-Al/Db-oxy? (viii) It appears that the activation energy for Db-oxy is greater than that for D-oxy. How should this be understood? (c) 2008 Published by Elsevier Ltd.