THE EFFECT OF YTTRIUM ON THE EARLY STAGES OF OXIDATION OF ALUMINA FORMERS - COMMENTS

A new approach to the effect of yttrium on the early oxidation stages of alumina formers is proposed. It relies on the elucidation of processes occurring in unstable aluminas. Different mechanisms by which yttrium can accelerate or retard the phase transformation of unstable aluminas into alpha-Al2O3 are discussed. Using a model of the defect structure of unstable aluminas it is concluded that doped yttrium should retard the transformation as long as its amount does not exceed a critical value. It is shown that the proposed approach enables fairly consistent description of effect of yttrium and to explain various existing controversies.