ZIRCONIA POWDERS AND ZIRCALOY OXIDE-FILMS - TETRAGONAL PHASE EVOLUTION DURING 400-DEGREES-C AUTOCLAVE TESTS

Tetragonal zirconia powders and Zircaioy-4 samples with two different tin contents have been submitted to high temperature steam autoclave t reatments and characterized by X-ray diffraction. We proposed a model for the evolution of the content of tetragonal phase in oxide layers b ased on two competing phenomena: the creation of the tetragonal phase at the metal/oxide interface (determined by the oxidation kinetics of the alloy), and the disappearance of the tetragonal phase (transformat ion into monoclinic phase) due to annealing. This model is in good agr eement with experimental results. Particularly, it explains that the l ower the tin content, the lower the content of tetragonal phase for a given oxide thickness, and that there is a gradient in the oxide layer . It shows that an important parameter in stabilizing the tetragonal p hase is the grain size. We found the critical grain size to be about 3 0 nm.