On the Polymorphic Transformation of Zirconium Dioxide Associated with the Breakaway during High Temperature Oxidation of Zr and Some Zr Alloys

The breakaway phenomena of Zr and some Zr alloys during high temperature oxidation were studied from a structural point of view, by means of high temperature X-ray diffraction, thermal gravimetry, optical microscopy, and differential thermal analysis. The structural change in oxide scale at breakaway is in close relation with the polymorphic transformation of zirconium dioxide (tetragonal↔monoclinic). The continuous observation of change in oxide scales with high temperature X-ray diffraction clarified that black colored pre-breakaway oxide layers contained a considerable quantity of metastable tetragonal oxide, and that the color change of oxide layers from black to white and the polymorphic transformation to monoclinic oxide occurred simultaniously at breakaway. The contents of tetragonal modification in pre-breakaway oxide scales on Zr alloys containing Ti or Sn were larger than that on pure Zr, and this agrees reasonably with the facts that the breakaway occurs at lower temperatures on those alloys, and that those elements lower the transformation temperature of zirconium dioxide. In the stable region of the tetragonal form, the breakaway did not occur in O2.