In situ XRD analysis of the oxide layers formed by oxidation at 743 K on Zircaloy 4 and Zr-1NbO

Two alloys, having different oxidation behaviour (Zy4 and Zr-1NbO), have be en investigated during oxidation at high temperature (743 K) and low oxygen pressure (10 kPa) by in situ X-ray diffraction (XRD). Tetragonal phase con tent and 'pseudo-stresses' on the monoclinic phase have been measured as a function of the oxide layer thickness. The tetragonal phase contents are si milar for both alloys and decreased with the oxide layer thickness. Pseudo- stresses were much more compressive on Zr-1NbO alloy, with limited changes at the corrosion kinetics transition. On cooling, the tetragonal fractions do not change, while 'pseudo-stresses' decreased in different ways for the two alloys. With respect to stress analysis, no correlation was found betwe en 'pseudo-stresses' and tetragonal phase content. In addition, due to the thermoelastic properties of the highly anisotropic phases of the zirconia, large internal thermal stresses are expected to develop during any temperat ure changes. The orders of magnitude of them are similar to the stresses in duced by swelling during oxidation from Zr to ZrO2. (C) 2000 Elsevier Scien ce B.V. All rights reserved.