X-ray determination of residual stresses and phase quantification in ZrO2 oxide layers formed on Zircaloy-4

The residual stress state and the microstructure of ZrO2 oxide layers formed on Zircaloy-4 plates in steam at 400 degrees C were studied using x-ray diffraction techniques. One of the most important features of the corrosion mechanism of Zircaloy is the development of compressive stresses within the oxide which strongly affect the microstructure of the growing oxide and the diffusion of oxidizing species. These compressive stresses can also favor the formation of tetragonal zirconia over monoclinic zirconia. Therefore, the accurate determination of the stress state and the composition of the oxide is essential for the understanding of the corrosion mechanism and the modelling of its kinetics. The samples were prepared by oxidation of Zy-4 plates in an autoclave at 400 degrees C in a vapor pressure of 10.3 MPa. Using conventional x-ray diffraction, it can be shown that the amount of tetragonal zirconia in the oxide layer decreases continuously with the oxidation time, however, with this type of radiation it is very difficult to obtain the phase distribution and the stress level profile in the oxide layer. To obtain more accurate information, specific diffractometers on the French synchrotron radiation facility (LURE Orsay) were used to work at higher wavelengths. The following experimental results were obtained : - High compressive stresses were found in the oxide layer and a strong effect of the oxide texture was observed. - There seems to be a trend for the tetragonal zirconia to be preferentially located at the interface metal/oxide.