Studies of zirconium alloy oxide layers using synchrotron radiation

The aim of this study is the determination of the residual stress state and the analysis of the microstructure of ZrO2 oxide layers (mainly the tetragonal to monoclinic phase ratio) formed on different zirconium alloy plates oxidized in steam at 400 degreesC for several lengths of time, using x-ray diffraction techniques. The marked texture and the large stress and phase distribution gradients in the zirconia layer did not allow to use a conventional x-ray apparatus, especially for local analyses. These difficulties have been overcame using the French Synchrotron Radiation (SR) facility LURE (Laboratoire pour 1'Utilisation du Rayonnement Electromagnetique) at Orsay, France. The distribution in the oxide layer was determined by varying both the incident angle and the wavelength of the beam. A preferential localization of the tetragonal phase close to the metal/oxyde interface was observed when comparing the x-ray diffraction peaks of the monoclinic phase (mainly ((1) over bar 11)) to the (111) reflection of the tetragonal phase. The same type of analysis has been performed to determine the residual stresses. These studies reveal the existence of a high in plane compressive stress state in the oxide layer and a lower one in the metal. A significant effect of the composition in the metal plates was observed on the tetragonal to monoclinic phase ratio as well as on the evolution of the compressive stress level in the ZrO2 layer with oxidation time. But these stresses are measured at room temperature and because of the anisotropy of thermal expansion coefficients, elastic constants and texture of Zr and ZrO2, these stresses are certainly not representative of the ones spreading out during the growth of the oxide scale. Therefore, in a first approximation a macroscopic calculation of the thermal stress has been performed, but the results obtained can not explain the stress reduction due to cooling observed elsewhere. In fact, the local stresses developing both in the metal and the oxide for particular crystallographic orientation (especially epitaxial orientations) have to be taken into account during the cooling. They may be determined using a thermoelastic self-consistent model.