Raman Spectroscopy Study of the Tetragonal-to-monoclinic Transition in Zirconium Oxide Scales and Determination of Overall Oxygen Diffusion by Nuclear Microanalysis of O18

Two allotropic forms of zirconium oxide, monoclinic and tetragonal, have been identified in the scales formed on zirconium alloys. The transition from tetragonal to monoclinic has been followed by X-ray measurements and Raman laser spectroscopy. Information on the average content of the tetragonal phase was obtained by X-ray diffraction, whereas Raman laser analyses on tapered sections revealed its distribution through the scale thickness. Oxidation exposures were made in an autoclave, using H2O18 and D2O18 to determine the overall diffusion coefficients. In particular, oxide scales have been studied on Zircaloy-4 with three different precipitate sizes, and on a Zr-1Nb alloy, after exposure in an autoclave for between 3 and 100 days. The specimens were analyzed in detail in the vicinity of the kinetics transition point, where the acceleration of corrosion occurs. Raman spectroscopy analyses enabled the crystallographic nature of the ZrO2 to be determined. Close to the interface, the tetragonal phase content is about 40%, when after the transition the tetragonal phase is transformed into monoclinic. The O18 diffusion treatment was carried out in an autoclave at 400°C under pressure on specimens previously oxidized for between 3 and 100 days in natural water vapor pressure. The diffusion profiles were determined by nuclear microanalysis using the O18(p,α)→N15 reaction. Based on these profiles, the volume and grain boundary diffusion coefficients were calculated for each material and for each oxidation time. The results show clearly that oxygen transport occurs principally by grain boundary diffusion. The boundary diffusion coefficient is 108 times higher than those for bulk diffusion (Db is equal to 10-13 to 10-15 cm2/s). The results of these analyses are in agreement with the changes in kinetics observed at the transition point. The stabilization of the tetragonal phase by stress is discussed.