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Zircaloy-4, in three different metallurgical forms (stress relieved, recrystallized, and β-quenched), was oxidized at 400°C, in steam, up to 95 days. For each sample, the fraction of tetragonal zirconia was measured by X-ray diffraction and Raman spectroscopy. These two techniques show the presence of several zones containing tetragonal zirconia: a zone rich in the oxide near the metal-oxide interface and an other zones with lower concentrations in the rest of the pre-transition layers. For the post-transition samples, the external sublayer contains only a small amount of tetragonal zirconia.
Measurements of residual stresses by X-ray diffraction in the metal underlying the oxide show that the metal is under tensile stress state and that the stress values vary with oxidation duration. The level of the stress depends on the metallurgical form of the initial metal. The low penetration of X-rays in the material also made it possible to show the presence of a very high stress gradient near the metal-oxide interface that can explain the high proportion of tetragonal zirconia near the interface.
The study of the incorporation of intermetallic precipitates in the oxide and their chemical changes was carried out by electron microprobe analysis on taper cross sections of the oxide. This technique makes it possible to perform a large number of point analyses that yield satisfactory statistics for the variation in the iron/chromium (Fe/Cr) atom ratio of the precipitates in the oxide.
The intermetallic precipitates are incorporated into the oxide layer and then undergo a chemical change starting at a particular distance from the metal/oxide interface. The characteristic values of the Fe/Cr ratio before oxidation (1.6 for stress relieved and recrystallized conditions and 0.8 for β-quenched samples) are progressively spread out during oxidation. This change could correspond to an oxidation of intermetallic precipitates with segregations of iron at the precipitate-oxide interface, as shown in the literature.
The oxidation of the precipitates is accompanied by a volume change that should lead to the formation of a stress field around the precipitates and could stabilize the neighboring tetragonal phase. When the precipitates are completely oxidized, the stress field disappears and there is a transformation of the tetragonal phase to a monoclinic form, leading to the kinetic transition. Stress relaxation is shown by a decrease of the tensile stresses in the metal underlying the oxide that is undergoing kinetic transition.
zirconium alloys, annealed state, recrystallized state, beta-quenched phase, tetragonal zirconia, residual stresses, stress gradients, intermetallic precipitates, X-ray diffraction, stress measurements, Raman spectroscopy, electron microprobe analysis, zirconium, nuclear materials, nuclear applications, radiation effects
Research engineer, Commissariat à l'Energie Atomique (CEA), DTA-CEREM-DECM-SRMA Centre d'Etudes de Saclay, Gif sur, Yvette