In-situ time-resolved study of structural evolutions in a zirconium alloy during high temperature oxidation and cooling

In-situ time-resolved Synchrotron X-ray diffraction analyses were performed on zirconium alloy (Zircaloy-4) sheet samples, during their heating, isothermal oxidation at 700, 800 and 900 degrees C under a flowing mixture of He and O₂ and cooling. The oxide growth and the evolution of the oxide structure as a function of time and temperature were studied with suitable time resolution. Oxide layer thicknesses of approximately 10 mu m were formed during the experiments. The incident X-rays penetrated the whole oxide thickness. The samples were examined after the experiments by field emission gun scanning electron microscopy, electron backscatter diffraction and electron-probe microanalysis. The results showed that the oxide contains a mixture of monoclinic and tetragonal zirconia evolving during heating, oxidation and cooling. The average volume fraction of tetragonal zirconia decreases during oxidation. This fraction is larger at 900 degrees C than at 700 and 800 degrees C. For oxide layers thinner than approximately 5 mu m, this fraction is larger at 800 degrees C than at 700 degrees C, but it is rather equivalent for both temperatures when the oxide thickness ranges between 5 and 8 mu m. Some of the tetragonal zirconia crystals transforms into the monoclinic phase during cooling after oxidation. This fraction of transformed tetragonal zirconia is larger after oxidation at 900 degrees C than after oxidation at 700 and 800 degrees C. It is suggested that these evolutions of the oxide crystallographic structure are related to micro-stresses and to temperature dependences of the critical size of zirconia crystals below which tetragonal zirconia is stabilized. (authors)