Tem Examination Of Oxides Formed On Zirconium Alloys

Open Access
Author:
Le Taillandier De Gabory, Benoit
Graduate Program:
Nuclear Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
None
Committee Members:
  • Arthur Thompson Motta, Thesis Advisor
Keywords:
  • Zirconium
  • Zircaloys
  • Zircaloy-4
  • ZIRLO
  • oxide
  • corrosion
  • TEM
  • microscopy
  • microstructure
  • texture
  • precipitate
  • interface
Abstract:
In this thesis research, oxide layers formed on Zr-based alloys in high temperature water autoclave environments are characterized, in an effort to understand the mechanism resulting in the variations of corrosion rate observed before and during the first kinetic transition. The study included oxides formed on Zircaloy-4 and ZIRLO when exposed to 360 °C pure water at saturation pressure, with samples archived just before and just after the first kinetic transition. From these oxides, thin foil samples are prepared from precise locations using the Focused Ion Beam In-Situ Lift-Out technique. These electron-transparent foils are then characterized using transmission electron microscopy. The study characterizes the oxide grain size and morphology, oxide texture, incorporation of metallic precipitates and the evolution of these with respect to the distance to the oxide/metal interface. The results are found to be in good agreement with previous research: the oxide is mainly monoclinic, with a small fraction of tetragonal phase. A preponderance of columnar oxide grains is observed, which extend to the oxide/metal interface. Equiaxed grains can be seen near the oxide/water interface (first oxide grown) and in the region corresponding to the oxide grown at the onset of transition. Lateral in-plane cracks are seen in these regions as well. Electron diffraction results show that the small equiaxed grains are associated with the tetragonal phase. Precipitates are incorporated in metallic form in the oxide. They later become oxidized, forming small equiaxed oxide grains in Zircaloy-4 with high tetragonal content, or amorphous precipitates in the oxides grown on ZIRLO. Special attention was given to the oxide/metal interface to determine the phases present, and measure the evolution of oxygen content in the oxide precursor layers. The interface exhibits an intermediate layer identified as ZrO, a discontinuous layer of “blocky” Zr3O grains embedded in the ZrO layer, and a suboxide layer identified as an oxygen saturated Zr phase on the metal matrix side. All these phases have an oxygen content comprised between the content in the oxide (66 O at%) and in the α-Zr phase (29 O at%). The oxide/metal interface evolves with exposure time, the width of the intermediate layers increasing until the transition. The oxide/metal interface structure is significantly affected by transition, during which the intermediate layers are consumed, causing their widths to decrease significantly. Combining EELS scans and Atom Probe Tomography results to obtain the evolution of the oxygen content across the interface, with TEM observations to characterize the different phases and diffusion layers allowed us to obtain a more comprehensive picture of the mechanisms driving the corrosion.