Zircaloy microstructure investigation by advanced microscopy techniques
PhD Student: Aaron Chote
Zircaloy-4 (Z4) is the principal alloy used in fuel cladding in the nuclear industry. Zirconium has a high affinity for oxygen at high temperatures and consequently instantaneous oxidation at the surface of zirconium creates an oxide layer. The oxide growth process occurs in three stages and changes the properties of the layer. Initially the oxide layer acts as a protective barrier slowing down corrosion (the pre-transition state). The layer further evolves to a stage where hydrogen and oxygen transport occur through defects created as a result of increased mechanical stresses (post-transition).
The ZrO2 oxide layer consists of two polymorphs (monoclinic and tetragonal) that exist in differing amounts dependent on the transition regime; tetragonal dominates in the pre-transition and monoclinic in post-transition.
To understand the influence of interfaces (grain boundaries and phase boundaries) distribution in the oxide layer, precession electron diffraction (PED) is required, as the grains are often on the scale of a couple of nanometres. PED will yield grain orientation data that we will analyse to obtain the grain boundary plane distribution (GBPD) in the growing oxide layer. Our evaluation will provide quantitative information on which interfaces are the most frequent ones, as opposed to qualitative information.