Microstructure of Zirconium Alloys Oxidized in Steam

Licentiatavhandling, 2009

Zirconium alloys are widely used in nuclear reactors as fuel cladding tubes because of their low thermal neutron capture cross-section, good corrosion properties and satisfactory mechanical properties. However, the main limiting factor of these materials is the oxidation behavior. With an improved corrosion resistance the alloys could be used for much longer times in the reactors. Therefore it is of great importance to try to understand the mechanisms of the oxidation process in order to improve the alloys for a better performance. In this study the oxidation behavior of zirconium alloys is studied, and special emphasis is put on the role of the intermetallic second phase particles (SPPs) containing iron, chromium and nickel and with a typical size of 50 nm. The alloys chosen for this study are materials that have long been used in reactors and for which long-term in-reactor data exists. The main method for investigations has been transmission electron microscopy (TEM) in combination with energy dispersive X-ray spectroscopy. As a first step, methods were developed for manufacturing large thin foil TEM specimens, containing the metal/oxide interface, using the focused ion beam in-situ liftout technique, and also for imaging the SPPs. The particles in the oxide are hard to spot with conventional bright field TEM due to substantial crystallographic contrast from the small oxide grain size. It was concluded that the best way to image the particles in the oxide is by using a high angle annular dark field detector in TEM. The morphology of the oxide and the metal/oxide interface was studied. It was found that the interface is undulating on a micrometer scale and sub-oxide was found close to the interface. The oxidation of SPPs was studied and it was concluded that the SPPs oxidize slower than the surrounding metal, and that the absent volume increase leads to void and crack formation as the SPPs become embedded in the oxide. On SPP oxidation, iron diffuses out of the particle into the surrounding oxide. The black oxide color of zirconium alloys has also been studied, using spectrophotometry. The conclusion is that the reason for the black appearance of oxidized zirconium alloys is the excitation of localized surface plasmon resonances in the metallic SPPs embedded in the oxide layer. When the oxide grows thicker it turns greyish as SPPs in the outer part of the oxide oxidize so that light has to travel through a thicker, partly cracked oxide before meeting absorbing particles.