Microstructure Investigation of the Oxidation Process in Zircaloy-2 - The Effect of Intermetallic Particle Size
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. With an improved corrosion resistance the alloys could be used for much longer times in the reactors, increasing fuel burn-up and decreasing the amount of radioactive waste. Therefore it is of great importance to try to understand the mechanisms of the oxidation process in these alloys.
In this study the oxidation behavior in steam autoclave of Zircaloy-2, an alloy used primarily in boiling water reactors, is studied. 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 main method for investigations has been transmission electron microscopy in combination with energy dispersive X-ray spectroscopy. Also atom probe tomography and spectrophotometry have been used. Focus has been on the microstructure of the oxide and the metal/oxide interface zone.
It was found 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 metal/oxide interface was found to undulate on a micrometer scale. This undulation gives rise to large stresses perpendicular to the metal/oxide interface. In the oxide, above wave crests, lateral cracks are formed, and it is shown that un-oxidized SPPs embedded in the oxide act as nucleation sites for these cracks. Therefore, a material with many small SPPs has more lateral cracks than a material with few large SPPs.
Adjacent to the oxide often a sub-oxide layer (~ZrO) is found, with varying thickness also in the same specimen. One sub-oxide layer with an average oxygen content of ~55 at. % was found to consist of fingers with ~60 at. % oxygen with a diameter of 5 nm and a length of 50 nm, penetrating into regions of ~50 at. % oxygen. From the oxide, oxygen diffuses into the metal and it was found that the width of the oxygen diffusion profile varies, with wider profiles underneath delayed parts of the interface. An oxygen enriched phase with ~30 at. % oxygen was found in some of the specimens.
Evidence of extensive plastic deformation in the metal underneath the oxide scale was found in the form of twinning, dislocation tangles and patches, cell formation and sub-grain formation. The heavily deformed layer is a few µm thick and no obvious difference could be seen underneath different oxide thicknesses or between alloys with different strength.
The black oxide color of zirconium alloys has 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.