The correlation between tetragonal phase and the undulated metal/oxide interface in the oxide films of zirconium alloys

The transition mechanism is of significance to the corrosion behavior of zirconium alloys. Two important phenomena may be the causes of transition, which are the undulated wavy interface and the existence of tetragonal zirconia at the metal/oxide interface. There is little detailed study on the correlation between these two features. Micro-scale Raman imaging of intensity and Raman shifts was conducted in this study to further evaluate the connection. It was studied based on the oxide films formed on Zr-0.5Sn-0.15Nb-0.5Fe-0.2 V and N36 claddings corroded in 500 degrees C/10.3 MPa steam. The stress in interfacial tetragonal zirconia has relationship with the amplitude to wavelength ratio of the undulated part and it is always found to be larger at the convex parts of interface. These parts are prone to crack formation. At one convex position that the oxide cracked, t-ZrO2 is found to be preserved beneath and above the crack. Stress is also preserved, but partially released. When interfacial t-ZrO2 exists, the stress in the surrounding monoclinic phase is observed to be small. This provides us a more detailed figure of the stress distribution in oxide film. Tetragonal phase is mainly distributed near the metal/oxide interface. More intense tetragonal signals are frequently found at the convex positions of interface. This might result from large tetragonal grains or aggregation of small t-ZrO2 grains. These grains are stabilized at convex parts because of the large compressive stress and might be unstable once the grain size exceeds a critical value or once cracks are formed at such positions. An illustration of the connection between t-ZrO2 and undulated interface is then proposed. Possible reasons that facilitate the formation of lateral cracks and possible reasons for the existence of t-ZrO2 at the convex positions are discussed. (C) 2019 Elsevier B.V. All rights reserved.