Microstructure of oxide layers formed on Zircaloy-2 in air at 450 degrees C

Conference item

Oxidation tests were conducted at 450 degrees C in air for Zircaloy-2 and modified alloys. The results showed breakaway oxidation, as has been reported previously. The microstructures of the oxide films were examined by transmission electron microscopy (TEM). Cross-sections were prepared from test samples with various thicknesses of oxide using ion beam thinning. The oxide structure was mainly columnar in both the pre- and post-transition oxides, but a heavily twinned structure was observed in the post-transition oxide. At the metal-oxide interface, omega-Zr was observed in the metal beneath the oxide in both the pre- and post-transition conditions. It is argued that the presence of the high-pressure omega-phase may be evidence of compressive rather than tensile stress close to the interface.Two types of the intermetallic precipitates were observed in Zircaloy-2, one was Zr-Fe-Cr type and the other was Zr-Fe-Ni type. The Zr-Fe-Cr type precipitates were found to survive unoxidised in the oxide near the metal-oxide interface, but they disappeared further away from the metal-oxide interface. No Zr-Fe-Ni type precipitates were observed in the oxide. It is suggested that tensile stresses from expansion during progressive oxidation of the Zr-Fe-Cr type intermetallic particles embedded in the oxide may be the cause of the breakaway in air.

Authors: Ishii, Y; Sykes, J

• Publication status: Published
• Journal: MATERIALS AT HIGH TEMPERATURES
• Volume: 17
• Issue: 1
• Pages: 23-28
• Publication date: 2000-01-01
• Event title: 4th International Conference on the Microscopy of Oxidation
• DOI: 10.1179/mht.2000.005
• EISSN: 1878-6413
• ISSN: 0960-3409
• Keywords: Zircaloy-2; microstructure of oxide layers