Microstructural analysis of zirconium alloys oxides

Precipitates present in Zry-4, characterized by a hexagonal C 14 Laves phase Zr(Cr,Fe)2 microstructure, undergo chemical composition changes when the alloy is oxidized in open furnace. EDS (Energy Dispersive Spectroscopy) and WDS (Wavelength Dispersive Spectroscopy) determinations in non-oxidized specimens, showed the presence of Zr, Cr and Fe within the precipitates. However, after the oxidation process, no Fe was detected. In order to obtain more experimental evidence of this process, a Laves phase Zr(Cr, Fe)2 alloy was prepared with the composition most frequently observed in the precipitates The alloy was oxidized at 650 degree C in open furnace. Average oxide thickness was about 30 microns. Particles and holes, attributed to missing particles, are also observed on the oxide surface. Mean precipitate size was about 3 microns. EDS mappings show a higher concentration of Cr within the precipitates. However, their Zr content was similar to that observed in the oxidized matrix. In the regions corresponding to the holes, EDS determinations showed a higher concentration of Fe. The experimental results are consistent with a selective oxidation process of the alloying elements of the Zr(Cr,Fe)2 phase. At the beginning of the oxidation process, Zr transforms to ZrxO2-x.The other elements, Cr and Fe, remain in metallic state until the oxygen potential within ZrxO2-x increases to a value high enough to allow the oxidation of Cr. This element dissolves first into the ZrxO2-x and then migrates to the surface. As oxidation proceeds, the oxygen potential within ZrxO2-x increases until a value corresponding to the dissolution of the Fe contained in Zr(Cr,Fe)2 to the surrounding zirconium oxide. Since our determinations showed a higher concentration of Cr on the external oxide surface, it is assumed that Fe atoms migrate to a region beneath that surface. (author)