Reassessment of oxidation-induced amorphization and dissolution of Nb precipitates in Zr−Nb nuclear fuel cladding tubes

Abstract

The surface oxide film of a Zr−2.5Nb alloy subjected to long term corrosion at 633 K in simulated primary coolant of pressurized water reactors has been analyzed. The primary concerns were whether Nb precipitates exhibit amorphization upon oxidation, and whether they dissolve into the matrix, as suggested by previous studies. Their behavior is of particular interest, from the viewpoint of engineering, as the mechanism of improving corrosion resistance of Zr fuel cladding by Nb addition, and from the viewpoint of basic materials science, as the critical condition of solid-state amorphization. If amorphization and dissolution proceed simultaneously, it would follow that amorphization occurs at conditions where both O and Nb atoms are mobile; under such conditions diffusion-induced amorphization has never been observed. It was found that the Nb precipitates exhibited amorphization without dissolution. Some of the inconsistencies among the previous studies were found to be artifacts of materials characterization methods. The final configuration of precipitates was amorphous Nb2O5, which is distinct from the other Nb oxides in terms of its dielectric nature with a wide band gap. The matrix initially contained a large amount of Nb greater than the solubility. Although the excess Nb atoms did not precipitate by thermal aging alone, oxidation was found to enhance their precipitation at this temperature. It appears that amorphization can occur even when the motion of atoms is not frozen-in.