Oxidation of β-Zr and related phases in Zr-Nb alloys:: an electron microscopy investigation

The oxidation of the metastable beta-Zr phase in Zr-Nb alloys, primarily Zr-2.5Nb, in 673 K steam or in lithiated water at 583 K, was investigated using electron microscopy and microanalyses. In the SEM, oxidised beta-Zr regions in the Zr-2.5Nb alloy were imaged via a field effect contrast mechanism. In the TEM, microanalyses consistently showed the presence of Nb associated with the oxidised beta-regions in suitably prepared samples. The beta-Zr was found to form a Nb2Zrx-2O2x+1 oxide, while the beta-Nb exhibited delayed oxidation with respect to alpha-Zr, forming a metallic sub-oxide initially and becoming amorphous when oxidised. For the partially decomposed beta-Zr, the omega-phase was found to form monoclinic ZrO2, while the Nb-enriched beta-Zr followed the behaviour of either beta-Zr or beta-Nb depending on the Nb concentration. (C) 2000 Elsevier Science B.V. All rights reserved.