High temperature and high pressure oxidation behavior of Zr-2.5Nb pressure tube material - Effect of beta phase composition and surface machining

Pressure tube material, Zr-2.5Nb, of pressurized heavy water reactors was given selective heat-treatments. The objective was to generate microstructures with different compositions of the second phase beta namely, Nb depleted beta(Zr) phase and Nb rich beta(Nb) phase. The material with bZr was then subjected to surface machining. The presence of phases after different heat-treatments was confirmed by X-ray diffraction and the resultant microstructures were characterized by transmission electron microscopy and electron back scattered diffraction. The Nb content in the beta phase after heat-treatments and residual stresses before and after surface machining were measured using X-ray diffraction. Oxidation was carried out in steam at 400 degrees C and 10 MPa up to 30 days and the oxides were characterized by Raman spectroscopy. It is shown that the presence of Nb rich beta(Nb) in the microstructure and faster diffusion of Nb into b phase brought about by surface machining resulted in an enhanced oxidation resistance. (C) 2015 Elsevier B.V. All rights reserved