Texture and Phases in Oxide Films on Zr-Nb Alloys

Abstract

ABSTRACTOxide films, 0.2-2.0 μm in thickness on Zr-2.5Nb and 11 μm thick on Zr-20Nb alloys, formed in steam at 673 K, have been examined using TEM, XRD and Raman spectroscopy. Columnar grains of mostly monoclinic Zr02 in oxide films on Zr-2.5Nb exhibit a dual texture: a fibre mode with an axis close to the 102m pole and a [001]m growth mode with an orientation relationship [100]m // [4510]α and (010)m // (0001)α with the α-Zr metal. In both modes, “tetragonal” (and/or cubic) ZrO2 was present. Raman spectroscopy differentiated two non-cubic “tetragonal” forms of ZrOz within the [001]m growth texture. In thin oxides (0.5 μm or less), this corresponds to the tetragonal ZrO2 observed in ceramic zirconia and is characterised by a Raman band near 260 cm−1. The 278 and related 438 cm−1 Raman bands observed here in some oxide films (and in other Zr corrosion oxides) are attributed to a separate, non-cubic phase structurally related to the tetragonal ZrO2. The intensities of the 278 and 438 cm−1 bands are dependent not only on the amount of this modified-tetragonal phase but also on the oxide texture (related to the metal texture) and the beam orientation. The lack of Raman response from the “tetragonal” ZrO2 within the fibre mode of texture indicates either a low volume fraction or a cubic-like structure. For oxide on Zr-20Nb, XRD and Raman spectroscopy show a mixture of monoclinic and “tetragonal” ZrO2; the Raman results indicate the “tetragonal” ZrO2 has a high crystal symmetry or nearly cubic structure.