Oxidation of Zirconium-Yttrium Alloys

Abstract

AbstractThe phase and elemental distributions on zirconium alloys containing 5, 10, 25, 35, and 50 at.%. yttrium, oxidized in dry air at 1200 and 1600°C, have been studied by X-ray diffraction, fluorescence, and electron-microprobe techniques. Zirconium-yttrium alloys, which are entirely solid at 1200°C, and zirconium 5 and 25 at.% yttrium alloys, which aire two-phase solid plus liquid at 1600°C, form a multiphase scale of ZrO2, Y2O3, and ZrN. The phase distribution varies with yttrium content, while the average metal composition remains constant throughout the oxide scale and base metal. In contrast, zirconium 35 and 50 at.% yttrium alloys, which are entirely liquid at 1600°C, form a single-phase Y2O3 layer.The mechanism of air oxidation of solid zirconium-yttrium alloys, up to 50 at. % yttrium, is one of anion diffusion to the oxide-metal interface with little or no diffusion of the metallic elements. The oxidation of liquid alloys differs in that, as a result of yttrium diffusion to the oxide-liquid interface, Y2O3 is the only oxide formed. Ternary Zr-Y-O isotherms are presented to illustrate the phase distribution and mode of formation.