Oxidation of niobium sheets at high temperature

Abstract This work shows the role of oxygen pressure on growth kinetics, morphology, phase composition and texture of oxide scale in the oxidation of niobium sheets at 900 °C. The particular morphology observed at low pressure shows the existence of competition between edge and surface growth mechanisms. Pressure and conversion rate influence the nature of preferential orientations of the Nb2O5 oxide layer: a [110] fibre axis is stabilized at high pressure but, at low pressure, the higher is the conversion rate, the more evident is this fibre axis. This behaviour is observed not only after cooling but also in situ. At high pressure, the evolution is not only temporal but also spatial: with …

Spatiotemporal dynamics in the oxidation of groups IV?V metals: study of zirconium

Abstract This paper focus on the mechanism of zirconium oxidation at 850 °C by X-ray diffraction (XRD) and the interactions between chemical (oxide growth, phase proportion) and physical processes (oxide scale texture, mechanical stresses). The structural gradients in the oxide scale composed of monoclinic ZrO 2 (higher proportion) and tetragonal ZrO 2 were revealed by XRD at variable incidence. The spatial evolution of the monoclinic zirconia texture through the oxide scale was shown and quantified. Relaxation of the oxide compressive stresses was observed for different oxidation times and correlated to kinetic breakaway evidenced by other methods. A structural mechanism of the oxide forma…

Determination of Zircaloy -4 Radiocristallographic Elastic Constants from Room Temperature to 300°C

A new experimental apparatus, allowing the determination of radiocristallographic elastic constants (REC) as a function of the temperature is presented. The material studied the zirconium alloy Zircaloy-4, showing low elastic anisotropy, an isotropic treatment of the problem is possible. As a consequence, the classical sin 2 ψ method is used to determine the REC from room temperature to 300°C.

Oxidation of Titanium, Zirconium and their Alloys, Texture of their Oxide Scales

Study of stress effects in the oxidation of Zircaloy-4

Abstract The oxidation of zirconium alloy Zy-4 is due to an anionic diffusion of oxygen leading to the formation of an oxide layer of zirconia. According to the Pilling and Bedworth ratio of the Zr/ZrO 2 system, the oxide scale is under compressive stresses. This stress state may play an important role during the corrosion process, from the phases as well as from the kinetics point of view. In order to better understand the eventual relationship between oxide phases, oxidation kinetics and stresses, an experimental apparatus was developed in our laboratory. It allows the application of external tensile stresses, in situ oxidation of the sample in controlled environment and X-ray diffraction…

Mechanical, Kinetic and Morphological Correlations during the Scale Growth of Niobium and Zirconium Oxides

Oxidation of niobium at 700°C and zirconium at 850°C, in air or oxygen pressures from 20 to I 000 hPa, lead to the formation in one case of a stratified Nb 2 O 5 oxide scale and in the other case a more compact ZrO 2 oxide scale. Analyses of the evolutions of phases, crystalline textures and mechanical stresses during oxidation are determined by X-ray diffraction techniques. Interactions between chemical processes, crystalline building and mechanical processes constitute together the driving force of the reaction and can explain the differences in morphologies of oxide scales.

INTERACTION BETWEEN MECHANICAL STRESSES AND OXIDATION OF THE NIOBIUM