Raman Spectroscopy Study of the Tetragonal-to-monoclinic Transition in Zirconium Oxide Scales and Determination of Overall Oxygen Diffusion by Nuclear Microanalysis of O18

Abstract

Two allotropic forms of zirconium oxide, monoclinic and tetragonal, have been identified in the scales formed on zirconium alloys. The transition from tetragonal to monoclinic has been followed by X-ray measurements and Raman laser spectroscopy. Information on the average content of the tetragonal phase was obtained by X-ray diffraction, whereas Raman laser analyses on tapered sections revealed its distribution through the scale thickness. Oxidation exposures were made in an autoclave, using H2O18 and D2O18 to determine the overall diffusion coefficients. In particular, oxide scales have been studied on Zircaloy-4 with three different precipitate sizes, and on a Zr-1Nb alloy, after exposure in an autoclave for between 3 and 100 days. The specimens were analyzed in detail in the vicinity of the kinetics transition point, where the acceleration of corrosion occurs. Raman spectroscopy analyses enabled the crystallographic nature of the ZrO2 to be determined. Close to the interface, the tetragonal phase content is about 40%, when after the transition the tetragonal phase is transformed into monoclinic. The O18 diffusion treatment was carried out in an autoclave at 400°C under pressure on specimens previously oxidized for between 3 and 100 days in natural water vapor pressure. The diffusion profiles were determined by nuclear microanalysis using the O18(p,α)→N15 reaction. Based on these profiles, the volume and grain boundary diffusion coefficients were calculated for each material and for each oxidation time. The results show clearly that oxygen transport occurs principally by grain boundary diffusion. The boundary diffusion coefficient is 108 times higher than those for bulk diffusion (Db is equal to 10-13 to 10-15 cm2/s). The results of these analyses are in agreement with the changes in kinetics observed at the transition point. The stabilization of the tetragonal phase by stress is discussed.

Evolution of stress fields and phase content in corroded zirconium cladding materials

“…Several researchers have noted that the oxide comprises a metastable tetragonal phase and an energetically stable monoclinic phase of ZrO2 [7][8][9][10][11][12][13] . The tetragonal phase develops locally when oxidation occurs; it has a smaller grain size and is believed to be stabilised by the high compressive stress in the oxide layer .…”

“…Shift Raman   (1) where  is the piezo-spectroscopy constant; and  is stress. The piezo-spectroscopy constant was obtained by carefully loading a monoclinic laboratory-prepared zirconia bar similar to the calibration materials use by previous works [8,11,26] and Lorentzian profiles; however, this peak fitting method induced some bias in the peak position owing to the peaks' asymmetry. The peak fits have an accuracy of ± 0.05 cm -1 , which is equivalent to an accuracy of ±28 MPa in the calculated stresses.…”

“…The data suggests that the ZIRLO has gone through transition at about 114 days and the Zr-4 will go through transition at about 160 days as indicated in Figure 1. The origin of the transition is still uncertain, although in general it is believed to be linked to a martensitic-type phase transformation from tetragonal to monoclinic, with an associated 5% volume increase which causes the macroscopic cracks [8,18,19] .…”

“…Corrosion in zirconium alloys is a complicated electrochemical process which is influenced by several factors, including the properties of the metal/oxide interface, water chemistry, pressure, irradiation, and time. Although many reported works [1][2][3][4][5][6][7][8][9] have studied numerous factors affecting the corrosion processes, and considerable understanding of the corrosion mechanisms have been secured, there are many effects that are still not fully understood: in particular, the relationships between the residual stresses that are generated between the oxide film and the substrate, the structural phase content, and the kinetics of corrosion. Better understanding can lead to the development of alloys * kokboonc@sunway.eduy.my † ab6856@coventry.ac.uk…”

Advanced Corrosion-Resistant Zr Alloys for High Burnup and Generation IV Applications

“…The tetragonal phase fraction has been previously linked to corrosion behavior in Zr alloys [17,18]. It is difficult however to separate cause and effect in this case.…”