3D Visualisation of Crack Distributions in Oxidised Zirconium Alloys by FIB-Slicing
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3D Visualisation of Crack Distributions in Oxidised Zirconium Alloys by FIB-Slicing Chris R. M. Grovenor1, Na Ni2, Sean S. Yardley1, Gareth Hughes1, Sergio Lozano-Perez1, John M. Sykes1 1 Department of Materials, Oxford University, Oxford, United Kingdom. 2 Department of Materials, Imperial College, London, United Kingdom.
ABSTRACT Zirconium alloys have been used as fuel cladding and structural fuel assembly components in nuclear reactors since the 1950s, and show a characteristic variation in oxidation rate and layered crack morphology during aqueous corrosion. It is common to associate the first phenomenon with the appearance of the second. We have used 3D serial sectioning to study the morphology and distribution of cracks in corroded ZIRLO samples at different stages of oxidation, and have shown that cracks nucleate and grow at all stages of the oxidation process not just at the kinetic transition. We have used this data to analyse the nucleation of cracks with reference to the shape of the oxide/metal interface and the distribution of second phase precipitates. INTRODUCTION Zirconium alloys have given service for many decades in water-cooled nuclear reactors, but corrosion in high temperature water is still the limiting factor for achieving high fuel burnup. Oxidation of zirconium alloys proceeds by the inward diffusion of oxygen ions from the oxidizing media to the metal/oxide interface, resulting in a diffusion-limited initial oxidation rate and cubic or parabolic kinetics [1]. Later, these alloys display breakaway behaviour characterized by a loss of protectiveness at a thickness of a few microns leading to a sharp increase in oxidation rate and rapid deterioration in service [1-3]. The final breakaway is often preceded by repeated stages of parabolic kinetics where first slowing and then acceleration of the oxidation rate occurs [4-6]. One characteristic observation is the appearance of roughly periodic lateral cracks in the oxide [7, 8]. The location of the cracks is closely linked to the metal/oxide interface roughness [8], and some authors have suggested that the formation of cracks is associated with transitions in oxidation kinetics [5, 8]. By performing serial sectioning and imaging, modern dual-beam focused ion beam (FIB) instruments can produce datasets suitable for the reconstruction of microstructures in 3D [9]. The analyzed 3D volume can be > 1000 µm3 and at the same time image resolution at the nanometer scale can be achieved. In this paper we show that 3-D characterization of cracks and the metal/oxide interface using sequential FIB sectioning can provide a quantitative description of the microstructure, from which parameters such as crack density, crack distribution and interface roughness can be extracted with good statistics and correlated with the mechanisms of cracking and transitions in oxidation kinetics.
EXPERIMENTAL DETAILS Samples of ZIRLO™ alloy tube (0.89 wt% Sn, 0.09% Fe, 0.87% Nb, 0.01% Cr) were oxidized in an autoclave at 360 °C in simulated pressurized water reactor (PWR) pri
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