The microstructure and microtexture of zirconium oxide films studied by transmission electron backscatter diffraction and automated crystal orientation mapping with transmission electron microscopy

A Garner,A Gholinia,P Frankel,M Gass,I Maclaren,M Preuss

doi:10.1016/j.actamat.2014.07.062

A Garner, A Gholinia + Show 4 more

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https://doi.org/10.1016/j.actamat.2014.07.062

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Journal: Acta Materialia	Publication Date: Aug 24, 2014
Citations: 126	License type: cc-by

Affiliation: University of Manchester, University of Glasgow

Abstract

A detailed characterization of nanostructured thin zirconium oxide films formed during aqueous corrosion of a nuclear-grade zirconium alloy (Zircaloy-4) has been carried out by means of two novel, ultra-high-spatial-resolution grain mapping techniques, namely automated crystal orientation mapping in the transmission electron microscope (TEM) and transmission electron backscatter diffraction (t-EBSD). While the former provided excellent spatial resolution with the ability to identify tetragonal ZrO2 grains as small as ∼5nm, the superior angular resolution and unambiguous indexing with t-EBSD enabled verification of the TEM observations. Both techniques revealed that in a stress-free condition (TEM foil prepared by focused ion beam milling), the oxide consists mainly of well-oriented columnar monoclinic grains with a high fraction of transformation twin boundaries, which indicates that the transformation from tetragonal to monoclinic ZrO2 is a continuous process, and that a significant fraction of the columnar grains transformed from stress-stabilized tetragonal grains with (001) planes parallel to the metal–oxide interface. The TEM analysis also revealed a small fraction of size-stabilized, equiaxed tetragonal grains throughout the oxide. Those grains were found to show significant misalignment from the expected (001) growth direction, which explains the limited growth of those grains. The observations are discussed in the context of providing new insights into corrosion mechanisms of zirconium alloys, which is of particular importance for improving service life of fuel assemblies used in water-cooled reactors.

Highlights

The motivation for understanding the microstructure and texture of zirconium oxide films comes from the requirement to improve the aqueous corrosion performance and minimize hydrogen pickup of zirconium alloys, which are used to clad nuclear fuel in light and heavy water reactors
A precession angle of 0.8° was used in order to increase the number of diffraction spots and reduce dynamical effects, which has been shown to improve the quality of phase/orientation maps using this system [31]
Despite the relatively small area of investigation, the results presented in this work give a significant insight into the mechanisms of the formation of zirconium oxide on Zircaloy-4 in pressurized water reactor (PWR)-like conditions

Summary

Introduction

The motivation for understanding the microstructure and texture of zirconium oxide films comes from the requirement to improve the aqueous corrosion performance and minimize hydrogen pickup of zirconium alloys, which are used to clad nuclear fuel in light and heavy water reactors. The microstructure of the oxide has been found to directly affect the corrosion performance of zirconium alloys; for example, alloys with low corrosion rates have been observed to exhibit large columnar grains [1]. Their superior corrosion performance is attributed to a reduction in grain boundary area in oxides with large grains, as the grain boundary diffusion coefficient is 108 times higher than the bulk diffusion coefficient of the oxide [2].

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