EBSD Study of Uranium Alloys
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EBSD Study of Uranium Alloys Daria Drozdenko1, Peter Minarik1, Mykhaylo Paukov1, Volodymyr Buturlim1, Ilya Tkach1, Ladislav Havela1 1 Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 12116 Prague 2, Czech Republic ABSTRACT Metallographical examination of Uranium alloys can benefit from electron back scatter diffraction (EBSD) technique. Various methods of surface preparation for microstructural characterization are described and compared. The aim of the study was to optimize the preparation technique for surfaces of U alloy splats for EBSD mapping, particularly in the context of U1-xMox alloys, as properties of Ȗ-U surfaces (e.g. with more Mo) are very different than for mostly Į-U type (low-alloyed U). INTRODUCTION In the course of investigation of U alloys and their hydrides, microstructure and texture analysis has become important. In particular, it is essential to reveal presence of individual phases of uranium, which are difficult to detect by X-ray diffraction in small concentrations, and their spatial distribution. Moreover, we often need to determine the grain size, features of grain boundaries, inclusions, impurity segregation, and twinning. Uranium metal exists in three allotropic phases (orthorhombic Į, tetragonal ȕ, and bcc Ȗphase) [1]. As shown in earlier work [2], conventional alloying route of retaining the bcc structure to low temperatures can be improved by rapid cooling, which reduces the necessary concentration of alloying elements, and yields samples in the form of foils suitable for further studies. To identify presence of small fractions of different phases, detailed microstructure analysis by Scanning Electron Microscope (SEM) is needed. Besides a common electron microprobe analysis, which gives details of elemental distribution, we need to distinguish possible different phases of the same or very similar composition. The electron back scatter diffraction (EBSD) technique can be used in such situation, providing mapping of the phases and crystallographic orientation of each grain. For such technique a high surface quality is a prerequisite. Special surface preparation techniques have to be applied as U alloys tend to oxidize rapidly in air [3-6]. The information depth of EBSD is 50-100 nm, so we do not need the surface clean in atomic sense, as required e.g. for photoelectron spectroscopy. Nevertheless, even thin oxide layer on the surface often makes the EBSD mapping unsuccessful and should be removed. In general, the first step of surface preparation for microstructure characterization is mechanical grinding and polishing. Then, to remove a surface oxide and to reveal the microstructure, different methods could be used. One of the possible methods is electrochemical etching or polishing, widely used for cast U alloys with large grain size of (40 ÷ 200 μm) [7-9], where different electrolyte solutions based on orthophosphoric acid were used. Parameters of the electrochemical procedure (voltage, temperature, time) may vary in dependence on alloying concentration. It was fou
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