Uranium Surface Preparation for EBSD and Optical Microscopy

Abstract

Depleted uranium has a variety of commercial and military applications in which mechanical properties are tailored for a specific use. Metallographic examinations, as carried out with both optical and orientation contrast microscopy (electron backscatter diffraction or EBSD), are important tools in controlling grain size, inclusion content, and microstructure for optimizing properties. The final preparation step for optical metallography generally involves acid-etching. A successful electropolishing technique has been reported for preparing EBSD specimens [1]. Conventional specimen preparation generates a mixture of acids and radioactive waste, so-called "mixed waste" as defined by the Resource Conservation and Recovery Act (RCRA). Mixed waste is more costly and more difficult to disposition than either separate component. The desire to eliminate mixed waste streams from the metallography laboratory led to the development of ion-beam polishing and etching techniques as the final step in the preparation of uranium specimens.All specimens were mounted in epoxy and mechanically ground and polished, using 0.05-µm alumina as the final polish. For both optical and EBSD methods, one goal was the elimination of corrosion and the control of corrosion on the post-ion-beam surface. The optical specimens required etching to reveal microstructural features, whereas, all polishing damage had to be removed from the EBSD specimens. A commercial ion-etching and coating system was used. Xenon gas was used as the etching and polishing species, rather than argon, because argon ions tended to roughen the surface, forming micron-sized bumps. The heavy xenon ions provided greater momentum to remove the uranium atoms. Optical specimens mounted in epoxy could be adequately etched at 6kV by rocking the specimen between 0° (normal incidence) and 30°, while rotating about the surface normal, for two minutes. After the ion-beam etching, a 2 to 3-nm-thick layer of carbon was sputterdeposited onto the surface at the rate of 0.01nm/s. Figure 1 shows a bright field image of a uranium surface treated as described here. The uniform background due to the absence of oxidation will permit the digitization of inclusions for automatic analysis. A typical specimen could be handled in air for several days without visible oxidation.The removal of polishing damage from EBSD specimens was important because of the shallow depth from which EBSD patterns arise. The surface from mechanical polishing did not produce EBSD patterns. Additionally, since etching was highly orientation dependent, it was necessary to prevent excessive etching so that rough morphology did not interfere with the data collection process. Thus, EBSD specimens were ion-milled under conditions that promoted a more uniform material removal. EBSD specimens were ion-milled at a xenon ion-beam incidence of 75° and 5 kV beam potential for approximately 2 hours, followed by carbon deposition, as described above. Under those conditions, only a light etch resulted; however, the surfaces produced hi...