Dislocation and microstructure analysis of tungsten

Abstract

The demand for neutron scattering and imaging techniques for material characterization under controlled conditions is continuously growing over the last couple of years. For such measurements large scale neutron facilities are needed and the European Spallation Source (ESS) will soon be the world's most powerful neutron source, build in Lund, Sweden [1]. Here, neutrons are produced through a spallation process in tungsten, which due to its high atomic number has a high neutron production density and localized neutron production. The use of tungsten avoids issues related to corrosion effects related to water cooling and it is relatively environmentally friendly compared to other target materials. Its disadvantages, however, are its low ductility and a high ductile‐to‐brittle transition temperature. Tungsten is the most critical non‐structural material and its integrity during operation is essential for maintaining helium circulation and retaining of transmutation elements, therefore it must operate reliably and predictably for the planned lifetime of the target. In order to estimate the target life, reliable data is needed on the mechanical properties of tungsten, both in unirradiated and irradiated conditions. Dedicated irradiation programs were established to examine the behavior of tungsten under representative operational conditions, which studies are performed in PSI. In this current study, in order to evaluate the properties of irradiated tungsten, initially as a comparison we must observe the microstructure and mechanical properties of unirradiated tungsten. Evaluating the microstructure of the unirradiated tungsten material normally would not be complicated; such transmission electron microscopy (TEM) samples can be created relatively easily by cutting 3 mm discs and then thinning them by jet polishing. However, in this case we developed samples in a more complex way, with the use of focused ion beam (FIB) and then flash electrochemical polishing the lamella in order to establish a method where in the future the irradiated tungsten samples can also be examined safely. Thanks to FIB, with the reduction of the sample size the activity of the sample also gets reduced dramatically, ensuring safe handling. The sample is lifted out internally inside the FIB by a micromanipulator and a 8 x 8 μm 2 and 200 nm thick lamella is created. Then, by flash polishing (in 0.5% NaOH, 2 °C) this lamella is more thinned down to the thickness of approx. 60 nm, which is necessary to remove the FIB induced damages on the surface of the lamella. Electron microscopy observation is performed with a JEOL 2010 type TEM operated at 200 keV and equipped with EDX. Bright field (BF and weak‐beam dark field (WBDF) imaging conditions were used at (g, 4g) or (g, 5g), while g=110. In order to obtain quantitative information of the dislocations, they are counted on several low magnification pictures on different areas of the sample.