Advanced Characterization of Irradiation Induced Defects in Tungsten Using STEM Optical Sectioning

Tungsten (W) is the primary material of choice to be used for plasma-facing components (first wall and divertor) of fusion reactors because of its superior high-temperature properties, neutron irradiation resistance and low retention of implanted tritium. In a fusion reaction, high energy neutron irradiation changes the mechanical and physical properties of materials due to the accumulation and evolution of the primary defects into stable nanometre-sized interstitial and vacancy clusters. According to molecular dynamics (MD) simulations, 97% of the defects formed under irradiation are smaller than 2 nm, and they contribute significantly to the hardening effects in the irradiated material. Despite the importance of this fact, experimental studies of these defects at the sub-nanometer level are yet to be reported.<br/>Scanning transmission electron microscopy (STEM) optical sectioning can be used to observe the strain or atomic displacements induced by dislocations [1, 2]. Here, we applied this technique to observe irradiation induced defects in tungsten. For STEM observation, 30 nm thick specimens have been prepared from single crystal tungsten using the wedge polishing technique in order to avoid ion irradiation artefacts during specimen preparation. The TEM lamellae have been subsequently irradiated <i>in situ</i> with 150 keV W ions up to 0.005 dpa. Several set of experimental data have been acquired at 30, 50, 70 mrad convergence angle and different acceleration voltages on a Thermo Fisher Themis at 300 kV and on a Nion UltraSTEM 100 at 100 kV. Focal series of STEM images of a few nanometer large irradiation induced defects were acquired with a focus step of 2 nm.<br/>Well established TEM characterization techniques, relying on dynamical or kinematical two beam imaging as such dark-field TEM imaging have limitations in detecting and characterizing radiation damage smaller than 2-3 nm. In this study, we show that STEM optical sectioning in the aberration-corrected STEM can be applied to successfully characterize dislocation loop smaller than 2 nm. The depth location of the defects within the specimen can be qualitatively analyze and the nature of the loops (vacancy versus interstitial) can be identify through quantitative analysis of the atomic plane displacements. In order to gain more insight into the strain field around the defects in three dimension, geometrical phase analysis (GPA) was performed on focal series of atomic resolution STEM high angle annular dark field (HAADF) images. The GPA study demonstrates that to accurately measure strain around the defect in three dimensions, it is necessary to take into account channeling of the electron beam down the atomic columns. The effects of experimental parameters such as incident electron beam energy, convergence angle and annular dark-field collection angle will be discussed and finally, a comparison of experimental data with atomic modeling obtained from MD simulation will be presented. We expect that the outcomes of this work will pave the way for the application of advanced characterization techniques to study more complex irradiation defects, such as collision cascade, which will in turn contribute to gain mechanistic understanding of irradiation damage.<br/>[1] Yang, H. et al. Nat. Comm. 6:7266 (2015)<br/>[2] Guerrero-Lebrero, M. P. et al. IMC 2014 Proceedings, ISBN 978-80-260-6720-7<br/>[3] Hÿtch, M. J., Snoeck, E., Kilaas, R. Ultramicroscopy 74:131–146 (1998)<br/>[4] This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training program 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. SuperSTEM is the UK National Research Facility for Advanced Electron Microscopy, supported by the Engineering and Physical Sciences Research Council (EPSRC).