Abstract text (incl. figure legends and references)

The development of appropriate materials for fusion reactors that can sustain high neutron fluence at elevated temperatures remains a great challenge. Tungsten is one of the promising candidate materials for plasma-facing components of future fusion reactors, due to several favorable properties as for example a high melting point, a high sputtering resistivity, and a low coefficient of thermal expansion. Pure W was neutron irradiated at 600 °C, 900 °C, 1000 °C, 1100 °C, and 1200 °C with a dose of ~1 dpa, and its microstructure was subsequently analyzed using transmission electron microscopy (TEM). The study provides the basis for a deeper understanding of the microstructural evolution of W under neutron irradiation.

Three types of defects forms in W as result of neutron irradiation: (i) voids, (ii) dislocation loops and (iii) W-Re-Os containing precipitates. The TEM study includes a detailed examination of the defect structure and, in particular, the distribution of transmutation-induced Re and Os, whose content is expected to be in the range of ~2% for Re and 0.2% for Os under the applied irradiation conditions. Analytical investigations show that the voids, precipitates and loops are surrounded by 8-15 nm Re- and Os-rich "clouds" (Fig. 01). The "clouds" do not produce any diffraction contrast in TEM images and can only be visualized by analytical methods. Grain boundaries act as sinks for all types of point defects, i.e., vacancies and interstitials, as well as Re and Os atoms, leading to the formation of a 10-20 nm wide zone adjacent to the grain boundaries that is free of voids and precipitates (the so-called depleted zone). In addition, we found that Re and Os are often inhomogeneously distributed along grain boundaries, which is due to segregation at boundary dislocations (Fig. 1).

The elemental maps in Fig.1, show the general distribution of transmutation elements on the large scale and are less useful for identifying concentration differences within individual clouds. The minor intensity variations are not well reproducible and identifiable in the color map. For this reason was used the intensity profile analysis across defects to show differences in Re and Os distribution in W irradiated at 600 °C (Fig.2). The elemental profiles in Fig. 2e shows that the Re intensity decreases slightly in the center, reflecting the position of the void. Precipitates, on the contrary, show a higher local Re and Os concentration in the center (Fig. 2e`). The intensity profiles across the marked loop are plotted in Fig. 2e``. It is notable that Re exhibits a uniform distribution in the area inside the loop, while Os tends to segregate at the dislocation line (Fig. 2e,e``).

This study presents the results of extensive microstructural analyses of W samples, which were neutron irradiated at temperatures between 600 °C and 1200 °C, up to a damage dose of ~1.0 dpa. The formation of dislocation loops, voids, and precipitates consisting of Re-Os- χ-phases was observed and characterized in detail.

Fig. 01. STEM-EDX spectrum images show the distribution of transmutation-induced Re (green) and Os (blue) inside W irradiated at 600 °C (a,a`), 900 °C (b, b`) and 1100 °C (e, e`).

Fig.02. Re, Os and combined Re/Os elemental maps obtained in W irradiated at 600 °C (a-d). The Re and Os intensity profiles of void (magenta arrow), a precipitate (yellow arrow) and a dislocation loop (white arrow) shown in parts (e-e``).