Self-passivating smart tungsten alloys for DEMO: a progress in joining and upscale for a first wall mockup

Abstract

Self-passivating, so-called smart alloys are under development for a future fusion power plant. These alloys containing tungsten, chromium and yttrium must possess an acceptable plasma performance during a regular plasma operation of a power plant and demonstrate the suppression of non-desirable oxidation of tungsten in case of an accident. The up-scaling of the bulk smart alloys to the reactor-relevant sizes has begun and the first samples with a diameter of 50 mm and thickness of 5 mm became available. The samples feature high relative density of above 99% and good homogeneity. With production of bulk samples, the research program on joining the smart alloy to the structural material was initiated. In a present study, the novel titanium–zirconium–beryllium braze was applied successfully to join the smart alloy to the Rusfer-reduced-activation steel. The braze has survived at least a hundred of cyclic thermal excursions in the range of 300–600 °C without mechanical destruction.

Brazing Tungsten/Tantalum/RAFM Steel Joint for DEMO by Fully Reduced Activation Brazing Alloy 48Ti-48Zr-4Be

“…Rusfer/Ta seam can be divided into the following zones: (1) martensite grains, (2) ferritic grains, (3) interaction zone with sharp phases, (4) interaction zone with elongated grains, (5) pure tantalum. Formation of the ferritic zone was also observed in [15,27,45]. The thickness of the layer increases closer to the fillet, because in Figure 6, a 50 µm ferritic layer formed, but in the central zone of a sample [27], this was 20 µm.…”

“…Formation of the ferritic zone was also observed in [15,27,45]. The thickness of the layer increases closer to the fillet, because in Figure 6, a 50 µm ferritic layer formed, but in the central zone of a sample [27], this was 20 µm. In this work (Section 3.2), we present optical images of the central area of the joint, and the thickness is also close to 20-30 µm in different samples.…”

“…By EBSD analysis we determined that the Rusfer/TiZr4Be/Ta seam consists of bcc-Fe, ZrC, Ta 2 Be, ZrFe 2 and bcc-Ta phases. The same microstructure was observed in [27], but no accurate investigation was made there. A ZrC phase formed in the Fe-bcc/ZrFe 2 interface, no further appearance of it was observed, and the thickness of the phase varied between 2-10 µm.…”

“…By EBSD analysis we determined that the Ta/TiZr4Be seam consists of bcc-Ta, Ta2Be, tetragonal-Ta (β-Ta), a solid solution based on bcc-Ti, a ZrC. Formation of ZrC can be related to the mixing of the melts between Rusfer/Ta a Ta/W seams throughout a fillet region [27] that occurs due to high wettability of the br ing alloy. The same morphology of the Ta2Be phase was observed while brazing Ta Figure 8 shows the Ta/W seams' analysis: a-backscattered electron image, b-EBSD phase map, c-g-EDS maps.…”

“…To solve this problem, we are attempting to use a fully reduced activation composition, 48Ti-48Zr-4Be wt.%. This alloy has already been successfully used when brazing a self-passivating tungsten alloy [27].…”