Mechanical and microstructural analysis of tungsten exposed in JET deuterium plasmas

A tungsten Langmuir probe exposed in the JET divertor during the ITER-like wall campaigns (ILW) has been studied to evaluate changes in mechanical properties and microstructure. The tip of the probe that was exposed to plasma was cross-sectioned and polished for post mortem analysis. Analysis involved a comparison with a non-exposed probe to determine the effect of plasma exposure on material microstructure and mechanical properties. Visually the probe appeared to have melted and re-solidified during its time in the vessel. Secondary electron (SE) images of cross sections showed the formation of bubbles near the exposed surface that ranged from 50 µm to sub-micron sized. Electron backscatter diffraction (EBSD) revealed that the average grain size had increased from 33 µm to 570 µm. The investigation also showed that hardness had increased from 5.2 to 6.1 GPa and pop-in behaviour was supressed after exposure. This was initially attributed to the uptake of deuterium (D) but nuclear reaction analysis (NRA) indicated that no deuterium remained in the sample and hinted that some other type of defect was modifying the mechanical properties.