Delayed surface degradation in W-Ta alloys at 400°C under high-fluence 40 eV He plasma exposure

The surface modifications in W-xTa alloys (x = 0, 6, 11 wt.%), as potential material candidates for tokamak components facing the fusion plasma, have been assessed at a temperature of ~400 °C, by exposing the material surface to 40 eV He for increasing He fluences up to 10^27m-2 and flux of ~10^23 m-2 s-1. Surface wave-like structures appear in all samples at a fluence of 10^26 m-2, whereas at the higher fluence of 10^27m-2 the material’s surface is characterised by the presence of blisters and ablations. This suggests a new critical fluence for blistering in pure He plasma to be between 10^26-10^27 m-2. There is also a near-surface layer with a thickness of ~200 nm that contains He bubbles of 2.20-2.70 nm in average size. Increasing Ta content leads to a small reduction in bubble size, and to clear reductions in the presence of blisters and ablations. Ta alloying can improve the hardness and yield strength and reduce the He diffusivity in the material. This therefore delays bubble formation and surface blistering and ablations that are affected by these mechanical properties. This was exemplified at the highest fluence by a large mitigation of surface material ablation.