Helium bubble behaviour in W-Mo alloys by in-situ TEM under He+ irradiation

We have assessed the effect of molybdenum content on the helium bubble behaviour in W-xMo materials (x = 0, 3.6, 10.2, 39 at.%), by using in situ transmission electron microscopy to monitor the bubble evolution during He + irradiation at 800 °C to a fluence of 1 × 10 20 ions/m 2 , and post-irradiation annealing to 1200 °C. Irradiation induces the formation of a bubble population that evolves with He + fluence by increasing in mean diameter and bubble count. The 10.2 at.% Mo content delays the detection of the first bubbles in the microstructure to a higher fluence level, as compared to the other three material compositions, and also reduces the detected bubble counts and fraction. This mirrors the predicted Mo effect on He diffusivity in tungsten, and points to the mobility of interstitial He atoms and potentially small He-vacancy clusters as the main factor controlling the formation and growth on helium-containing bubbles, rather than the amount of surviving vacancy-type defects after the radiation-induced displacement cascade. The bubble population increases gradually in mean diameter during post-irradiation annealing to 1200 °C. However, the small bubbles become unstable and re-emit helium to the matrix above 1100 °C, for all studied compositions apart from 39 at.% Mo. These results open the way for tailoring of the Mo content for enhanced He tolerance, with potential implications for fuel retention as well, of tungsten-based components in fusion tokamak environments. • In-situ He + irradiation at 800 °C and subsequent annealing to 1200 °C of W-xMo alloys. • 10at.% Mo gives the highest critical fluence for He bubble detection at 800 °C. • Average bubble size increases with temperature up to 1100 °C for all materials. • Above 1100 °C small bubbles become unstable in all materials other than W-39at.%Mo. • The bubble evolution is mainly attributed to molybdenum's effect on He diffusivity.