Nanoindentation has been performed on tungsten, unimplanted and helium-implanted to $ 600 appm, at temperatures up to 750 1 C. The hardening effect of the damage was 0.90 GPa at 50 1 C, but is negligible above 450 1 C. The hardness value at a given temperature did not change on re-testing after heating to 750 1 C. This suggests that the helium is trapped in small vacancy complexes that are stable to at least 750 1 C, but which can be bypassed due to increased dislocation mobility (cross slip or climb) above 450 1 C.
