High temperature indentation of helium-implanted tungsten

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