The plasma-facing components of future fusion power plants must be designed to accommodate property changes from radiation damage and helium implantation. An understanding of these property changes is therefore extremely important. Ion irradiation and nanoindentation are common techniques to study these irradation effects, however until now mechanical testing has been limited to room temperature. This is a far cry from the temperatures at which these materials will be in service.
A MicroMaterials Nanotest has been fitted with a vacuum system (<10 mbar) and independent tip and sample heating to 750°C. This allows the testing of oxidising materials, such as tungsten—the key plasma facing material - that cannot otherwise be tested at temperature. Typical thermal drift rates of <0.10 nm/s are achieved for >90% of the indents performed up to 450°C and rates of <0.15 nm/s are achieved for >90% of the indents performed up to 750°C. These are the lowest drift rates obtained from any high-temperature nanoindenter system. Using this system, the mechanical properties of pure tungsten and tungsten implanted with helium ions have been explored over a range of temperatures and strain rates. This gives a quantitative assessment of the mechanical properties of the radiation-damaged material, and the effect of the presence helium. Testing at different strain rates gives an understanding of the damage structures present causing the change in mechanical properties.
The hardness of pure tungsten decreases strongly with increasing temperature, from ~6 GPa at 50°C to ~3 GPa at 250°C, after which it remains constant. Helium implantation to levels of 600 appm produces an increase in hardness of ~4 GPa at 50°C that decreases to ~2 GPa by 750°C. This decrease in hardening with temperature is thought to be due to the increased mobility of dislocations to bypass helium-vacancy clusters that cause the hardening effect. These results are important for the design of plasma-facing components for any future nuclear fusion device.