Effect of He Plasma Exposure on Recrystallization Behaviour and Mechanical Properties of Exposed W Surfaces—An EBSD and Nanoindentation Study

Highlights: EBSD, nanoindentation and AFM have been used to understand the post-plasma exposure recrystallization behaviour of W. These techniques have shown that exposure to plasma at all temperatures from 300 °C to 800 °C leads to a retardation in recrystallization when annealed, especially to an annealing temperature of ~1200 °C. It was found the exposure to plasma at 300 °C was able to retard the recrystallization process up to an annealing temperature of 1400 °C, while higher plasma exposure temperatures (500–800 °C) were effective in slowing recrystallization only up to annealing temperatures of 1200 °C. Plasma exposure and annealing did not have measurable effects on the pile-up around nanoindentation, possibly because plasma exposure itself decreases pile-up, and also retards recrystallization, which would reduce pile-up if allowed to be completed. Fusion reactors are designed to operate at extremely high temperatures, which causes the plasma-facing materials to be heated to 500 °C to 1000 °C. Tungsten is one of the target design materials for the plasma-facing diverter components in Tokamak designs, such as ITER, because of its excellent high-temperature strength and creep properties. However, recrystallization due to high temperatures may be detrimental to these superior mechanical properties, while exposure to He plasma has been reported to influence the recrystallization behaviour. This influence is most likely due to the Zener effect caused by He bubbles formed near the surface, which retard the migration of grain boundaries, while at the same time modifying the surface microstructure. This paper reports a study of the effect of plasma exposure at different sample temperatures on the recrystallization behaviour of W at different annealing temperatures. The characterization after plasma exposure and annealing is pursued through a series of post-exposure annealing, followed by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) characterization and nanoindentation to determine the mechanical properties. Here, it is shown that the hardness is closely related to the recrystallization fraction, and that the plasma exposure at a sample temperature of 300 °C slows down the recrystallization more than at higher sample temperatures of 500 °C and 800 °C. Atomic force microscopy (AFM) was subsequently used to determine any changes in pile-up height around the nanoindents, to probe any indication of changes in hardenability. However, these measurements failed to provide any clear evidence regarding this aspect of mechanical behaviour.