Thermal stability of the microstructure in rolled tungsten for fusion reactors

Plasma-facing components of future fusion reactors will have tungsten-based materials as armor. Annealed pure tungsten is brittle at room temperature restricting its use as plasma-facing material, whereas plastically deformed tungsten behaves in general more ductile at ambient temperatures. During operation as plasma-facing material at high temperatures, the deformation structure induced by plastic deformation becomes unstable. Restoration processes as recovery, recrystallization, and grain growth will alter the microstructure and impair the desired mechanical properties. In particular, recrystallization will reinstate the intrinsic brittleness of tungsten. Achieving a thorough understanding of the occurring restoration mechanisms (for long times at temperatures as close to the desired operation temperatures as possible) and quantifying the temperature-dependent recrystallization kinetics are essential for assessing the materials performance and an informed materials selection. The thermal stability of differently rolled pure tungsten plates is reviewed with the aim of predicting the materials lifetime; the impact of different activation energies on the selection of armor materials highlighted. The concept of a recrystallization temperature constituting a threshold temperature below which recrystallization does not occur is dismissed.