Identifying microstructural changes responsible for retarded grain growth during tungsten recrystallization after helium plasma exposure

Helium plasma is known to affect recrystallization in tungsten, with lower temperatures during plasma exposure leading to slower crystal grain growth. To understand why this occurs, tungsten samples were first exposed to helium plasma at surface temperatures between 300°C and 800°C, before annealing at temperatures between 1100°C and 1400°C. Annealing after helium exposure at 300°C was confirmed to lead to smaller crystal grains than annealing after exposure to helium at 500°C. Small 1-2 nm radius nanobubbles formed readily in tungsten after helium plasma exposure, but disappear after annealing at temperatures of 1100°C and above. The formation of cracks and open volumes beneath the surface was observed exclusively in tungsten exposed to helium-plasma at 300°C, with extensive surface cracks visible after annealing. These cracks were not observed for higher temperature helium exposure and likely form due to the strong tendency of bubbles to cluster along grain boundaries for helium exposure at 300°C. Despite this, nano-mechanical testing revealed a similar influence of annealing conditions on tungsten hardness for all plasma exposure conditions studied. The crack formation is likely caused by interactions between solute helium and residual defects from surface polishing.