Thermal stability of thin rolled potassium-doped tungsten sheets during annealing at temperatures up to 1400 °C

Abstract

Microstructural changes will occur in tungsten when used as plasma-facing material in future fusion reactors due to the high operation temperatures. Potassium doping has proven to delay such undesired changes in drawn tungsten wires. The same strategy is adapted to thin cold-rolled tungsten sheets containing 80 ppm potassium. Their thermal stability is characterized by isochronal annealing for two hours as well as isothermal annealing at selected temperatures for longer times. Mechanical degradation is quantified by micro hardness testing; microstructure and texture evolution are investigated using scanning electron microscopy and electron backscatter diffraction. During annealing for two hours at temperatures between 800 °C and 1400 °C, the potassium-doped tungsten sheets undergo recovery. The strong rotated cube texture present in the as-rolled condition intensifies enormously during annealing for two hours, but does not change significantly during further annealing. Isothermal annealing at temperatures between 1300 °C and 1400 °C causes significant loss in hardness during the first two hours, but only smaller changes further on, indicating that solely extended recovery is occurring. No evidence for primary recrystallization is found as recrystallization is retarded by a combination of high angle boundaries being absent due to the strong texture and their immobilization by potassium bubbles. The recovery kinetics is quantified based on Kuhlmann’s recovery model and compared with observations on differently rolled thin plates of pure tungsten. Despite different amounts of recoverable hardness, the recovery parameters resolved for potassium-doped tungsten fit nicely to the master curve for recovery obtained earlier on pure tungsten plates confirming that recovery is indeed the dominating restauration mechanism and that doping with 80 ppm potassium does not alter the recovery mechanism fundamentally.