Stagnant recrystallization in warm-rolled tungsten in the temperature range from 1150 °C to 1300 °C

Pure tungsten is the prime candidate for armor material of fusion reactors; its application at the expected operation temperatures for longer times will result in changes in the microstructure, in particular due to recrystallization, undermining the otherwise outstanding properties of tungsten. Investigating the thermal stability of tungsten depending on the manufacturing technology is therefore crucial. The thermal response of a sintered, hot isostatically pressed tungsten plate warm-rolled to 80% thickness reduction is assessed in the temperature range between 1150 °C and 1300 °C. Isothermal annealing treatments were performed at six different temperatures. With increasing annealing time, the macro hardness decreased and different stages corresponding to different stages of the microstructural evolution and the progress of recrystallization could be identified and confirmed by electron backscatter diffraction. For all six annealing temperatures a stagnation period in the evolution of the macro hardness was observed where the degradation of mechanical properties halted for a significant amount of time, before it resumed. Microstructural investigations revealed that the stagnation occurred when tungsten was still only partially recrystallized. For the time to half recrystallization, an activation energy of 548 kJ/mol comparable to the activation energy of bulk self-diffusion is inferred.