Temperature gradient based annealing methodology for tungsten recrystallization kinetics assessment

In future thermonuclear fusion reactors, like ITER and DEMO, plasma-facing components with tungsten armor material will have to sustain high thermal fluxes (10 MW/m² in steady state and 20 MW/m² in quasi-steady-state). For such extreme conditions, tungsten may reach temperatures higher than 1000 °C and, consequently can be prone to recrystallize, which can limit the lifetime of the divertor targets under cycling thermal loadings. Characterization of recrystallization kinetics involves the use of different methods and devices (furnace, laser heating) depending on the annealing temperature regime to be analyzed. For these methods, the heating and the subsequent characterization of around 10 samples (5 × 4 × 4 mm) per annealing temperature is needed. In this article, a method is proposed to quantify the recrystallization kinetics between 1300 °C and 1600 °C using a limited number of samples and heating conditions. The basic idea is to induce a steady-state temperature gradient in tungsten rods by heating one side of the rod using a laser heating system. Tungsten rods (50 × 4 × 5 mm) were annealed with this method. The resulting microstructures are characterized using optical microscopy and hardness measurements. The different stages of tungsten softening are clearly evidenced. These first results can be considered as a proof of concept for the use of such a methodology to assess tungsten recrystallization at high temperature.