Experimental investigation of the microstructural changes of tungsten monoblocks exposed to pulsed high heat loads

Extending the lifetime of tungsten based plasma facing components for future fusion reactors remains an everlasting challenge. In this work, the microstructure of tungsten monoblocks exposed to a few thousand cycles of combined pulsed heat loads of 10 and 20 MWm ⁻² (achieved via an electron beam) is thoroughly investigated. The heat exposure assisted surface roughening is observed to be significant. Build-up of thermal stresses in the monoblock results in severe geometrical distortions. The evolution of the microstructure of the tungsten monoblocks due to recrystallization is found to be substantial in the top 5.5 mm from the surface, and the relation between the recrystallization threshold and temperatures throughout the monoblock is investigated. Additionally, no traces of recrystallization-induced crack formation within the monoblock is observed. The recrystallization-induced microstructural evolution is investigated in terms of grain size, grain boundary distribution, and the recrystallization induced softening as determined from micro-hardness measurements. An adequate quantitative match between the changes in the microstructural features such as grain size, grain boundary character, and the related hardness is obtained. Moreover, the depth dependent microstructural recrystallized fraction in the monoblock is examined via hardness and EBSD measurements, and a comparison between the different methods is presented. The presence of a preferred crystal orientation of the recrystallized grains is observed and discussed in terms of the initial texture.