Microstructural evolution of tungsten under thermal loads: A comparative study between cyclic high heat flux loading and isochronous furnace heating

•Steady-state heating and cyclic high heat flux loading of DEMO divertor W mock-ups was examined.•HHF loading caused overgrown grains on the surface of monoblocks at the 2100 °C temperature zone.•Post-SSH analysis after 1500–––2100 °C resulted a homogenous recrystallized and narrow grain-size distribution.•Vickers hardness values of W after 500 cycles of 20 MW/m2 and hot water cooling did not drop remarkably. Recrystallization is one of the most pronounced microstructural changes of tungsten when subjected to cyclic high heat flux (HHF) loads. In the framework of the European DEMO divertor development an intensive R&D programme is being performed. Many HHF tests of tungsten monoblocks have shown that recrystallization and grain growth have been deemed a major degradation feature leading to brittleness and reduced strength. In the previous recrystallization studies, tungsten was normally heat-treated in a furnace in slow uniform heating. However, in a HHF test or in the assumed fusion reactor operation, the water-cooled tungsten armor is rapidly heated by cyclic HHF pulses generating a steep temperature gradient (20 MW/m2 loading results in about 200 K/mm) and thermal stresses. This difference raises the question as to whether a furnace heat-treatment condition properly simulates the cyclic HHF loading case in terms of recrystallization behaviour if the heat exposure conditions are kept comparable to each other. The present paper addresses this issue. To this end, a comparative microstructural study was performed for two different groups of tungsten samples: one tested under well-defined furnace heat-treatment conditions (1500 ℃, 2100 ℃) and the other one taken from the monoblocks (at the positions of the same corresponding temperatures) of a water-cooled mock-up tested under HHF loads at 20 MW/m2, 500 cycles. Extensive quantitative image analysis was carried out based on detailed microstructural and crystallographic characterization and micro-hardness was measured. The HHF loaded surface is dominated by the formation of extremely large grains in contrast to furnace heating at the same temperature. The samples heated at 2100 °C exhibited a remarkable difference in recrystallization and grain growth behaviour and hardness values between the two heating cases. Two commercial tungsten grades (AT&M, ALMT) showed a similar behaviour to each other.