Abstract
Morphological and nanomechanical alteration of tungsten in extreme environments, like those in edge localized modes in nuclear fusion environments, up to 46.3 GWm−2 heat fluxes were experimentally simulated using electrothermal plasma. Surface and subsurface damage to the tungsten is seen mainly in the form of pore formation, cracks, and resolidified melt instabilities. Mirco voids, rosette-type microfeatures, core-shell structure, particle enrichment, and submicron channels all manifest in the damaged subsurface. The formation of voids in the subsurface was determined to originate from the ductile fracture of hot tungsten by plastic flow but not developed to cracking. The voids were preferentially settled in grain boundaries, interfaces. The directionality of elongated voids and grains is biased to the heat flow vector or plasma pathway, which is the likely consequence of the thermally driven grain growth and sliding in the high-temperature conditions. The presence of a border between the transient layer and heat-affected zone is observed and attributed to plasma shock and thermal spallation of fractural tungsten at high temperature. Plasma peening-like hardening effects in tungsten were observed in the range of 22.7–46.3 GWm−2 but least in the case of the lowest heat flux, 12.5 GWm−2.
Highlights
Tungsten is a leading material for the divertor of tokamak-style fusion reactors due to its high melting point (~3422 °C), good thermal conductivity (~175 Wm−1°C−1 at room temperature and ~120 Wm−1°C−1 at 800 °C), reasonably good mechanical strength (EY ~ 450 Gpa, nanohardness ~8 GPa), high sputtering threshold energy (Eth ~ 200 eV for deuterium), resistance to hydride formation, and low tritium retention[1]
Damage to tungsten by hydrogen and helium ions, thermal shock from plasma exposure, and high heat effects are all important to understand for predicting the materials performance in nuclear fusion reactor environments[2,3,4]
An extremely high heat flux environment known as an edge localized mode (ELM)[5] deliver 1–10 GWm−2 flux over submillisecond time frames for 10–100 sec, and is cause for concern for long term reactor operation
Summary
Tungsten is a leading material for the divertor of tokamak-style fusion reactors due to its high melting point (~3422 °C), good thermal conductivity (~175 Wm−1°C−1 at room temperature and ~120 Wm−1°C−1 at 800 °C), reasonably good mechanical strength (EY ~ 450 Gpa, nanohardness ~8 GPa), high sputtering threshold energy (Eth ~ 200 eV for deuterium), resistance to hydride formation, and low tritium retention[1]. Despite these advantages, there are significant concerns as to the survivability of tungsten in a fusion environment. Previous studies with capillary discharging devices have reached heat flux ranges from 10 to ~140 GWm−2 making them excellent tools for simulating ELM conditions[7]
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