Tungsten Recrystallization Behavior Under Steady and Transient Hydrogen Plasma Loading

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Because of its extraordinary material properties, like its high melting point and thermal stress resistance, low erosion and swelling rate, and high radiation damage resistance, highly deformed pure tungsten has been chosen as the plasma facing surface material for the ITER reactor divertor. The study of tungsten’s recrystallization behavior and damage response during operation conditions is thus important because the divertor will have to withstand high heat fluxes and temperatures during service which induce recrystallization. This phenomena alters the microstructure of the material, inducing degradation in its properties, like loss in mechanical strength and embrittlement making it prone to large plastic deformation, surface roughening, crack networks formation and propagation. Understanding this behavior under the ITER reactor operation circumstances is paramount for the success of the reactor.

The aim of this project was to test regimes simulating steady state operation and high frequency and temperature transient pulses called ELMs (Edge Localized Modes) striking the divertor. Recent research has shown that the degradation and behavior of tungsten under these transient conditions does not consistently follow the expected parameters characterized in the literature. According to it, recrystallization, grain growth, and crack formation seem to be suppressed by the plasma loading under these regimes, thus a new understanding of the material behavior for these circumstances must be developed. To do this, ITER grade tungsten samples were subjected to a hydrogen plasma beam at DIFFER’s Magnum-PSI with temperatures at the strike point ranging from ~1000 to ~1500 °C and high frequency pulses that increased the surface temperature by ~200 to ~300 °C above the steady state temperature. The surface thermal shock response to the plasma pulses was characterized by means of infrared and pyrometer readings at the samples’ surface during exposure. Temperature and power density calculations were correlated with identified damage morphologies on the targets and a damage map for the experiments was elaborated, which showed that the most severe damage (cracks and crack networks) begin to appear in the range of the measured recrystallization temperature of the samples, which was lower than expected.

Using Vickers hardness, the recovery and recrystallization kinetics of the material were characterized by means of logarithmic decay and a modified version of JMAK recrystallization kinetics that includes an incubation time for the onset of recrystallization. Recrystallization kinetics were found to accelerate as the hydrogen exposure progresses, thus yielding lower effective activation energies for recrystallization when comparing furnace one hour exposures, plasma one hour exposures, and plasma four hour exposures. This pointed to the presence of hydrogen actively reducing the activation energy for self-diffusion. Simulations of hydrogen diffusion were performed to test this hypothesis, and even though the total concentration is low, given that the high experimental temperature does not permit trapping of the hydrogen, the diffusing atoms may still play a role in accelerating the recrystallization kinetics.

Based on the results of this research, it is proposed that interstitially diffusing hydrogen segregating to voids or grain boundaries is modifying the behavior of surrounding tungsten crystal lattice. Specifically, the mobility of the grain boundaries may be increasing because the hydrogen’s presence would be promoting the creation of ledges in the grain boundary resulting in an overall free energy reduction for grain boundary diffusion. This localized defect formation would require a lower concentration of hydrogen than that required for solute drag or other suppression mechanisms, and might be mechanism behind the behavior observed in this work’s experiments.