Correlation of hardness and surface microcracking in ITER specification tungsten exposed at QSPA Kh-50

Abstract

In this work, we have investigated the evolution of the hardness of tungsten under successive thermal shock pulses induced under plasma exposure at quasi-stationary plasma accelerator QSPA-Kh50. The applied conditions represent localized modes of plasma instabilities expected under operation in ITER. The base temperature of 300 °C and deposited heat load of 0.45 MJ/m2 is known to be close to the cracking threshold, which is chosen in this study on purpose. Nanoindentation and microstructural characterization (identification of microcracks) is applied to the samples exposed to 10, 50, 70 and 100 pulses to reveal ability of nanoindentation technique to capture the threshold for the microcrack formation. Knowing that under the selected exposure conditions, the subsurface region of the material is a subject to the recrystallization, which is induced by the overheating during the plasma discharge, nanoindentation measurements are performed on the same tungsten grade annealed at 1300 °C, 1500 °C and 1800 °C to achieve different degree of recrystallization. It is shown that multiple microcracks appear after the 50th cycle which correlates with the reduction of the hardness corresponding to the massive grain growth. FEM analysis is applied to identify stress/temperature distribution across the sample depth to clarify the nucleation location, expected to occur in the region with the highest stress concentration close to the ductile to brittle transition temperature.