Mechanism of nanocracks formation in tungsten and tungsten-nickel-iron alloy under helium ion irradiation

Plasma-facing materials used in fusion energy facilities, such as pure tungsten (W) and tungsten-nickel-iron (W-Ni-Fe) alloys, must withstand helium irradiation, which can lead to nanocrack that eventually develop into surface blistering and large-size crack. The mechanisms behind this phenomenon remain elusive, highlighting the need for a detailed investigation. This study employed positron annihilation spectroscopy (PAS) and transmission electron microscopy (TEM) to analyze the development of nanocracks. The results elucidate that helium irradiation initially engenders vacancy-type defects within the material matrix. As the irradiation dose escalates, implanted helium atoms are captured by these vacancies, leading to the formation of helium-vacancy complexes that subsequently expand into larger aggregates and eventually evolve into helium bubbles. The coalescence of these bubbles results in the generation of high-pressure helium entities, which precipitate nanocrack formation once the internal pressure surpasses the structural limits of the material, ultimately culminating in surface blistering.