Cracks and blisters formed in nanocrystalline tungsten films by helium implantation

Abstract

Tungsten is regarded as a potential plasma facing material (PFM) in nuclear fusion reactors owing to its high melting point, good thermal conductivity, and low sputtering yield. However, various types of defects such as interstitial atoms, vacancies, dislocation rings, cavities, helium bubbles, and “fuzz” occur in W under helium irradiation. In this study, we obtained α- and β-phase W films with nanochannels and used a helium ion microscope to conduct helium ion implantation experiments on the films. The implantation dose and energy were 1.5 × 1022 ions/m2, and 30 keV, respectively. There was no helium bubble formation in the β-W film until the completion of the implantation. The uniformly distributed columnar crystals growing perpendicular to the substrate constitute remarkable nanochannels. These effectively inhibit the aggregation of helium atoms. For α-W films prepared at high temperatures, the escape efficiency of the implanted helium atoms was reduced owing to the chaotic and reduced number of nanochannels, and helium bubbles are eventually formed. Cross-sectional images revealed that helium atoms tended to gather near the grain boundaries and gradually followed the nanochannels formed by the grain boundaries. The expansion eventually formed numerous longitudinal fissures. The results show that grain boundaries play a decisive role in the radiation resistance of nanostructured materials. Furthermore, the preparation of regular nanochannels that reach the sample surface is an effective method to improve the helium escape capability.