Dynamic response of He bubbles in Fe9Cr1.5W0.4Si F/M steel under in-situ stepwise nanocompression

Inert-gas bubbles in metals have attracted great attention due to their fundamental role in swelling and degradation of materials. Studies of bubble behavior can provide valuable insights for developing advanced materials. Although extensive research has been conducted on bubble behavior under irradiation or thermal conditions, there is still a lack of investigation under both stress and temperature conditions. In this work, in-situ uniaxial compression tests were performed on helium-bubbled Fe9Cr1.5W0.4Si F/M steel using transmission electron microscopy at 923 K to investigate the responses of bubbles with increasing strain. The results revealed a series of bubble responses, including frequent annihilation, occasional coalescence, and a "forced" coalescence achieved by connecting multiple bubbles under high strain. These responses relied on bubble migration and interaction, influenced by the factors such as the He-V ratio, relative bubble position, and sink effect. Furthermore, the migration mechanism of compressed bubbles at high temperatures is considered to be a combination of surface diffusion induced by thermal gradient and volume diffusion induced by stress gradient. These two mechanisms are expected to compete with each other in different temperature and stress. These findings offer crucial experimental insights into the behavior of bubbles in solids exposed to stress and thermal conditions.