Ultrafast surface diffusion driven by abnormal relaxation mode in the marginal glass-forming Fe-based metallic glasses

Atomic surface mobility of glasses plays an important role in understanding glass dynamics and determining many fundamental processes on the surface. However, the diffusion dynamics at the free surface in marginal glasses remains unknown due to limited glass formation ability. In this study, we systematically investigate surface diffusion and relaxation behavior in four marginal glass-forming Fe-based metallic glasses with great application potential. Surface diffusion rates in marginal glass- forming Fe-based metallic glasses are significantly faster than those of stable metallic glasses. For the first time, an abnormal β_t relaxation mode with thermal activation character is identified between α and β relaxation. Strikingly, the activation energy of surface diffusion matches that of β_t relaxation. A mechanism involving cooperative cluster motion associated with β_t relaxation is proposed to explain the ultrafast surface diffusion. These results establish a direct correlation between surface diffusion and bulk relaxation, providing a basis for tailoring surface properties in metallic glasses.