Universal secondary relaxation and unusual brittle-to-ductile transition in metallic glasses

It has long been recognized that the relaxation spectrum of glassy solids is intrinsically connected to their disordered structural features and deformation behaviors. However, such connections still remain elusive for metallic glasses. Here, through the extensive study of a variety of metallic glasses over a wide range of temperatures, we provide the compelling evidence for the existence of a universal fast secondary relaxation process, which occurs at a temperature far below the glass transition point with a low activation energy (0.3-0.6 eV). Furthermore, it is demonstrated that the initiation of plasticity in metallic glasses is strongly correlated with the fast relaxation process. By exciting the fast relaxation process, multiple shear banding is triggered as opposed to single shear banding in the metallic glasses, which leads to an unusual brittle-to-ductile transition in malleability, being fundamentally different from the ordinary one commonly observed in crystalline alloys. Furthermore, our results shed the quantitative insights into the atomistic mechanisms that differentiate brittle from plastic metallic glasses at different temperatures.