Fundamental links between shear transformation, β relaxation, and string-like motion in metallic glasses

Plastic deformation and relaxation dynamics are two major topics in glass physics. Secondary (β) relaxation has been assumed to be a relevant plastic mechanism in amorphous solids, e.g., metallic glasses (MGs), at the macroscopic scale. However, due to the constraints of the time scale of traditional computer simulation, it is still an open question whether the correlation can be justified at the atomic level, or is it just a correlation in a mean-field sense? In this work, molecular dynamics simulations augmented with metadynamics are conducted to study the basic atomic rearrangement mechanism up to experimentally relevant timescales (up to milliseconds). We show that in a unique Al90Sm10 MG with pronounced β relaxation, the atoms initiating the plastic deformation are exactly the ones apt to develop string-like cooperative motions, as expected from the perspective of β relaxation. For a Y65Cu35 MG without obvious β relaxation, string-like motions are rarely observed in the deformation of participating atoms, although their atomic displacements are also aligned in a correlative manner. Therefore, our laboratory long-time-scale observation enables the in situ construction of a fundamental link among the versatile concepts of shear transformation, β relaxation, and string-like motion in amorphous materials.