Unusual vortex-like atomic motion observed for viscoelasticity in metallic glass

Viscoelasticity in metallic glasses (MGs) is commonly attributed to atomic slips or the activation of local shear transformation events. However, based on our extensive molecular dynamics simulations of creep in MGs, we find that a significant portion of viscoelasticity in MGs is mediated through back-and-forth anharmonic motions of atoms. Interestingly, we also show that such anharmonic oscillatory atomic motion stems mainly from collective atomic rotations, which generates local vorticities with an intensity ten folds of the vorticity induced directly by local shear. As deformation proceeds, the vortex-like atomic motion shows an increasing correlation with local atomic slips or the activation of shear transformation zones. Our findings indicate that the viscoelastic deformation during creep in MGs is a two-step process, which echoes very well with the recent discovery of two secondary relaxations in a variety of MGs.