  In this work we use molecular dynamics simulations to study the stress
overshoots of metallic glass Cu$_{50}$Zr$_{50}$ in three scenarios
(unloading-reloading, slide-stop-slide, and stress serrations) that are
associated with shear band relaxation. We found that, after the elastic
recovery effect is factored out, atomic volume in the shear band barely changes
during compressive relaxation but decreases during tensile relaxation, while
local fivefold symmetry increases consistently for both cases. We propose that
the atomistic mechanism for the related stress overshoots is due to the
relaxation of structural symmetry, instead of free volume, in the shear band.
Upon unloading, a propagating shear band continues for some time before
arrested, which results in a stress undershoot and could contribute to material
fatigue under cyclic elastic loads. We did not directly observe stress
serrations via molecular dynamics simulations due to the very high simulated
strain rates. While athermal quasistatic simulations produce serrated flow
stress, we note that such serrations result from global avalanches of shear
events rather than the relaxation of the shear band.
