Cycle deformation enabled controllable mechanical polarity of bulk metallic glasses

Tuning anisotropy in bulk metallic glasses, ideally isotropic, is of practical interest in optimizing properties and of fundamental interest in understanding the amorphous structure and its instability. By employing the quasi-elastic asymmetric mechanical cycling method, we effectively induce the polarized plastic response of a model bulk metallic glass, without damaging the sample or introducing significant annealing or rejuvenation effects. Moreover, the polarized anelastic limit can be well controlled by regulating the amplitude of mechanical cycling. Through the atomic-level analysis of nonaffine displacement, it is found that the asymmetric cycling can only exhaust the plastic atomic rearrangements, and the survived anelastic rearrangements govern the anelastic limit of the training direction. While usual structural indicators are not sensitive probes for the induced anisotropy, the changes in local yield stress distributions capture the polarization well. This polarized plastic response of metallic glasses is originated from the plastic-event-exhaustion induced asymmetry of local potential energy surface, rather than the frozen-in anelastic strain. Our study is of fundamental importance, which furthers our understanding of the mechanical deformation of metallic glasses and sheds some light on the prospects for improved properties through induced anisotropy.