  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
mechanical polarity 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, we find that only plastic atomic rearranged events corresponding
to the training direction can be exhausted by asymmetric cycling and the
survived anelastic events dominate the directional anelastic limit. The
polarized distribution of local yield stress reveals that the mechanical
polarity is attributed to the plastic-event-healing induced asymmetry of local
potential energy surface, rather than frozen-in anelastic strain. Furthermore,
the healing of plastic events associated with the redistribution of local
residual stress indicates the origin of polarity induced by asymmetric cycling.
Our study is of fundamental importance, which furthers our understanding of the
mechanical deformation of metallic glasses and shed some light on the prospects
for improved properties through induced anisotropy.
