Transition from softening to hardening related with strain rate in Zr-based metallic glasses

•The dynamic fracture strength with the increasing strain rate shows the transition from strain-rate softening to strain-rate hardening.•The viscosity in the shear bands greatly reduces and the free volume keeps increasing, which results in the occurrence of strain-rate softening.•3.The shear bands are difficult to extend due the strong interaction between multiple shear bands and dendritic crystal, leading the occurrence of strain-rate hardening.•The transition from softening to hardening mainly depends on the competition between free volume and local crystallization.