On the effect of hydrostatic stress on plastic deformation in metallic glasses

Previous experimental observations have demonstrated that large stress gradient results in high plasticity, while the underlying mechanism and constitutive laws are rarely reported. In this work, a micro-mechanics based model was proposed to account for the contribution made by hydrostatic stress. This micro-mechanics model was then incorporated into the free volume model describing the viscous softening behavior and a generalized constitutive law was established. Using a user material subroutine (UMAT) and von-Mises criterion, the new constitutive equations were implemented into a finite element code to simulate the material response and free volume evolution procedure under tensile and compressive loading when hydrostatic stress was taken into account. Furthermore, comparison was made between current simulation and previous theoretical or experimental results, good agreement was successfully obtained. Therefore, it is concluded that the current constitutive law is a good candidate for describing the deformation behavior of metallic glass under complex stress states.