Computational modeling of plastic deformation and shear banding in bulk metallic glasses

The study of plastic deformation of bulk metallic glasses (BMGs) reveals two modes. At low temperatures and high stresses plastic deformation is inhomogeneous, in which case the failure of BMGs is mostly due to the formation of a very thin shear band. Inside this band, the evolving strain is highly localized. At high temperatures and low stresses, plastic deformation is homogeneous and the BMGs exhibit a Newtonian viscosity. It is generally acknowledged that the shear-induced free volume evolution plays an important role in the failure of BMGs under both modes of deformation. In this paper, a free volume-based constitutive model is proposed, which accounts for the transition of inhomogeneous to homogeneous deformation and non-Newtonian to Newtonian viscosity. A time integration algorithm is presented in this paper with a description of its implementation in the commercial code ABAQUS through the user-defined material window UMAT. 2 and 3-Dimensional finite element simulations are performed and the model is validated with results of uniaxial compression and micro-indentation experiments for a wide range of temperatures and strain rates. The simulation results exhibit hydrostatic pressure dependence, and suggest that a mixed-power-strain-rate-dependent free volume creation mode is more effective than the commonly used strain-rate-dependent free volume creation law, when considering large temperature ranges.