A constitutive theory for the mechanical response of amorphous metals at high temperatures spanning the glass transition temperature: Application to microscale thermoplastic forming

An extremely promising microscale processing method for bulk metallic glasses called thermoplastic forming has emerged in recent years. At present, there is no generally accepted theory to model the large-deformation, elastic–viscoplastic response of bulk metallic glasses in the temperature range relevant to thermoplastic forming. What is needed is a unified constitutive framework that is capable of capturing the transition from a viscoelastic–plastic solid-like response below the glass transition to a Newtonian fluid-like response above the glass transition. We have developed a large-deformation, constitutive theory to fill this need. The material parameters appearing in the theory have been determined to reproduce the experimentally measured stress–strain response of Zr41.2Ti13.8Cu12.5Ni10Be22.5Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vitreloy-1) in the strain-rate range [10-5,10-1]s-1, and in the temperature range [593, 683] K, which spans the glass transition temperature ϑg=623K of this material. We have implemented our theory in a finite element program, and this numerical simulation capability is used to determine appropriate processing parameters in order to carry out a successful micron-scale hot-embossing operation. By carrying out a corresponding physical experiment, we demonstrate that micron-scale features in Vitreloy-1 may be accurately replicated under the processing conditions determined by use of the numerical simulation capability.