A fictive stress model and nonlinear viscoelastic behaviors in metallic glasses

Transition behaviors from linear to nonlinear viscoelasticity under constan t strain-rate deformation near the glass transition have been investigated for Pd- and Zr-based alloy glasses. The transition occurs at critical strai n-rate, and the steady-state viscosity may decrease by many orders of magni tude above the critical strain-rate. Concurrently with the transition, the growth of the stress shows a stress-overshoot: the stress increases initial ly attaining a maximum, then decreases and attains a steady-state flow. The transition between steady-state Newtonian and non-Newtonian flows can be a nalyzed by a stretched exponent relaxation function, and both the normalize d viscosity and the flow stress can be represented by a master curve in ter ms of the product of the strain-rate and the Newtonian viscosity. These res ults imply that the occurrence of the transition from the Newtonian to non- Newtonian is explicitly determined by the flow stress. A model, based on th e hypothesis of stress-induced structural relaxation and the concept of fic tive stress for the nonlinear viscoelastic behaviors has been proposed. The model calculation has reproduced fairly well the experimental results of t he Pd- and Zr-based glasses, in particular, the development of the stress-o vershoot behavior. In addition, the model reveals a stress-overshoot and un der-shoot oscillation at very high strain-rate. This oscillatory nonlinear behavior has been observed in many polymer solutions, and also the latest s tudy in metallic glasses, The model calculations of other nonlinear viscoel astic behaviors, such as stress relaxation during stress growth after abrup t cessation of steady-state how, and a stress regrowth after a brief interv al of relaxation, are also presented.