Transition from linear to nonlinear viscoelasticity during deformation in a Zr-based glassy alloy

The transition behavior from linear to nonlinear viscoelasticity during constant strain-rate deformation of a Zr-based glassy alloy near the glass transition temperature is investigated and a calculation based on concept of a fictive-stress model is performed. The experimental results show that the viscoelastic behavior of the glassy alloy is characterized by a very narrow relaxation-time distribution due to its simple atomic structure. Hence, the transition between steady-state Newtonian and non-Newtonian flows can be analyzed by a stretched exponent relaxation-function of strain rate. The condition at which the transition occurs in the Zr-based glassy alloy is investigated with a new model proposed on the basis of the hypothesis of stress-induced structural relaxation and a concept of fictive stress that expresses the structure of the glassy material indirectly. Stress-strain curves calculated from the model agree quantitatively well with experimental results. The calculated curves at sufficiently high strain-rates in the nonlinear viscoelastic regime show a stress-oscillation. This has been observed in many polymers, but not in glassy alloys. In the Zr-based glassy alloy, the oscillation is observed as predicted by the model.