Obsevations Ofstress-Induced Structuraldisorder and Fictive Stress in Bulk Metallic Glasses

Abstract

ABSTRACTThe transformation from Newtonian to non-Newtonian viscous flow at the glass transition of Pd- and Zr-based alloy glasses has been investigated in compressive tests under either a constant strain rate, or a constant load. The transition occurs at a critical stress being nearly independent of temperature. The mechanism of the transition thus has been attributed to the stress-induced structural relaxation. This paper describes the evidence of stress-induced disorder as indicated by the change in the viscosity with stress and the evolution of specific heat of the alloy glasses subjected to non-Newtonian steady-state viscous flow. Also presented in this paper is the in-situ observation of structural disorder, by direct measurements of the temperature change of sample, in particular the soften process during a constant load deformation. The heat of evolution is then calculated, and found to scale as the logarithm of the normalized viscosity during entire deformation. This result implies that the relationship between the structural disorder, as indicated by the enthalpy change and viscosity is the same in the transient state during deformation as well as in the steady-state flow process. This is conceptionally very important in that it enables us to introduce a fictive stress which indirectly represents the glass structure.