A fictive stress model calculation of creep deformation of Zr-based bulk metallic glass in the glass transition region under a constant load

A model calculation based on the fictive stress is conducted on the creep deformation of a Zr₅₅Al₁₀Ni₅Cu₃₀ bulk metallic glass near the glass transition temperature under a constant applied load. The model successfully reproduces qualitatively the main features associated with creep viscoelastic behavior at various initial applied stresses, σ₀. Whenever the value of σ₀ exceeds the critical flow stress, σ_c (=80MPa), the flow stress curve, logσ-log t, shows a dip due to stress-induced softening, and the stress dependent viscosity, η(σ), initially decreases. It attains a minimum value, η_min(σ_min), as the glass attains an equilibrium structure corresponding to the imposed stress, σ. For stress, σ, less than the viscosity minimum stress, the η(σ) curves all decrease and tend to merge together, and may be fit with a master curve established previously. This reveals that the model curves depend mainly on the choice of structural relaxation time, γ_fic, and little on the value of Young's modulus used in a simple Maxwell model calculation. We found a systematic deviation between the model and experimental curves, such that the model tends to overestimate the structural softening in case with smaller initial applied stress. The cause of this deviation is unclear and is a subject for future studies.