Newtonian and non-Newtonian viscosity of supercooled liquid in metallic glasses

We have investigated the viscosity of the supercooled liquids over a wide range of strain rate by constant-strain-rate tensile tests for La₅₅Al₂₅Ni₂₀, Zr₆₅Al₁₀Ni₁₀Cu₁₅ and Pd₄₀Cu₄₀P₂₀ (at.%) metallic glasses. The supercooled liquids exhibited Newtonian viscosity (equilibrium state) that transferred to a non-Newtonian viscosity (non-equilibrium state) with increasing temperature. The critical strain rate of the Newtonian viscosity increased with increasing temperature in accordance with the Arrhenius relation. The strain rate dependence of viscosity was described well with a master curve expressed by a stretched exponential function η/η0 = 1 - exp(C/ε^β). The Newtonian viscosity was successfully described with the free-volume model of Cohen and Grest. The fragility of the Newtonian viscosity revealed that the supercooled liquids of the metallic glasses were classified into the relatively strong liquids such as Ge₁₀As₁₀Se₈₀, B₂O₃ and Na₂O-2SiO₂ glass formers.