Viscosity of the zirconium-titanium-copper-nickel-beryllium bulk metallic glass forming alloy above the liquidus temperature

The viscosity of Zr41.2Ti13.8Cu12.5Ni 10.0Be22. (Vit1) has been measured above the liquidus temperature, T liq = 1026 K, using a high vacuum high temperature rapid annealing furnace equipped with a Couette Concentric Cylinder Rheometer. Steady state measurements have been taken over a temperature range of 1075-1300 K and a shear rate range of 100-102 s-1. It has previously been discovered that there exists a pronounced decrease in viscosity with increasing shear rate which is contrary to the general belief that metallic systems above the liquidus temperature should show Newtonian behavior due to high atomic mobility. This suggests that there is short or medium range order present in the liquid state that can be destroyed by shearing. This current study has discovered that this shear rate dependence of the viscosity of Vit1 decreases with increasing temperature and approaches the Newtonian behavior and viscosities of simple monatomic or binary liquid alloys at 1225 K. Once this state is reached the viscosity will remain Newtonian in the liquid state and no order is reformed until the sample is cooled into the supercooled region. This indicates a strong temperature history dependence of the viscosity. It has also been discovered that initially crystalline Vit1 has an order of magnitude lower viscosity than initially amorphous Vit1 at 1075 K after melting. This difference decreases with increasing temperature until similar viscosities are obtained at 1175 K. The Vogel-Fulcher-Tammann relationship shows decreasing fragility of Vit1 with increasing shear rate and increasing temperature. It was also seen that temperature has a larger and more permanent effect on the fragility than shear rate. The development and results of converting viscosity data to configurational entropy using the Adam-Gibbs entropy model for viscous flow are discussed. This shows that the configurational entropy present after melting is on the order of the entropy of fusion.