Shear rate dependence of viscosity and configurational entropy of the Zr_(41.2)Ti_(13.8)Cu_(12.5)Ni_(10.0)Be_(22.5)) metallic glass forming liquid

摘要

In this study the viscosity of Zr_(41.2)Ti_(13.8)Cu_(12.5)Ni_(10.0)Be_(22.5) (Vit 1) has been measured as a function of temperature and shear rate above the liquidus temperature, T_(liq) = 1026 K, using a high temperature Couette Rheometer. It has been discovered that there exists a pronounced decrease in viscosity with increasing shear rate. This is contrary to the general belief that metallic systems above the liquidus temperature should show Newtonian behavior. It has also been discovered that this shear rate dependence of the viscosity decreases with increasing temperature and approaches the Newtonian behavior and viscosities of simple monatomic liquids or binary alloys at approximately 1300 K. These results are discussed in terms of configurational entropy, S_c, by using the Adam-Gibbs entropy model for viscous flow. It is suggested that a large amount of short-range order still exists above the liquidus temperature. At high shear rates this order is destroyed causing the configurational entropy to increase by a magnitude that is on the order of the entropy of fusion.