Equilibrium viscosity, enthalpy recovery and free volume relaxation in a Zr44Ti11Ni10Cu10Be25 bulk metallic glass

We report on the free volume of a Zr44Ti11Ni10Cu10Be25 bulk metallic glass (BMG) in terms of its experimentally determined volumetric relaxation, enthalpy recovery and equilibrium viscosity below the glass transition temperature T-g. The results obtained present a very consistent picture of structural relaxation as analyzed by calorimetric and volumetric methods. At annealing temperatures below T-g a linear correlation is found between the experimentally determined enthalpy of recovery and free volume reduction, giving a constant value for the formation enthalpy of the free volume in the glass. The free volume in the equilibrium liquid is calculated from viscosity measurements carried out below T-g. These data are described using the Cohen-Turnbull and Cohen-Grest models of the free volume. Additionally, an interpretation of the free volume based on the Adam-Gibbs equilibrium viscosity model is proposed. Analysis of the free volume and enthalpy functions of the equilibrium liquid reveals a temperature-dependent formation enthalpy for the free volume that varies from similar to 1 eV at the melting point to similar to 2 eV near the glass transition. Finally, we analyze the melt viscosity of the Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 BMG and find that the Adam-Gibbs model gives the best prediction of the free volume at high temperatures. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.