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 Tg. The results obtained present a very consistent picture of structural relaxation as analyzed by calorimetric and volumetric methods. At annealing temperatures below Tg 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 Tg. 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 ∼1 eV at the melting point 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.