Thermodynamic and kinetic studies of the Cu-Zr-Al(-Sn) bulk metallic glass-forming system

The thermodynamic and kinetic properties of Cu45.6Zr45.3Al9.1, Cu47.5Zr45.1Al7.4, and Cu47.3Zr45.8Al6.4Sn0.5 bulk metallic glasses are investigated to help understanding the origin of the glass-forming ability in the Cu-Zr-Al(-Sn) glass-forming system. The thermodynamic driving force for crystallization is determined by using specific heat capacity data and several thermodynamic parameters, e.g., enthalpies and entropies, measured via differential scanning calorimetry and differential thermal analysis. Moreover, a thermomechanical analyzer and a custom-built Couette rheometer were used to measure shear viscosity and to determine the fragilities in the low-and high-temperature regimes, close to the glass transition and in the stable melt respectively. For each alloy, the thermodynamic driving force and low-temperature fragility exhibit good correlation with the critical casting diameter. Melt viscosity was measured up to approximately 150 K above the liquidus temperature, i.e., from 1100 K to 1350 K. At temperature above 1350 K, reactions between the graphite crucible and the melt were observed to occur and may interfere with the viscosity measurement. Considerable differences in fragility between the stable melt and the deeply supercooled liquid were observed and discussed in terms of a fragile-to-strong crossover. (C) 2020 Elsevier B.V. All rights reserved.