Thermodynamics of polymorphism in a bulk metallic glass: Heat capacity measurements by fast differential scanning calorimetry

The Au-based bulk metallic glass-forming alloy Au70Cu5.5Ag7.5Si17 is the first atomic system in which monotropic polymorphism has been experimentally found. In this study, strategies for measuring thermodynamic quantities such as heat capacity, transformation enthalpy and entropy, and Gibbs free energy for stable and metastable phases are evaluated based on this system. The transformation enthalpies between phases are determined via differential scanning calorimetry (DSC); the corresponding heat capacities are measured via stochastic temperature-modulated DSC (TOPEM); and the equilibrium melting temperatures of metastable phases, only detectable at high rates, are determined via fast DSC (FDSC). Whereas the slow heating rate of conventional DSC limits the characterization of metastable phases, the combination of both DSC and FDSC allows for their detailed heat-capacity determination. Furthermore, various approximation functions that have previously been used to determine the temperature dependence of the thermodynamic driving force for nucleation and crystallization are compared with the experimental data.