Thermodynamics and fragility of Fe-based glass forming melts

Abstract Methods to determine the fragility index, m , both in thermodynamic and kinetic terms are reviewed and applied to two Fe-based glass formers, Fe 40 Ni 40 P 14 B 6 and Fe 41 Co 7 Cr 15 Mo 14 C 15 B 6 Y 2 . The thermodynamic fragility is obtained from entropy curves based on experimental data of the liquid specific heat and of the enthalpy of transformation from glass or liquid to crystal phases. The thermodynamic fragility appears systematically higher than the kinetic one because of the large scatter of data and their extrapolation to obtain the Kauzmann temperature of vanishing entropy. A practical method to estimate the viscosity of glass formers in the undercooling regime was derived from the fragility index and its correlation with parameters of the VFT equation. It provides reasonable agreement with experimental data.