Correlation Among Thermodynamics, Fragility, And Glass Forming Ability Of Metallic Glass Forming Liquid

Glass forming, which related with many cutting-edge issues of thermodynamics and dynamics, is one of the important and difficult aspects for the basic theory of condensed matter. For metallic glasses forming melts, through research of thermodynamics and the correlation of thermodynamics and dynamics, the property of the final materials can be predicted to certain extend, and thus the further industral production can be directed. In this thesis, the focal points are thermodynamics, fragility and glass forming ability (GFA). Differential Scanning Calorimetry (DSC), X-ray diffraction (XRD) and high-temperature viscometer are employed, to investigate the prosperities of La-based, Sm-based and Al-based metallic glasses.Fragility indexes for La-based glass-forming liquid of different thermodynamic definitions are calculated based on temperature dependence of excess entropyΔSliq-cry. It is ambiguous to evaluate fragility by interceptingΔSliq-cry-temperature curves. We have found that the thermodynamic fragility MΔs, denoting by theΔS drop at onset melting temperatures Tm, rather thanΔS drop at glass transition temperatures Tg, relates linearly with kinetic fragility m.The dynamic viscosity of Al-Yb and Al-Ni-Yb superheated melts is measured using a torsional oscillation viscometer. The amorphous ribbons produced by the melt spinning technique are characterized by DSC. the activation energy for viscous flow (E), which reflects the change rate of viscosity, has a good negative relation with the GFA in both Al-Yb and Al-Ni-Yb systems. However, there is no direct relation between liquidus viscosity (ηL) and GFA. The superheated fragility M can predict GFA in Al-Yb or Al-Ni-Yb alloy system.An index A, presented by the decrease slop of specific heat capacity when supercooled liquid translates into melts, is proposed to investigate the relationship of thermodynamics and GFA. It is found that for Al-based marginal metallic glasses (MMGs) A is lager (>35×10-3J/mol*K2), while for bulk metallic glasses (BMGs) A is smaller(<35×10-3J/mol*K2). However, in the La-based, Sm-based and Al-based alloy systems experimentally investigated in this thesis, lager A corresponds to higher GFA. Gibbs free energy difference between liquid and crystal as a function of temperature are calculated. Driving force for crystallization, Gibbs free energy difference, appears to be a crucial factor in the GFA of different alloy systems. However, the kinetic factors must be taken into consideration investing alloys with similar composition.