Thermodynamic description and determination of nucleation rate and their application on the modeling of the glass formation of Pd-based alloys

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Crystal nucleation from an undercooled melt is one of the fundamental processes during solidification and is far from being well understood. It is the only event that prevents the formation of an amorphous phase from an undercooled melt. If the liquid phase is stable upon cooling and/or the competing crystalline phases are difficult to nucleate and/or grow, then glass formation from the melt is facilitated. Pd-based bulk metallic glasses show good glass forming ability, excellent corrosion resistance, high strength, super ductility, and high thermal stability. This joint research project involves the thermodynamic description, the quantitative determination of the nucleation rates of crystalline phases from the undercooled melt, the derivation of the nucleation barriers of crystalline phases, and their application on the modeling of the glass formation range/ability (GFR/GFA) of the Pd-based alloys by coupling the Davies-Uhlmann kinetic formulations with the CALPHAD approach, for achievement the alloy design of the Pd-based bulk metallic glasses.In this project, a hybrid approach of CALPHAD, key experiments, and first-principles methods will be employed to obtain the thermodynamic description of the Pd-Si and Pd-Cu-Si system. The nucleation in undercooled melts of Pd-Si and Pd-Cu-Si melts will be determined by differential scanning calorimetry (DSC) experiments. Free energy barriers for nucleation of a spherical nucleus will then be extracted from the measured nucleation rates, which are used to evaluate the critical cooling rate for glass formation from the time-temperature-transformation (TTT) curves, which are a measure of the time necessary for the formation of detectable amounts of a crystalline phase from an undercooled liquid as a function of temperature. The driving force of crystallization and the mixing enthalpy of liquid will also be calculated and used to evaluate the GFA of these alloys. Thus, the aim of this coordinated approach is to address the glass-forming ability of bulk metallic glass forming metallic alloys on the basis of a profound and quantitative description of the thermodynamics and kinetics of the nucleation process in these materials, and also to strengthen the Sino-German collaboration