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

过冷熔体的结晶成核是凝固过程中的基本过程之一，目前还远未得到很好的理解。这是唯一能防止过冷熔体形成无定形相的事件。如果液态在冷却时是稳定的，和/或竞争的晶相很难成核和/或长大，那么从熔体中形成玻璃是容易的。Pd基块体金属玻璃具有良好的非晶形成能力、优良的耐腐蚀性、高强度、超塑性和高热稳定性。这项联合研究项目包括热力学描述、晶相过冷熔体成核速率的定量测定、晶相成核势垒的推导以及它们在Pd基合金玻璃形成范围/能力(GFR/GFA)模拟中的应用。通过将Davies-Uhlmann动力学公式与CALPHAD方法相结合，实现了Pd基大块金属玻璃的合金设计。用差示扫描量热法(DSC)研究了Pd-Si和Pd-Cu-Si过冷熔体中的形核过程。然后，将从测量的成核速率中提取成核的自由能垒，该成核速率用于从时间-温度-转变(TTT)曲线评估玻璃形成的临界冷却速度，TTT曲线是从过冷液体形成可检测到的晶相所需的时间作为温度的函数。还将计算晶化驱动力和液体混合热，并用它们来评价这些合金的GFA。因此，这种协调方法的目的是在深入和定量地描述块体金属玻璃形成金属合金的成核过程的热力学和动力学的基础上，解决这些材料的玻璃形成能力，并加强中德合作。

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