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一个基本过程，目前还没有得到很好的理解。这是防止过冷熔体形成非晶相的唯一因素。如果液相在冷却时是稳定的，并且/或者相竞争的结晶相很难成核和/或生长，那么熔体形成玻璃就很容易。pd基大块金属玻璃具有良好的玻璃成形能力、优异的耐腐蚀性、高强度、超延展性和高热稳定性。本联合研究项目包括热力学描述、过冷熔体中结晶相成核速率的定量测定、结晶相成核势垒的推导，以及将davis - uhlmann动力学公式与CALPHAD方法相结合，应用于pd基合金的玻璃形成范围/能力（GFR/GFA）的建模，实现pd基大块金属玻璃的合金设计。在本项目中，将采用CALPHAD、关键实验和第一性原理方法的混合方法来获得Pd-Si和Pd-Cu-Si体系的热力学描述。用差示扫描量热法（DSC）研究了钯硅和钯铜硅过冷熔体的成核过程。然后从测量的成核速率中提取球形核成核的自由能势场，用于评估从时间-温度转变（TTT）曲线中形成玻璃的临界冷却速率，TTT曲线是对从过冷液体中形成可检测数量的结晶相所需时间的测量，作为温度的函数。计算结晶驱动力和液体混合焓，并以此评价合金的GFA。因此，这种协调方法的目的是在对这些材料的成核过程的热力学和动力学进行深入和定量描述的基础上，解决大块金属玻璃成形金属合金的玻璃化能力，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