Heterogeneous Nucleation during Solidification of Undercooled Liquid Metals

在大多数情况下，金属合金的凝固是由非均相形核事件引起的。研究均相成核需要一个没有任何外部成核剂的样品，而研究非均相成核需要一个只含有已知和表征成核剂的样品。为了避免在散装系统中遇到的困难，采用液滴样品技术来检测凝固过程中的成核催化剂。在这种方法中，散装液体被分散成许多细小的液滴，以限制外来核剂的影响。通过在包晶或共晶合金中使用在成核前表现出高度过冷的液滴样品，并在液+固两相场中平衡合金，研究了不同原生固体对随后在液体中成核的催化作用，而不受外来基质的可能影响。建立了一种涉及纯金属和掺入稳定固体颗粒的液滴样品的新方法，扩大了可以检测成核催化剂的范围。利用这种方法，可以在定义良好的条件下设计可重复的非均相成核实验研究，以分析成核动力学。该过程使用球形帽模型进行建模，该模型有许多缺陷，这些缺陷被解决，以便对催化反应进行真正的理解。在过去的研究中，在一个明确的系统中进行了连续冷却期间的成核动力学测量。结果表明了成核位置动力学的重要性，但也指出了连续冷却方法对成核动力学的局限性。因此，基于等温保温处理的另一种更有效的方法已经开发出来，并将应用于这项更新工作。一个合适的系统基于双镉合金，其中单成核动力学已被分离，并检查和选择的样品改性，包括基体或核的合金，以提供失配对成核动力学行为的影响的定量评估。成核是金属合金凝固过程中的一个重要方面。这项工作代表了迄今为止进行的凝固过程中最全面的定量非均质成核动力学评估。＊＊＊

9712523 Perpezko Under most conditions, metal alloy solidification is initiated by a heterogeneous nucleation event. While the study of homogeneous nucleation requires a sample free from all external nucleants, the study of heterogeneous nucleation requires a sample which contains only well known and characterized nucleants. To circumvent difficulties encountered in bulk systems, a droplet sample technique is utilized to examine nucleation catalysts during solidification. In this approach a bulk liquid is dispersed into many fine droplets to limit the influence of extraneous nucleants. By using droplet samples in peritectic or eutectic alloys that exhibit a high undercooling before nucleation and equilibrating the alloys in the liquid-plus- solid two-phase field, the catalytic effects of the different primary solids on subsequent nucleation in the liquid are examined without the possible influence from a foreign substrate. A new approach with droplet samples involving pure metals with incorporated stable solid particles has been established which extends the scope of nucleation catalysts that can examined. With this approach, reproducible experimental studies of heterogeneous nucleation can be designed under well-defined conditions for analysis of the nucleation kinetics. The process is modeled using the spherical cap model which has a number of deficiencies that are addressed in order to develop real understanding of catalysis reactions. In past research nucleation kinetics measurements during continuous cooling in a well defined system have been conducted. The results indicate the importance of nucleation site dynamics, but also point out limitations of continuous cooling approach to nucleation kinetics. As the result, an alternate, more effective approach based on isothermal holding treatments has been developed and will be applied in this renewal effort. A suitable system based on Bi-Cd alloys, where single nucleation kinetics has been isolate d, is examined and selected sample modification to include alloying of both the substrate or the nucleus is pursued to provide quantitative evaluation of the effect of disregistry on nucleation kinetics behavior. %%% Nucleation is an important aspect in metal alloy solidification. This effort represents the most comprehensive quantitative heterogeneous nucleation kinetics evaluation during solidification that has been conducted to date. ***