Heterogeneous Nucleation during Solidification of Undercooled Liquid Metals

小行星9712523 在大多数条件下，金属合金凝固是由异质形核事件引发的。虽然均匀成核的研究需要不含所有外部成核剂的样品，但非均匀成核的研究需要仅含有众所周知和表征的成核剂的样品。 为了避免在散装系统中遇到的困难，液滴样品技术被用来检查在固化过程中的成核催化剂。 在这种方法中，大量液体被分散成许多细小的液滴，以限制外来成核剂的影响。 通过使用液滴样品在周晶或共晶合金，表现出高的过冷度成核和平衡的合金在液体加固体两相场，不同的初级固体对随后的成核在液体中的催化作用进行检查，而没有可能的影响，从外国基板。 一种新的方法与液滴样品，涉及纯金属与稳定的固体颗粒已被建立，扩大了范围的成核催化剂，可以检查。 通过这种方法，可以在明确定义的条件下设计可重复的异相成核实验研究，以分析成核动力学。 该过程是使用球冠模型，它有一些缺陷，解决，以发展真实的理解催化反应建模。 在过去的研究中，在一个明确定义的系统连续冷却过程中的成核动力学测量已经进行。 结果表明了形核位置动力学的重要性，但也指出了连续冷却方法对形核动力学的局限性。 因此，一种替代的，更有效的方法的基础上等温保温处理已被开发出来，并将应用于这一更新的努力。 一个合适的系统的基础上的Bi-Cd合金，其中单一的成核动力学已被隔离D，检查和选择的样品修改，包括合金化的基板或核追求提供定量评价的影响，不注册的成核动力学行为。 形核是金属合金凝固过程中的一个重要方面。这一努力代表了迄今为止在凝固过程中进行的最全面的定量非均匀形核动力学评价。 ***

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. ***