Heterogeneous Nucleation during Solidification of Undercooled Liquid Metals

点击翻译按钮获取中文摘要

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. ***