Microstructural Evolution in Directional Solidification

摘要

The current status of microstructure formation during the directional solidification of binary alloys is summarized with an emphasis on the issues that still require fundamental understanding. Most rigorous theoretical models are based on diffusive growth conditions. However, significant convection effects are generally present during directional solidification that makes it difficult to validate the models. It will be shown that a significant fraction of current experimental data is obtained under conditions in which fluid flow in the melt is either significant or dominant. In peritectic systems, the presence of fluid flow can give rise to novel microstructures that are not found under diffusive growth conditions. For single-phase alloys, fluid flow can significantly alter microstructural scales and morphological transition conditions. We shall present experimental results in Al-Cu, Sn-Cd and Pb-Bi systems, and show how the results obtained in presence of fluid flow can be analyzed quantitatively through the characterization of microstructure in terms of local conditions at the interface. In addition, an experimental technique will then be discussed in which not only diffusive growth conditions can be achieved, but convection effects can also be gradually introduced to examine systematic changes in microstructural characteristics as a function of level of convection.