Multiphase Solidification and Multi-crystal Microstructure in Single Crystal Superalloys

单晶高温合金在凝固过程中由于多组分合金体系和多相析出而形成复杂的组织。在初步的研究工作中，我们发现并证明了γ / γ素数共晶可能是由熔体枝晶间区域的γ枝晶单独产生的，因此具有随机取向。在添加碳的高温合金中，大多数γ / γ素质共晶优选地在熔体中现有的碳化物上成核，因此也根据其碳化物基体的取向显示出随机取向。因此，所谓的SC铸件的微观组织实际上是多晶的，尽管无法检测到宏观晶界。本课题将系统研究单晶凝固高温合金的多相凝固及其形成的多晶显微组织，这对基础研究和工业应用都具有重要意义。研究了高取向取向共晶的成核行为，以解释为什么共晶更容易在熔体中成核，甚至在晶格差异很大的碳化物上成核，而不容易在晶格参数近似相同的γ枝晶上成核。最后，建立了描述高温合金中γ / γ素质共晶凝固的模型概念。基于该模型，将尝试通过改进凝固工艺来避免或至少显著减少这种取向错误缺陷，以获得更好的微观组织单晶度和最佳的SC铸件质量。

The microstructure formation during solidification of single-crystal (SC) superalloys is complex due to the multicomponent alloy system and the multiphase precipitation. In our preliminary research work it was found and demonstrated that the gamma/gamma-prime eutectic may arise from the gamma dendrites in the interdendritic region of the melt separately and thus has a random crystal orientation. In superalloys with carbon additions, most of the gamma/gamma-prime eutectics preferably nucleate on the existing carbides in the melt, thus also revealing random orientations as according to the orientation of their carbide substrates. As a result, the microstructure of the so-called SC castings is, in reality, polycrystalline, although no macroscopic grain boundaries can be detected. In this project, the multiphase solidification and the resulting polycrystalline microstructure in single-crystal solidified superalloys will be systematically investigated since it is of great importance to both fundamental research and industrial applications. The nucleation behavior of the highly misoriented eutectics will be studied to explain why the eutectics preferably nucleate from the melt or even on carbides having very different crystal lattices, and do not easily nucleate on the existing gamma-dendrites, which have approximately the same lattice parameters. Finally, a model concept is developed and build-up that describes the solidification of gamma/gamma-prime eutectics in superalloys. Based on this model, attempts will be made to avoid or at least significantly reduce this misorientation defect by improving the solidification process, with the aim of obtaining a better microstructural monocrystallinity and an optimal quality of the SC castings.