Multiphase Solidification and Multi-crystal Microstructure in Single Crystal Superalloys

点击翻译按钮获取中文摘要

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.