对一种第三代单晶高温合金的单晶叶片和试棒铸件进行了检测，没有观察到共晶组织在横向上向铸件外表面偏聚的现象，但是发现它在垂直方向上发生了明显的不均匀分布。铸件朝下的底面区域共晶很少，而朝上的顶面区域则有大量的共晶聚集。其原因在于定向凝固过程中γ′相形成元素Al + Ti + Ta会在枝晶间液体中富集，并通过扩散和对流的方式向上迁移，最后在铸件的上表面聚集而生成大量的γ/γ′共晶组织。铸态共晶上表面聚集的影响因素除了合金成分，还有凝固条件和铸件形状。对铸件固溶热处理后组织的研究表明，铸件外表面区域的残余共晶量明显少于内部区域。这是因为铸件表面Al和Ti元素的挥发造成了γ′形成元素的贫化，导致共晶组织固溶速率加快。

In recent years, surface γ/γ′-eutectics have been observed to form continuous layers along the peripheries of the outer surfaces of Ni-based single crystal (SC) castings. This type of solidification defect poses specific concerns for turbine blade production because large surface eutectics are very difficult to completely dissolve during practical heat treatment. In the present work, a third generation SC superalloy was used to cast turbine blades and cylindrical bars. However, no eutectic layer was observed on the outer surface of the blade aerofoils and bars in hundreds of castings. Instead of the lateral eutectic enrichment on the outer side surface, a non-uniform distribution of eutectics along the solidification length was detected. Eutectics were found to accumulate on the upper surface of solidification units such as platforms whereas less eutectics were observed on the bottom surface. Especially on the inclined upper surface at the junctions between platform and airfoil, a large amount of eutectics were accumulated so that only a small part could dissolve during conventional solution heat treatment. This phenomenon, called the eutectic accumulation on the upper surface, could be attributed to the upward solutal flow of eutectics forming elements through diffusion and convection, which is finally stopped beneath the upper surface. The factors influencing the upward eutectic accumulation are the chemical composition of the alloys, the process conditions, and the local geometry of the castings, such as the platform thickness and the inclination of the upper surface. Another surface effect was also observed on the eutectic dissolution during heat treatment process. It was found that the eutectics in the outer surface area were dissolved completely during heat treatment process than that in the internal one. The possible explanation is that at high temperatures, the eutectic forming elements Al and Ti evaporate from the casting surface faster than the other elements, as experimentally confirmed by chemical measurement.