The microstructure, size, elemental composition and hardness of the surface eutectic layer formed during directional solidification of a Ni-based single crystal superalloy were studied. The formation mechanisms of the surface eutectic on the outer surface of the casting were also discussed. The metal/mould interactions did not play any role in the formation of the surface eutectic. The formation cause of surface eutectic layer was attributed to the interdendritic residual liquid for excretion caused by solidi？cation shrinkage.
Effects of Re on the formation of surface eutectics have been investigated by using Ni-base single crystal superalloys with different Re additions. It was found that Re promotes the segregation of Al and Ta to the eutectic melt, leading to an increase of the surface and internal eutectics. In addition, the addition of Re also increased the freezing range, the local solidification time, and the permeability of the dendritic network within the mushy zone. These factors ultimately promoted the outflow of the interdendritic residual liquid with the action of solidification shrinkage, and led to the formation of more surface eutectics. In contrast, the addition of Re had no obvious influence on the surface eutectic microstructures.
