The microstructure and composition of the interdendritic liquid along the mushy zone of superalloy Inconel 718 that was directionally solidified at various solidification rates between 2 and 100 μm s−1 have been investigated by SEM and EDAX techniques. The interdendritic liquid segregation profiles along the mushy zone are presented. The liquid density difference and Rayleigh number in the interdendritic liquid were calculated and analysed as well. It was found that when the solidification rates increased in the range 10–70 μm s−1, segregation of Nb decreased, but segregation of Mo was most serious at 20 μm s−1. The liquid density difference increased the most for rates from 20 to 40 μm s−1 as temperature decreased. The maximum relative Rayleigh number was highest at 10°C below the liquidus temperature at 20 μm s−1, which indicated the conditions where fluid flow most easily occurred for Inconel 718. The relative Rayleigh number synthetically considers the factors affecting fluid flow and can give a reasonable prediction for fluid flow tendency.

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