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The interdendritic segregation along the mushy ZO*e of five directionally solidified and quenched alloys (MAR-MOOZ, MAR-M247, IN718, Tl and C-276) has been measured by SEM/EDAX techniques and the corresponding concentration profiles are presented. These profiles have also been translated into liquid density profiles by a numerical model. Chemical compositions and estimated liquid densities of freckles found in MAF-MOOZ, IN718, Tl and C-276 are also reported. It was deducted from the observations in this study that freckles tend to initiate relatively close to the tip of the dendrites, about 15 to 20°C below the liquidus temperature of the alloy, where the fraction liquid is typically of the order of 0.4 to 0.6. The Rayleigh number as a criterion for freckling is described and it is shown that freckles could result from density inversion gradients of the order of 0.03 (g/cm3)/"C, whereas 0.005(g/cm3)/oC is probably too low. The importance of incorporating minor alloying elements such as C, or Si when studying liquid buoyancy related to freckling has also been outlined. Finally, emphasis is put on the crucial role played by carbon in some freckle-prone alloys as a powerful trigger for freckling, both through segregation and through precipitation of heavy elements into carbides.

Liquid Density Inversions During the Solidification of Superalloys and Their Relationship to Freckle Formation in Castings