EFFECT OF COOLING RATES ON SEGREGATION AND DENSITY VARIATION IN THE MUSHY ZONE DURING SOLIDIFICATION OF SUPERALLOY INCONEL 718

The microstructure and segregation of experimental samples of the superalloy Inconel 718 solidified at different cooling rates have been investigated by SEM and EDAX. The equilibrium phase fraction diagrams of Inconel 718 and the segregated liquid composition were calculated by Thermo-Calc. The experimental and calculated results were in good agreement with those of an Inconel 718 ingot, and they indicated that, at a cooling rate less than 3°C/min, heavy Nb and Mo segregation resulted in Laves phase formation directly from the liquid. The density of the liquid surrounding the Laves phase decreased and the region between the two Laves precipitates (inter-precipitate) often had the lowest density. A density difference (or micro inter-precipitate density inversion) existed between the inter-precipitate liquid and the original liquid at the same or higher temperatures where there were no liquid-precipitated Laves.