Solute enrichment induced dendritic fragmentation in directional solidification of nickel-based superalloys

Dendritic fragmentation should be strictly prevented to avoid introducing new equiaxed grains of arbitrary orientations (known as freckles) in the single crystal blade, whose mechanism properties are extremely sensitive to the grain boundaries. However, the mechanism of the formation and distribution of freckles in superalloy castings remains debatable. Here the interactions among thermal-solutal convection, solute segregation and dendritic structure in the directional solidification is revealed via a cellular automaton-finite volume model developed and validated for nickel-based single-crystal superalloys. The simulated channel distribution, morphology of residual liquid region, dendritic tip velocity and freckle grains are in good agreement with the previous experimental observations. The present work directly predicts solute enrichment induced dendritic fragmentation, that is, the middle of the over-growing dendritic trunk is melted by the superheated liquid metal in the channel and thus the top of the dendrite becomes isolated fragments. These dendritic fragments rather than the newly nucleated grains are proved to be the root cause of freckle grains. The simulation results also provide new insight into the effect of the operating parameters on the solute distribution and dendritic structure. The undesired lateral heat flux in the directional solidification promotes the transfer of the solute-enriched solute and the migration of the channels, and thus results in the dendritic detachment. Increasing cooling rate makes significant effects on eliminating the freckle defect, because a higher cooling rate promotes the growth of the ternary dendritic arms on the side branches and inhibits the transfer of solute-enriched melt to the over-growing dendrite.