Abstract: Efficient selection of columnar dendrites during directional solidification is crucial for manufacturing single-crystal turbine blades of superalloy. Nucleation of new grains during the initial transient stage significantly inhibits rapid columnar dendrites selection and growth. The nucleation depends not only on the presence of potential particles within the melt but also on the undercooling that the melt can achieve. Therefore, the melt undercooling evolution behavior during columnar dendrite growth is studied by quantitative phase-field simulations of directional alloy solidification under varying temperature gradients and withdrawal rates. Simulations reveal that the undercooling that controls nucleation of potential particles increases with both temperature gradient and withdrawal rate during the entire solidification process. A distinct “blackout zone” that poses a significant risk to columnar dendrite selection and growth is found during the initial transient stage. Within this zone, both the undercooling and the extent of the undercooled region reach their maximum. 3D phase-field simulations with preset artificial nucleation agents are conducted to verify these findings. It demonstrates that nucleation can easily occur within the “blackout zone” and a high temperature gradient promotes nucleation, thereby accelerating the columnar-to-equiaxed transition (CET). Consequently, contrary to traditional understanding, merely increasing the temperature gradient (while still permitting melt constitutional undercooled) is suboptimal for rapid columnar dendrite selection. These results represent an important advance in understanding of dendrite growth during directional solidification and provide new insights into controlling microstructure in both single-crystal and inoculated castings.

Key words: Phase-field simulation, Undercooling, Columnar dendritic selection, Directional solidification, Blackout zone