Microscopic Modelling of Freckle Formation during Directional Solidification and Its Verification via In situ X-ray Observation

A 3D microscale solidification model, resolving complex interaction of solute partition and diffusion, interdendritic thermosolutal convection, dendrite formation and remelting, was used to study freckle formation for unidirectional solidified Ga-In alloys. The simulated results are validated against real time, in situ X-ray radiographic experiments, highlighting both similarities and differences between model and experiment. The redistribution of solute concentration and the formation of segregated solutal channels were both captured by the numerical model. Effects of thermo-solutal convection due to temperature and solute concentration variations were investigated and the formation of segregated solutal channels (freckles) was also examined in detail. With the assistance of the numerical solution, an improved understanding of the freckle channel formation at the microstructural level was provided.