Simulation of equiaxed growth ahead of an advancing columnar front in directionally solidified Ni-based superalloys

Growth of equiaxed grains ahead of an advancing columnar front leads to the formation of as-cast defects, such as stray grains in single crystals or tree rings in vacuum arc remelting (VAR) ingots. In this study a combined cellular automata-finite difference model was applied to simulate dendrite growth and the formation of equiaxed grains in directionally solidified nickel-based superalloys. Realistic dendritic structures and complex solute concentration profiles at the growth front were simulated. It was observed that the solute interaction between primary dendrites occurs well below their tips, while strong solute interaction occurs between the diffusion fields of secondary and tertiary arms. The influence of thermal gradient and growth velocity on CET was investigated and the results were combined on a CET map, showing that a decrease in thermal gradient and an increase in growth rate favour a CET.