Phase-field simulation of the columnar-to-equiaxed transition in alloy solidification

The columnar-to-equiaxed transition (CET) in directional solidification of alloys is simulated using the phase-field method. The method relies on the solution of a solute conservation equation and an equation for the propagation of the phase field on the scale of the developing microstructure. A parametric study is performed to investigate the effects of the applied temperature gradient and pulling speed, the seed spacing and nucleation undercooling for the equiaxed grains, and the crystalline anisotropy strength on the CET. The results qualitatively agree with a previously developed analytical model of the CET. At relatively high pulling speeds, a mixed columnar–equiaxed structure is found to be stable over a range of temperature gradients. Furthermore, the CET depends sensitively on the anisotropy strength. The simulations also reveal the presence of primary spacing adjustments during purely columnar growth due to nucleation of seeds, and deactivation of seeds by solutal interactions from nearby growing grains.