Influence of melt convection on the morphological evolution of seaweed structures: Insights from phase-field simulations

In the present study, the morphological evolution of instabilities in a binary polycrystalline alloy is investigated under the influence of melt convection through phase-field simulations. In the absence of interfacial anisotropy, a grain boundary initiates and amplifies a convex ridge, which sequentially undergoes splitting and generates a random fork like pattern in a supersaturated melt, namely seaweed. Categorized as regimes I and II, degenerate and strongly tilted seaweeds are the two distinguishable structures observed at low and high flow velocities, respectively. In the presence of weak convection fields, the solidification of individual crystallites is driven by the proportional and sideward amplification of the tips across the domain. In regime II, the combined influence of convection and solutal gradients at the grain boundary strikingly generates an oriented pattern. The role of surface energies on the development of perturbations across the two solid grains is studied via comparing the microstructures. For different velocities, we observe that the growth of tip splitting branches along a solidifying interface is restricted with the decrease in the grain boundary energy and the relative grain boundary groove angle. Furthermore, a quantitative image analysis is performed for all the microstructures to study the relation between the local orientation and the liquid phase convection. Symmetric and skewed nature of the distributions demonstrate the preferred local tilt angles for the structures in regimes I and II, respectively. Besides, a power law for the tip splitting frequency f∝Re^-0.02 is also established. This relation is consistent with seaweed morphologies, attributed by frequent and inhibited splitting events at various Reynolds numbers.