Tip-splitting instability and transition to seaweed growth during alloy solidification in anisotropically preferred growth direction

The evolution of the solid–liquid interface morphology during the initial transient of directional solidification is investigated by quantitative phase-field numerical simulation during cooling down of an Al–4wt.% Cu alloy growing in the preferred 〈100〉-direction. Simulations show that the shape of the non-planar solid–liquid interface varies with the instantaneous growth parameters; in particular, above a critical value of tip velocity, cell/dendrite tips undergo Mullins–Sekerka morphological instability resulting in tip splitting and transition from cells or dendrites to seaweeds. The numerical simulations demonstrate that, despite the 〈100〉-direction corresponding to high solid–liquid interface energy, seaweed formation is predicted when noise, either inherent to the numerical mesh and/or intentionally added to the phase field, is strong enough to blur the solid anisotropy at the cell/dendrite tip, which confirms the analytical predictions of Brener et al. (1996). Both the tip-splitting mechanism responsible for the seaweed morphological transition as well as the seaweed growth dynamics are characterized. The numerical predictions are then compared to the seaweed transition evidenced on an Al–4wt.% Cu alloy by in situ and real-time synchrotron X-ray radiography in the initial transient of directional solidification, and good agreement is found.