Keywords: dendritic-growth, MHD, thermoelectricity, enthalpy, undercooling, binary-alloy.

Experimental work of material processing under high magnetic field conditions has shown that the microstructure can be significantly altered. There is evidence that these effects can be attributed to the Lorentz force created through thermoelectric magneto hydrodynamic interactions [1]. However the mechanism of how this occurs is not very well understood.

The effects of a constant uniform magnetic field on thermoelectric currents during dendritic solidification are investigated using a two-dimensional enthalpy based numerical model developed by Voller [2]. Growth in stagnant flow is related to real material properties by correlating the tip velocity to experimentally measured values. A constant convection is then applied incident to the tip of the crystal. The fluid motion and change in dendrite morphology are found to be similar to those of other models [3].

Using an approximation of the dendrite growing in free space it was found that an applied magnetic field causes the resulting Lorentz force to generate a circulating flow influencing the solidification pattern. As the magnetic field strength increases it was found that secondary growth on the clockwise side of the primary arm of the dendrite was encouraged, while the anticlockwise side is suppressed due to a reduction in local free energy. The preferred direction of growth rotated in the clockwise sense under an anti-clockwise flow for both a binary alloy and a pure material. The tip velocity is significantly increased compared to growth in stagnant flow.

Grain growth consisting of multiple seeds growing in the same plane, gives a competition to release latent heat resulting in stunted growth. The initial growth is very similar to the single seed cases indicating that the dendrites must become close before the thermoelectric interactions are significant.

These changes to the dendrite morphology could significantly alter the macro properties, allowing for the development of new materials.

1

X. Li, Y. Fautrelle, Z.M. Ren, "Influence of an axial high magnetic field on the liquid solid transformation in AlCu hypoeutectic alloys and on the microstructure of the solid", Acta Materialia 55, 1377-1386, 2007. doi:10.1016/j.actamat.2006.10.007

2

V.R. Voller, "An enthalpy method for modeling dendritic growth in a binary alloy", submitted to Int J. Heat Mass Transfer, 51(3-4), 823, 2008. doi:10.1016/j.ijheatmasstransfer.2007.04.025

3

C. Beckermann, H.-J. Diepers, I. Steinbach, A. Karma, X. Tong, "Modeling melt convection in phase-field simulations of solidification", J. Computational Physics, 154, 468-496, 1999. doi:10.1006/jcph.1999.6323

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