Influence of an electric or magnetic field on the liquid-solid transformation in materials and on the microstructure of the solid

The influence of an electric or magnetic field on the liquid-solid transformation in materials and the microstructure of the resulting solid is reviewed. In the case of metals, electromigration in the liquid produced substantial changes in composition due to differences in ionic mobility. Further, both a small, continuous d.c. current (similar to 0.1 A cm(-2)) and high density (similar to 10(3) A cm(-2)) electropulsing refined the microstructure of castings. A magnetic field ( > 1 T) applied during directional solidification significantly reduced the convective flow in the melt and distorted the cellular array, but did not affect the dendritic array, nor the macrosegregation. The mechanisms by which these effects on the microstructure occur are not clear. In the case of electropulsing, the influence of the current appears to be to enhance the nucleation rate and at the higher current densities to deform and fracture dendrites by the pinch effect. The influence of a magnetic field appears to be largely due to the Lorentz forces which are established between the motion of the conductive melt and the applied field. Regarding semiconductors, an electric current of 1-10 A cm-2 enhanced the growth rate of GaAs single crystals on a substrate and reduced the dislocation density in the product. Theoretical treatments of these effects are in good accord with the experimental results. In the case of polymers, an electric field of similar to 1 V cm(-1) 'pulled' a camphor single crystal from a solution of camphor in CCl4. The mechanism by which this interesting phenomenon occurs still needs to be established. (C) 2000 Elsevier Science S.A. All rights reserved.