ON THE THEORY OF ALLOY SOLIDIFICATION IN A MAGNETIC FIELD

Abstract

The degree of segregation in unidirectionally solidified ingots, chill-cast in magnetic fields up to 34 kilo-oersteds, has been determined for alloys of the aluminium–copper and bismuth–antimony systems. It is found that the chill-face segregation in aluminium–copper ingots is increased approximately 50% above the no-field ingots for a 34-kilo-oersted field. The effect is independent of field direction. A 13-kilo-oersted field decreases the chill-face segregation by 60% in ingots of the bismuth (rich) – antimony system. A theory based on the diffusion inhibition effect of the magnetic field (previously reported) is presented to explain the segregation change. By applying irreversible thermodynamic principles, it is shown that the segregation changes in the field-solidified ingots are a manifestation of the decreased entropy production of the diffusion, convective mixing, and viscous flow processes of solidification in a magnetic field. For the aluminium–copper ingots there is order-of-magnitude agreement between the observed segregation change and the decreased diffusion rate in the magnetic field.