THERMOELECTRIC MAGNETOHYDRODYNAMIC EFFECTS ON SOLIDIFICATION OF METALLIC ALLOYS IN THE DENDRITIC REGIME

In solidifying metallic alloys, an internal thermoelectric current is present and may influence the solidification structure. On the microsc opic scale, this internal current causes Peltier cooling at the tip (h eating at the base) of a dendrite, which is a destabilizing effect. On the mesoscopic scale, the thermoelectric current in the presence of a n external magnetic field causes fluid motion within the mushy zone, w hich in turn generates coarseness of the dendritic structure and enhan cement of convective phenomena. Several ways have been used to investi gate these thermoelectric magnetohydrodynamic (TEMHD) effects. Experim ents realized with binary alloys of different thermoelectric powers de monstrate the relevance of these effects and their influence on the so lidification process. Numerical models give first estimates of the typ ical interdendritic flow velocities due to the TEMHD effects. For the microscopic phenomena, the theoretical analysis shows that the Seebeck and Peltier effects have a destabilizing influence on the liquid-soli d interface, even without any external electric or magnetic field. To characterize these phenomena, a new dimensionless parameter Se is intr oduced, which is a function of the properties of the solidified materi al.