The microstructures of Al-40%Cu (mass fraction) hypereutectic alloy directionally solidified under a longitudinal magnetic field were investigated. The results show that the magnetic field has a great influence on the morphology of the primary Al₂Cu phase at the growth rate of R=2 μm/s and a temperature gradient of G_L=42.6 K/cm at the solid/liquid interface. The effect of thermoelectromagnetic convection (TEMC) which drives the fluid flow is predominant under low magnetic fields and different at different scales according to the model of TEMC established.
It is found that TEMC causes severe deformation of the primary Al₂Cu phase opposed to the well-aligned faceted
primary phase in the absence of the field at microscopic scale and the primary phase tends to become a
faceted-non-faceted transition gradually with the increase of the magnetic field. Moreover, TEMC modifies the
mushy zone length at macroscopic scale because of the secondary convection from the bulk caused by the microscopic
TEMC. TEMC also affects the solute distribution in the front of the interface at different scales and the process
of heat transfer. Under higher magnetic fields, the effect of TEMC is suppressed, however, the considerable
thermoelectric Lorentz force makes the primary phase become the irregular cellular structure. Meanwhile, the
primary phases align along the magnetic field closely, which results from the remarkable magnetocrystalline
anisotropy of the Al₂Cu crystal.