Preferred orientation and microstructure evolution of Al ₃Ni phase in the Al–18 at% Ni alloy during directional solidification under a high magnetic field

The peritectic Al–18 at.% Ni alloys were directionally solidified at various pulling velocities without and with a 6 T high magnetic field. The preferred orientation and microstructure evolution of the peritectic Al₃Ni phase were investigated. Without magnetic fields, the alloys exhibited a dendritic and aligned microstructure. With the magnetic field, the aligned microstructure of the alloys was broken, forming a special transverse plate-like structure with some blocky eutectic structures distributed in the alloys. At 0 T and 5 μm/s, the Al₃Ni phase was preferentially oriented along the <010> direction, while the sample prepared at 100 μm/s exhibited no preferred orientation. At 6 T, when the pulling velocity was 5 μm/s, the solidification of the alloy changed from a peritectic reaction to a hypereutectic reaction, and the high magnetic field induced the preferred orientation of the Al₃Ni phase along the <001> direction. When the pulling velocity was increased to 20 and 100 μm/s, the alloy still mainly underwent the peritectic reaction. The primary Al₃Ni₂ phase oriented along the <0001> and the peritectic phase attached to it to form the <001> preferred orientation. Furthermore, the orientation degrees of the phases decreased with increasing pulling velocity. The changes of the preferred orientation and microstructure of the alloys during directional solidification in high magnetic field can be linked to the combing effects of the magnetic torque, thermoelectromagnetic force, and magnetic force, which can alter the crystal orientation, convection, solute and grain migration. Furthermore, the magnetic field effects on the crystal orientation and microstructure of the alloys during directional solidification is strongly dependent on the pulling velocity.