Effect of hydrodynamics on microstructure evolution of Nd-Fe-B alloys

The influence of melt convection on the microstructure formation of Nd-Fe-B alloys, mainly the volume fraction and grain size of the a-Fe phase, has been investigated experimentally using the electromagnetic levitation technique and a specially designed floating-zone arrangement. Samples were subjected to a strong rotation during levitation and compared to fixed samples without additional sample rotation in the levitation facility. A controlled influence on the melt flow is possible via tailored magnetic fields which enable enhancement or suppression of the melt convection using the floating-zone arrangement. A specially designed magnetic two-phase stirrer offers a strong influence on the melt flow in the floating-zone facility. The microstructure pattern, mainly the volume fraction and grain size of the a-Fe phase, vary strongly with the strength of the internal flow motion in both types of experiments. The melt flow in a levitated droplet was studied numerically under the additional effect of a global sample rotation giving rise to the assumption of strong suppression of internal motions. The electromagnetically driven flow in the inductively heated melt zone of the floating-zone arrangement turned out to be of strong influence on the resulting microstructure. Numerical simulations are essential in order to understand and design the various flow structures.