Influence of melt convection on the microstructure of levitated and undercooled Nd-Fe-B alloys

The electromagnetic levitation technique, which allows the in situ measurement of the solidification kinetics containerlessly, has been used in order to investigate the influence of melt rotation on the microstructure formation of Nd-Fe-B alloys mainly the volume fraction and grain size of the alpha-Fe phase. Samples were subjected to strong rotation during levitation and compared with fixed samples without additional (to the internal melt convection) sample rotation in the levitation facility. The determination of the alpha-Fe volume fraction by measuring the magnetic moment in a vibrating sample magnetometer resulted in a distinct reduction of the alpha-Fe volume fraction in samples with strong sample rotation. The reason is that sample rotation, superposed the internal melt convection, seems to retard rapid dendrite growth. The influence of cooling rate and undercooling level on microstructure formation and phase distribution has been investigated as well. (C) 2002 Elsevier Science B.V. All rights reserved.