Microstructual evolution and magnetic properties of the Nd–Fe–B alloys solidified from undercooled melt by containerless solidification

Using a 25 m drop tube, the containers solidification of Nd–Fe–B alloys with the nominal composition of the Nd14Fe79B7 melts was carried out. Molten alloy ejected into a drop tube was rapidly cooled and solidified during free fall. The samples accumulated at the bottom of the drop tube, the shape of which is spherical, contained a large amount of a metastable phase with a chemical composition near to that of the bulk alloy and a trigonal crystal structure similar to that of the binary Nd2Fe17 phase. The volume fraction of the metastable phase decreased as the diameter of the sample increased. In particular, the metastable phase was partially decomposed into very fine lamellar grains of Nd2Fe14B and some Nd-rich phase by a solid-state reaction in the sample with a diameter of 1200 μm. The DTA measurement shows that the phase transformation from the metastable phase to the stable Nd2Fe14B phase occurred at approximately 950 K. The thermomagnetic properties were also measured by VSM in order to characterize the metastable phase.