Suguru Sugiyama

Magnetic properties of metastable phase from undercooled Nd-Fe-B melts

Nd14Fe79B7 alloys were solidified from the undercooled melts by containerless processing. The resultant alloys contained a large amount of a metastable phase with a chemical composition near that of the bulk alloy and a crystal structure similar to that of the Nd2Fe17 phase. The metastable phase had low magnetic properties, a saturation magnetization of 86.7 emu/g, a remanence of 4.5 emu/g and a coercivity of 0.14 kOe. The magnetic properties of the sample annealed at 1020 K were markedly improved due to the increase in the amount of the Nd2Fe14B phase.

Phase selection in containerless solidification of FeSi2 during free fall in drop tube

The melts of the Fe-66.7%Si alloy ejected into a drop tube was solidified during its free fall. The spherical samples collected at the bottom of the drop tube were classified into several groups according to their diameters from 300 μm to 1000 μm. The microstructures of the samples were examined and analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential thermal analysis (DTA). In addition to the grain refinement of the constituent phases, it was observed that the primary phase changed from the equilibrium FeSi (ε) phase to the metastable Fe2Si5 (α) phase and then returned to the ε phase again with decrease in the sample diameter. This result indicates that the microstructure of the sample solidified from the melt during free fall is controlled by the phase competition between α and ε depending on the degree of the undercooling. If it is assumed that the phase having the highest growth rate is selected as the primary phase, the dendrite growth model proposed by Boettinger, Coriell and Trivedi predicts the changes of the primary phase from ε to α and then to ε with increase in the undercooling. This means that the metastable eutectic point as a function of undercooling is expressed by the curve just like a character, C, in the Fe-Si binary phase diagram.

Microstructure and magnetic properties of rare earth Nd-Fe system alloys produced by containerless processing

NdxFe100−1.5xB0.5x alloys (x=10, 11.8, and 14) were solidified from the undercooled melts by drop tube processing. The samples contained a large amount of the metastable phase with the composition near to their bulk composition in each alloy. The volume fraction of the metastable phase increased as the sample diameter decreased. The metastable phase was partially decomposed into very fine lamellar grains of the Nd2Fe14B and α-Fe or Nd2Fe14B and some Nd-rich phase by solid-state reaction in the samples with a diameter of 1200μm. The solidification behavior of the metastable phase was explained in terms of a hypothetic scheme of the pseudobinary Nd-Fe-B phase diagram