Melt-spun Nd-Fe-B magnets and the Nd1+εFe4B4 phase

The microstructure of Nd-Fe-B magnets made from melt-spun ribbons differs significantly from that of the sintered magnets. An ideal two-phase microstructure of small Nd2Fe14B grains surrounded by a Nd-rich intergranular phase is easily achieved in the melt-spun magnets while sintered magnets invariably contain the boron-rich Nd1.1Fe4B4 phase. This phase can form in the melt-spun alloys only when the cooling rates are slow and excess Nd and B are added deliberately. In those cases, this new phase appears as randomly distributed and randomly oriented grains of the same size as the Nd2Fe14B grains. The Nd1.1Fe4B4 grains are highly faulted in the Fe sublattice with their fault planes parallel to the (110) plane. Energy dispersive x-ray analysis and diffraction shows that this phase forms with varying Nd:Fe ratios in different grains. We have also found that the faults form in pure bulk samples of Nd1.1Fe4B4 but not in pure NdCo4B4 samples. Possible antiphase boundaries arising from modulated Fe sublattice are seen. The faulted microstructure is believed to result from (a) sluggish kinetics of formation of this phase, (b) incommensurate crystal structure, and (c) weak bonding between the Nd chains and chains of Fe-B tetrahedra that make up the structure. Lorentz microscopy shows this phase to act as a weak barrier to domain wall motion.