Microstructure of hot-pressed and die-upset magnequench magnets (abstract)

Hot pressing is a convenient method for preparing fully dense Nd-Fe-B magnets from the melt-spun ribbons. Die-upsetting the hot-pressed material produces an aligned high-energy product magnet.1 Transmission electron microscopy was used to study the microstructures of both the hot-pressed and the die-upset magnets. For a starting composition of Nd14Fe81B5, it is found that the magnetic Nd2Fe14B phase comprises over 95% of the material. In the hot-pressed magnet, most of the Nd2Fe14B grains are nearly spherical and their distribution nearly isotropic, the average grain size around 100 nm. In contrast, the die-upset magnets consist of flat parallel grains with their c axes parallel to the stress axis. In the case of hot-pressed materials, a thin intergranular layer of an amorphous Nd-rich phase (like the one found in the melt-spun alloys)2 is seen. In the die-upset material, the flat surfaces between adjacent grains are free of any such intergranular phase. In both these magnets, 100–200 nm size noncrystalline particles with Nd/Fe over nine are found, their number and amount being an order of magnitude higher in the die-upset magnet. Also found are a crystalline phase of approximate composition Nd7Fe3 (boron content unknown) and an fcc crystal structure with lattice parameter 0.56 nm at the grain junctions. No Nd1+εFe4B4 phase is ever found in any of these samples. Lorentz electron microscopy indicates that the second and third phases act as pinning sites for the magnetic domain walls in these materials.