Anisotropic magnets were obtained by hot deformation with the partial crystallized precursor prepared via spark-plasma sintering （SPS）. Amorphous powders with the nominal composition of Nd_28.72Fe_balCo_5.66 Ga_0.59B_0.92 （wt%） were used as the starting material. The results show that the amorphous powders would suffer varying degrees of crystallization even below the crystal- lization point during the SPS process under high pressure. And the pre-crystallized grains in precursors have great impacts on the microstructure and magnetic properties of the hot-deformed magnets. The final obtained anisotropic magnets exhibit homogeneous microstructure consisting of well-aligned and platelet-shaped Nd_2Fe_14B grains without abnormal growth. It can be found that a reasonable pro- portion of pre-crystallized gains could promote the pref- erential orientation in the magnet, leading to the achievement of optimal magnetic properties among the magnets with identical composition and best magnetic performance is achieved in the magnet hot deformed from the 490 ℃ high-pressure hot-pressed precursor.
The disproportionated phases of Nd Hx, Fe2B,and a-Fe from Nd2Fe14B were applied to prepare Nd Fe B magnets by two different routes. The results show that the route of annealing in horizontal vacuum sintering furnace cannot reach the purpose of complete recombination after the hot pressing and hot deformation process due to the lack of dehydrogenation channel. The route of applying low pressure of 4–25 MPa on the as-disproportionated green compact during the desorption recombination process in situ hot deformation in a spark plasma sintering（SPS）system can obtain completely recombined Nd Fe B magnet with good anisotropy and magnetic properties. The maximum magnetic properties,（BH）max= 201 kJ m^-3,Br= 1.142 T and Hcj= 469 k A m^-1, are obtained after being treated for 15 min at 750 ℃ under low pressure.
The effects of initial density on the magnetic properties of NdFeB magnets prepared by single-stage hot deformation were investigated in this work. The results show that the values of maximum energy product （BH）m and coercivity Hcj decrease with the increase of initial density. Under optimum condition, an anisotropic magnet with a maximum energy product of 264 kJ-m-3 was pro- duced using the initial density of 4.4 g-cm-3. The influence of initial density on the magnetic properties was discussed on the basis of microstructure and strain energy. It is concluded that the thicker platelet grains are obtained along the Nd2Fe14B base plane with the initial density increasing. It is mainly because that grain rotation is restricted by high strain energy, which results from high initial density.
Radially oriented Nd-Fe-B ring magnets were prepared by backward extrusion of MQ-C powder. The punch chamfer radius has a great impact on the microstructure and magnetic properties of the ring magnet. With the chamfer radius changing from 2, 5 to 8 mm, the cracks in the inner wall decrease obviously while the crystallographic alignment drops. Furthermore, the mechanism of caxis growth was suggested to be a combination of shear deformation in the corner and solution-precipitation under the stress parallel to radial direction. The alignment drops on the top of ring because the grains grow freely and some textured grains grow through nucleation and recrystallization. In the present work, the optimal punch chamfer radius is found to be 2 mm, and in this case, the remanence,coercivity, and maximum energy product of the ring magnet achieve 1.4 T, 670 kJám, and 342 kJám,respectively.
The partially recombined compacts with ultrafine grain size were taken advantage of preparing anisotropic nanocrystalline magnets with full density and homogenous microstructure and texture by reactive deformation under low pressure. Because of the ul- trafine grain size of the precursors, the partially recombined phases could quickly achieve recombination. The results suggested that the newly recombined Nd2Fe14B grains with fme grain size could undergo deformation immediately during the desorp- tion-recombination reaction, and then an obvious anisotropy and uniform alignment would be obtained. The magnetic properties, （BH）max=214 kJ/m3, Br= 1.26 T, Hcj=463 kA/m, were obtained after being treated for 5 min at 820 ℃ in high vacuum under low pres- sure less than 26 MPa. Microstructures of the magnets were analyzed using X-ray diffraction （XRD）, scanning electron microscopy （SEM） and transmission electron microscopy （TEM） respectively. Magnetic measurements were carried out using a vibrating sample magnetometer （VSM） with the maximum field of 2.88 T. Accurate phase contents were measured by a Mossbauer spectrometer.
