Microstructure And Properties Of NdFeB Based Spark Plasma Sintered And Hot Deformed Magnets And Diluted Magnetic Alloys

Sintered permanent magnets have attracted much attention and found their applicationsin a wide range of devices after the borth of the Nd-Fe-B magnets in1983for their low costand superior magnetic properties. As the conventional sintering is a pressureless sinteringtechnique, the as-sintered magnets generally contain porosity and nonmagnetic phasesinevitably, which have a negative effect on the thermal stability, corrosion resistance andfracture toughness. In addition, the conventional densification process is not suitable forpreparing nanocrystalline bulk magnets, since magnetic properties will be deteriorateddramatically by the grain growth due to the low heating rate, high sintering temperature（above1000°C）, and long holding time. To densify the nanocrystalline magnetic powders bysintering, a new method of sintering, spark plasma sintering （SPS）, has been introduced.In this work, commercial melt spun powders and micron-sized Zinc and Dysprosiumoxide powders were used as the starting materials. Bulk isotropic and anisotropicnanocrystalline Nd-Fe-B magnets were synthesized by SPS process and by SPS followed byhot deformation （HD）, respectively. The relationship between SPS temperature andmicrostructure has been investigated. The influences of Zn and Dy₂O₃additions on themagnetic properties and microstructure for isotropic and anisotropic magnets were studied indetail. In order to further improve the hard magnetic properties, the grain boundary diffusion（GBD） method has been emplyed for HDed magnets. Furthermore, nanocrystalline Nd-Fe-Bribbons with a range of excess Nd volume fractions were prepared by vacuum arc-melting andmelt spinning and the structure and magnetic properties of these diluted magnetic alloys havebeen studied.For isotropic nanocrystalline Nd-Fe-B magnets, the effects of SPS temperature wereinvestigated with SPS pressure of50MPa and SPS duration of5min Two distinguishablezones, coarse grain zone and fine grain zone, with different grain sizes were noticed in allSPSed magnets. The width and the grain size of the coarse zones increased quickly withincreasing temperature, while the grain size in the fine zone increased slowly. The density ofSPSed magnets also increased with the increasing temperature. The magnet sintered at750°Cpossesses an optimal microstructure. The microstructure and magnetic properties were studied for bulk magnets synthesizedby SPS with the additions of Zn and Dy₂O₃sintered at750°C. Low Zn contents reduced thedensity of Zn added magnets slightly. The porosity and coarse grains appeared in the particlesinterfaces containing Nd-Zn compounds when Zn content is up to2wt.%. With increasingDy₂O₃content, the porosity increased in the magnet, resulting in the decreased density. Bothadditives play a role in suppressing grain growth of Nd2Fe14B phase and Zn has a moresignificant effect. Compared with Zn, Dy₂O₃results in a more considerable increase of_jH_c.For Zn added magnets, the optimal magnetic properties were obtained with0.6wt.%Znaddition. For Dy₂O₃added magnets, the magnet with2wt.%Dy₂O₃addition showed theoptimal magnetic properties. For the SPSed magnet with a combined addition of2wt.%Dy₂O₃and0.6wt.%Zn, the optimal magnetic properties obtained wre J_r=0.73T,_jH_c=1142kA/m, and （BH）max=91kJ/m³. The temperature coefficients of remanence （α） and coercivity（β） were-0.133%K^-1)and-0.462%K^-1), respectively.Anisotropic nanocrystalline magnets were prepared by SPS followed by HD processsuccessfully. The microstructure of different areas in the additive free HDed magnets wasstudied. The centre of the magnets had the optimal microstructure with high degree oforientation. The influences of Zn addition on the microstructure, magnetic properties,temperature stability and anti-corrosion properties have been investigated. Compared with theadditive free HDed magnet, the degree of orientation and magnetic properties of the magnetwith2wt.%Zn addition decreased. This must be attributed to the excessive Zn addition. Thetemperature coefficients of remanence （α） and coercivity （β） are higher than the additive freeHDed magnets, indicating a better temperature stability. The magnet with2wt.%Zn additionalso showed better anti-corrosion properties.The grain boundary diffusion （GBD） method has been tried for the addtive free HDedmagnet in order to further improve the magnetic properties. The preliminary results show thatit is very difficult to transfer the GBD process to the SPS+HDed magnets. This may resultfrom a grain growth and discontinued Nd rich phase. On the contrary, Dy was successfullydiffused into N33sintered magnet for a continued Nd rich phase and a less grain growth.These results demonstrated that the distribution of Nd-rich pahse is very important to theGBD process. For diluted magnetic Nd_δFe1₁₄B alloys prepared under optimal conditions, with increasingNd content from12.69at.%to42.30at.%, the coercivity increased from1021kA/m to1713kA/m, the remanence decreased from0.80T to0.38T, and the values of Jsand Js/Js（Nd2Fe14B）also decreased gradually. The exchange coupling interaction may still exist in the Nd11Fe14Balloy for J_r/Js=0.51>0.5. The relationship between coercivity （_jH_c） and the particle spacing （R）was also investigated and the results show that_jH_cvalue is proportion to R by calculating andplot fitting. Further investigation are needed for this interesting type of alloys.