Preparation, Microstructure, And Properties Of Isotropic And Anisotropic Nanocrystalline NdFeB-based Magnets

Nd-Fe-B magnets have been widely used in various fields such as electromechanicalequipments, information, communication, and medical devices, due to its excellent magneticproperties. Nanocrystalline Nd-Fe-B magnets have attracted much attention not only becauseof their good magnetic properties but also due to their exceptional thermal stability andfracture toughness. The conventional densification methods for preparing nanocrystallinemagnets is still a major concern, since magnetic properties will deteriorate dramatically due toslow heating rate, high sintering temperature, and long holding time.In this work, isotropic and anisotropic nanocrystalline Nd-Fe-B magnets were fabricatedby spark plasma sintering (SPS) and SPS followed by hot deformation, respectively. Theprocessing-microstructure-magnetic properties were investigated in detail. The diffusion ofNd-rich phase and induced developing microstructure were discussed for the isotropic andanisotropic nanocrystalline Nd-Fe-B magnets. The characteristics of recoil loops and theirrelationships with the microstructure and properties for hot deformed nanocrystallineNd-Fe-B magnets were investigated.Isotropic nanocrystalline Nd-Fe-B magnets were synthesized by spark plasma sintering(SPS) using the melt spun ribbons as the starting materials. Due to the local high-temperaturefield induced by spark plasma discharge at the vicinity of the particle boundaries, the distincttwo-zone structure was formed in the SPSed magnets. The SPS temperature and pressure haveimportant effects on the widths of coarse and fine grain zones, as well as the grain sizes intwo zones. The changes in grain structure led to the variations in the magnetic properties.Melt spun Nd-Fe-B ribbons with two compositions and various powder sizes were employedas the starting materials. For the magnets with RE-rich compositions, the influence of powdersizes on the coercivity of SPSed magnets is very significant. For single phase Nd-Fe-B alloyswith stoichiometric2/14/1composition, the starting powder with a larger particle size isbeneficial to achieve better magnetic properties. The best combination of magnetic propertiesare J_r=0.82T,jH_c=1516kA/m, and (BH)max=116kJ/m³at the sintering condition of700oC/50MPa/5min. The values of α and β are－0.107%K^-1and－0.682%K-1, and therelative density is as high as99.5%. Anisotropic nanocrystalline magnets were prepared by spark plasma sintering (SPS)followed by hot deformation (HD) using SPS precursors. The influences of strain rate andcompression ratio on the microstructure, magnetic properties, temperature stability have beeninvestigated. The platelet-shaped grain dimension perpendicular to the pressing direction wasrelated to the remanence and temperature coefficient of coercivity for hot deformed magnets.A strong domain-wall pinning model was valid to interpret the coercivity mechanism andcharacteristics of initial magnetization curves for hot deformed magnets. The influences ofnon-uniform plastic deformation on the microstructure and magnetic properties were alsoinvestigated. Good magnetic properties with Jr=1.35T,jHc=829kA/m, and (BH)max=336kJ/m³have been obtained. The value of β is－0.682%K-1.The role of Nd-rich phase in developing microstructure and properties of isotropic andanisotropic Nd-Fe-B magnets has been investigated. Melt spun Nd-richNd13.5Fe73.5Co6.7Ga0.5B5.6and Fe-rich Nd_7.7Pr_2.6Fe84.1B5.5alloy powders were mechanicallymixed with different ratios. The mixed powders were consolidated into isotropic magnets andanisotropic magnets by spark plasma sintering (SPS) and SPS followed by hot deformation,respectively. The composition and microstructure of diffusion area between Nd-rich andFe-rich compositions for isotropic and anisotropic magnets were investigated. The gradientdistribution of Nd content from Nd-rich to Fe-rich area due to the diffusion of liquid Nd-richphase during the SPS and hot deformation was observed, which leads to gradually changes ingrain structure. The c-axis crystallographic alignment decreased with the decrease of Ndcontent.The characteristics of recoil loops and their relationships with the microstructure andproperties for hot deformed nanocrystalline Nd-Fe-B magnets were investigated. The recoilloops of all SPSed magnets are closed using single magnetic powders as the starting materials.However, the recoil loops of hot deformed anisotropic Nd-rich Nd-Fe-B magnets without softmagnetic phase were found to be open. The investigations showed that the aggregation ofNd-rich phase is related to the open loops. The Nd-rich phase aggregation with various sizes,lager than grains, which can produce non-uniform distribution of local demagnetization fields,reduced the local pinning field and lead to the inhomogeneity of magnetic anisotropy. Thisvariation of local magnetic anisotropy in the magnets should be responsible for the formation of open recoil loops. Less extent of Nd-rich phase aggregation obtained by increasing therelative density of SPS precursors, which can reduce the openness of recoil loops, and theenhanced magnetic properties can be obtained obviously. Using the SPSed magnets with thedensity of7.03g/cm³,7.33g/cm³, and7.50g/cm³as the precursors, hot deformed magnetswith the coercivity of226kA/m,288kA/m, and995kA/m were obtained under the samedeformation conditions, respectively.