Fabrication,Microstructure And Property Regulation Of Nanocrystalline Ce-Fe-B Based Magnets

Much demanded and overused are the critical rare earth elements such as Pr,Nd,Dy and Tb with increasing need of Nd-Fe-B-type rare earth permanent magnets in the world.The soaring price of these four critical rare earth elements and the roller-coaster-type price fluctuation in rare earth raw materials in general have caused a considerable pressure on the Nd-Fe-B manufacturing industry and instability in their downstream applications.However,the most abundant rare earth element Cerium?Ce?is overstock and the utilization efficiency is very low for a long time.So,it is quite important and necessary to do research and develop Ce-containing permanent magnetic materials with higher ratio of performance over cost for national strategic security and the reduction of the costs of raw materials.The thermal stability,metallurgical behaviors and valence state variation of Ce in high abundant rare earth based magnets are quite different from those in the Nd-Fe-B based magnets.In this dissertation,melt-spinning and spark plasma sintering?SPS?techniques were used to fabricate nanocrystalline Ce-Fe-B based alloys and magnets.The relationship is established among the compositions,structures,Ce valence weight and magnetic properties.The magnetic properties,elemental metallurgical behaviors,microstructure characterization and coercivity enhancement mechanisms are investigated systematically.Discussed also include the evolution of intergranular exchange coupling interaction strength.First of all,nanocrystalline Ce-Fe-B based alloys were fabricated by melt-spinning technique.Non-rare earth element is added to optimize the composition of alloys.The effect of non-rare earth element doping on phase structures,magnetic characteristics,microstructures and Ce valence weight are investigated.The results show that the addition of Nb,Cu,Ga element can enhance the coercivity of alloys effectively.The best overall magnetic properties are eventually obtained with the ingot composition of Ce₁₇Fe_74.5B₆Nb_0.5Cu_0.5Ga_0.5Co_2.0 alloys.The magnetic properties are remanent magnetization J_r=0.43 T,coercivity H_ci=486 kA/m,maximum energy product?BH?_max=30 kJ/m³,Curie temperature T_c=437.8 K.The synchrotron radiation X-ray results show that the Ce³⁺valence weight increases while the Ce⁴⁺valence weight decreases compared with the counterpart Ce₁₇Fe₇₈B₆ alloy.The increased valence weight of Ce³⁺is beneficial to the magnetic properties.Obvious remanent magnetization enhancement and fully closed recoil loops are observed for the optimized alloys.The improved magnetic properties are mainly due to the increased RE/Fe ratio,Ce³⁺valence weight and strengthened intergranular exchange coupling interactions.The isotropic nanocrystalline Ce-Fe-B based magnets were then prepared by SPS technique using the optimized alloys.The relationship among sintering processes,microstructures and magnetic properties is discussed in depth by mainly using TEM and PPMS techniques.It is demonstrated that the sintering temperature has a significant influence on the average grain size and width of coarse grain areas.The best magnetic properties are J_r=0.37 T,H_ci=227 kA/m,?BH?_max=16 kJ/m³,obtained at 650°C under 50 MPa for 2 min.Their exchanging coupling interactions are quite weak compared with those of the counterpart alloys.The microhardness of the Ce-Fe-B based SPSed magnets prepared at different temperatures exhibits no obvious variation.The electrochemical experiment results display that the corrosion resistance of the magnets is improved by sintering at high temperature.Dual main phase structured nanocrystalline Ce-Fe-B based magnets were prepared by blending low melting point NdCu alloy with Ce₁₇Fe_74.5B₆Nb_0.5Cu_0.5Ga_0.5Co_2.0 alloy before SPS process.Investigated are the magnetic characteristics,thermal stabilities,coercivity enhancement mechanisms and elemental metallurgical behaviors.It was examined by the magnetic measurements to reveal that the magnetic properties and thermal stabilities of magnets are effectively improved by NdCu addition.J_r,H_ci,?BH?_max are 0.47 T,476 kA/m,30 kJ/m³ with 20wt.%NdCu content.The temperature coefficients of the remanence and coercivity of the magnets are better off from-0.459%/K,-0.599%/K to-0.340%/K,-0.553%/K respectively in the range of300-400 K.Intergranular exchange coupling interaction is also strengthened.It is shown by the analysis of microstructures and composition identification that the ribbon boundary is rich in Ce,Nd but lean in Fe.A strong Cu enrichment and a slight Ce leanness are visible at the interface between ribbon and ribbon boundary.It is also found out that Nd atoms in the ribbon boundary prefer to diffuse into the ribbons with low rare earth content.The diffused Nd element replaces partially the Ce atoms in Ce-Fe-B main phase and forms?Nd,Ce?-Fe-B hard magnetic phase,which result in two Curie temperature points in the magnets.The grain boundary of fine grain area is rich in Nd,Cu,O but lean in Fe,Co.Micrometer scaled CeFe₂ grains exist at coarse grain area.The concentration of Nd,Cu is high while the content of Fe,Co and Ce is low at grain boundary of these areas.Finally,three main phase nanocrystalline SPSed magnets were prepared with different ratio of alloys of Ce-Fe-B and Pr-Nd-Fe-B by dual alloy method.The remanent magnetization J_r,coercivity H_ci,maximum energy product?BH?_max and microstructure are improved obviously with increasing percentage of Pr-Nd-Fe-B alloy.The magnetic properties of J_r=0.71 T,H_ci=915 kA/m,?BH?_max=72 kJ/m³ are obtained for the SPSed magnets with 80 wt.%Pr-Nd-Fe-B alloy.The temperature coefficients of the remanence and coercivity of the magnets are improved from-0.459%/K,-0.599%/K to-0.232%/K,-0.534%/K respectively in the range of 300-400 K.The coexistence of multi main phases leads to the enhanced intergranular exchange coupling interaction.The grain boundary of both coarse grain area and fine grain zone is rich in Ce,Pr,Nd,Cu but poor in Fe,Co.It is indicated by the electrochemical experiment that the addition of Pr-Nd-Fe-B is harmful to the magnet's corrosion resistance.To conclude,the fabrication,microstructure and property regulation of nanocrystalline Ce-Fe-B based alloys and magnets are studied systematically,which may shed light on the further research and development of abundant rare earth magnets.