Preparation And Properties Optimization Of Nanocrystalline Ce-Fe-B Based Magnets And Anisotropic Sm-Fe Based Magnets

A large amount of critical rare-earth（RE）elements like Pr,Nd,Dy,Tb have been employed in RE permanent magnets,raising the price of RE and leading to unbalance utility of RE source.To develope Ce-Fe-B magnets with moderate magnetic properties and SmFe₁₂-based magnets with high performance is a promising solution for using the RE more effectively.However,the performance of Ce-Fe-B magnets is still far from to viable application,and the route for producing SmFe₁₂-based magnets is also required to further develop.Herein,this study investgated the mechanism for enhancing the performance of Ce-Fe-B magnets.The comprehensive properties were successfully improved by introducing element interaction,constructing chemical heterogeneity,forming texture and engineering grain boundary structures.Moreover,the intrinsic properties of Th Mn₁₂-type phase were modified,and anisotropic Sm（Fe,Ti,V）₁₂-based sintered magnets were successfully prepared by conventional liquid sintering process.Detailed characterizations were conducted to study the microstructure,magnetic domain and magnetization reversal process of Sm（Fe,Ti,V）₁₂-based sintered magnets.The origin of coercivty was revealed,and the magnetic properties were optimized.The main results are list as:The magnetic properties of melt-spun nanocrystalline Ce₁₇Fe₇₈B₆ alloy was optimized by doping Ta or Si elsments.The mechanism for the properties enhancement was clearly clarified.For Ce₁₇Fe_78-xTa_xB₆（x=0-1）alloys,the coercivityμ₀H_c increases from 0.55 to 0.70 T with the increasing x value from 0 to 0.75.The microstructure characterizations indicate that Ta doping is beneficial to grain refinement.A second phase of Ta B₂ is observed in Ce₁₇Fe_77.25Ta_0.75B₆ alloy.The micromagnetic simulation confirms that non-magnetic particles within hard magnetic phase is crucial for preventing the further magnetization reverse by pinning effect and for enhancing the coercivity.Si substitution for Fe is found to not only suppress the formation of Ce Fe₂ phase,but also significantly improve the coercivityμ₀H_c and thermal stability of Ce₁₇Fe_78-xSi_xB₆（x=0-3.0）alloys.Hard magnetic grains are isolated by forming a structure with thin and continuous intermediate layer,consequently preventing the domain wall propagation and increasing coercivity.The Curie temperature of the main phase monotonically increases from 424 K for x=0 to 445 K for x=3.0,contributing to improved thermal stability.The highestμ₀H_c of 0.67 T,the lowest temperature coefficients（β=-0.551%/℃andα=-0.368%/℃）,and an acceptableμ₀M_r=0.44 T are obtained in Ce₁₇Fe_78-xSi_xB₆（x=3.0）alloy.The magnetic properties of Ce₁₇Fe₇₅Si₃B₆ alloy was further modified by the abundant RE elements Y or La substitution for Ce.The coercivity of nanocrystalline(Ce_1-xY_x)₁₇Fe₇₅Si₃B₆alloys was enhanced by constructing the chemical heterogeneity.It is found that,in these alloys,the atom diffusing abilities in 2:14:1 phase,Ce-rich phase,Y-rich phase,and the residual 1:2phase during the solidification are different,resulting in chemical heterogeneity.The formation of RE-rich phase contributes to the predominant mechanism for the abnormal increase ofμ₀H_c.The magnetic properties of(Ce_0.5Y_0.5)₁₇Fe₇₅Si₃B₆ alloy areμ₀H_c=0.54 T,（BH）_max=57 k J/m³（7.1 MGOe）and T_c=547 K.The addition of La in(Ce_1-yLa_y)₁₇Fe₇₅Si₃B₆ alloys improves theμ₀M_r,the maximum energy product（BH）_max,curie temperature T_c because of the higher saturation magnetizationμ₀M_s and curie temperature T_c of La₂Fe₁₄B phase.However,lower anisotropic fieldμ₀H_a of La₂Fe₁₄B phase leads to the decrease ofμ₀H_c for(Ce_1-yLa_y)₁₇Fe₇₅Si₃B₆alloys.La prefers to entering into the intergranular phase and forming bulk aggregations,which is harmful to coercivity and weakens the effect of La on suppressing Ce Fe₂ phase.The best combination of magnetic properties with（BH）_max=6.5 MGOe,μ₀H_c=0.42 T,μ₀M_r=0.60 T and coercivity coefficientβ=-0.324%/℃is obtained in(Ce_0.6La_0.4)₁₇Fe₇₅Si₃B₆ alloy.Anisotropic Ce Fe B-based magnets have been successfully fabricated using the composition of Ce₁₇Fe₇₅Si₃B₆ by the single-stage hot-deformation（SSHD）process.The simplified process skips the hot-pressing stage,contributing to small grain size and high coercivity of the magnets.The degree of texture（DOT）,coercivity and thermal stability of Ce Fe B-based hot-deformed（HDed）magnets were significantly enhanced by adding 10 wt.%Nd₇₀Cu₃₀ and grain boundary diffusion（GBD）.The magnetic properties of Ce Fe B-based HDed magnets with Nd-Cu addition areμ₀H_c=0.55 T,μ₀M_r=0.59 T,（BH）_max=58 k J/m³（7.3 MGOe）.By combining Nd-Cu addition and GBD,the magnetic properties of HDed magnets have been further improved toμ₀H_c=0.88T,μ₀M_r=0.60 T,（BH）_max=61 k J/m³（7.7 MGOe）.The evaluated micromagnetic parameterαby fitting to the Kronmüllar equation increases from～0.116 for SSHDed sample to 0.304 for the sample with Nd-Cu addition and 0.441 for the sample with both Nd-Cu addition and GBD,suggesting that the optimization of microstructure is the primary origin of high coercivity.The formation of RE-rich intergranular phase between two hard magnetic grains acts as the pinning sites for preventing magnetic domain wall propagation.The pinning-type coercivity mechanism was demonstrated in two Nd-Cu-treated samples.An anisotropic bulk SmFe₁₂-based sintered magnet with the nominal composition of Sm₈Fe_73.5Ti₈V₈Ga_0.5Al₂（at.%）was successfully developed,and it showed a sufficiently large coercivity of 1.0 T and moderate saturation magnetization ofμ₀M_s=0.74 T.The anisotropy field of the main 1:12 phase was determined to beμ₀H_a=10.2 T.Detailed multi-scale microstructure characterizations show that the magnet consists of Sm（Fe,Ti,V,Al）₁₂ grains with the Th Mn₁₂-type crystal structure and a size distribution of～3-15μm.The Sm（Fe,Ti,V,Al）₁₂ grains are enveloped by～3 nm thick Sm-rich amorphous intergranular phase.Secondary phases including metallic（Sm,Ga）-rich,Sm O_x,and Fe₂（Ti,V）phases coexist with the 1:12 phase.Measured angular dependence of coercivity follows Kondorsky type magnetization reversal,suggesting the coercivity results from the pining of magnetic domain walls.Magneto-optical Kerr effect（MOKE）microscopy revealed magnetization reversal starts at the grain boundary and interphase interfaces,and the thin amorphous intergranular phases act as the pinning sites against magnetic domain wall propagation.The microstructure transformation and extrinsic performance change in the anisotropic Sm（Fe,Ti,V）₁₂-based sintered magnets were investigated via reducing the Ti content and partially substituting Co with Fe.Reduction of Ti content in anisotropic sintered Sm₈Fe_73.5+xTi_8-xV₈Ga_0.5Al₂ magnet contributes to the elimination of secondary Fe₂Ti phase,resulting in the increase of remanence.The Sm₈Fe_73.5+xTi_8-xV₈Ga_0.5Al₂ magnet with x=3 achieved aμ₀M_r of 0.8T and the maximum energy product（BH）_max of 113 k J/m³（14.2 MGOe）,which are better than that of the ever reported anisotropic SmFe₁₂-based bulk magnets.The formation of thin intergranular phase（IGP）contributes to the relatively high coercivity of～0.6 T.In addition,it is demonstrated that the substitution of Co for Fe results in decrease of remanence and coercivity for sintered Sm₈(Fe_0.95Co_0.05)_73.5Ti₈V₈Ga_0.5Al₂ magnet.The microstructure characterizations indicated that the formation of twins inside 1:12 grains deteriorates the texture and remanence.Magneto-optical Kerr effect（MOKE）microscopy and micromagnetic simulation results revealed that the magnetization reversal starts at the twined grains,while Sm-rich intergranular phase hinders the propagation of reversed domains to the neighboring grains,preventing the deterioration of coercivity.However,the formation of soft magnetic SmFe₂-type phase is a major reason for the further decrease of coercivity to 0.55 T.The optimum microstructure to realize high-performance anisotropic bulk SmFe₁₂-based sintered magnets should be one without twins and soft SmFe₂-type phase,but with refined grain size,and thin non-magnetic or weak ferromagnetic IGP isolating the SmFe₁₂ grains.This study systematically investigated the correlations of the composition,process,microstructure,and magnetic properties for nanocrystalline Ce-Fe-B alloys and anisotropic Sm（Fe,Ti,V）₁₂-based sintered magnets.It provides guidelines for developing the novel RE permanent magnets with high performance/cost ratio.