Preparation And Magnetic Properties Of Nanocrystalline Re-Fe-B Permanent Magnetic Materials Based On High Abundance Rare Earth Elements Of Ce,La,and Y

The ever-increasing demand for Nd-Fe-B permanent magnets has led to the overuse of critical rare earth（RE）including Nd,Pr,Dy,and Tb,while the high abundance RE resources such as Ce,La,and Y is overstock.Therefore,the development of Ce,La and Y-based RE-Fe-B magnets is an effective way to realize the balanced utilization of RE.However,the magnetic properties,phase behaviors and metallurgical behaviors of Ce-Fe-B,La-Fe-B and Y-Fe-B systems are quite different with those of Nd-Fe-B system.Up to now,Ce,La and Y-based RE₂Fe₁₄B alloys have not been well understood yet,and its magnetic properties need to be further improved.In this dissertation,melt-spinning and hot-press/deformation techniques were employed to prepare Ce,La,and Y-based RE-Fe-B nanocrystalline alloys and magnets.The grain boundary diffusion process（GBDP）was used to optimize the grain boundary（GB）and improve the coercivity of anisotropic hot-deformed magnets.The precipitation behavior and regulation mechanism of Ce,La and Y-based RE-Fe-B alloys and magnets,the distribution and coordination of multiple RE elements,the optimization of the preparation process and magnetic properties for anisotropic magnets,and the internal correlation mechanism between GB regulation and coercivity were studied.Firstly,the interaction between Ce,La and Y elements on the phase constitution,magnetic properties and metallurgical behavior of RE-Fe-B alloys was discussed.The results show that the hard magnetic RE₂Fe₁₄B（2:14:1）phase cannot be formed in the ternary La-Fe-B alloy,and≥20 at.%substitution of Ce or Y for La can promote the formation of the 2:14:1 phase.Although the anisotropic field H_A of La₂Fe₁₄B compound is lower than that of Ce₂Fe₁₄B,partial substitution of Ce by La can increase the anisotropic constant K of Ce₂Fe₁₄B,resulting in the enhanced coercivity H_cj.In addition,20 at.%La substitution can effectively inhibit the precipitation of Ce Fe₂ phase in the RE-rich Ce-Fe-B alloy.Although the Y element is not conducive to the H_cj of the Ce-Fe-B alloy,it shows benefits on improving of alloy in 300-400K,as well as the Curie temperature T_c of 2:14:1 phase.The H_cj of Y-Fe-B alloy shows an increasing trend with temperature,which leads to the positive value ofβ.Partial substitution of La for Y can further improve theβvalue,indicating the enhanced thermal stability.Based on the understanding of ternary and quaternary alloys,the quinary（Ce,La,Y）-Fe-B alloy was designed and optimized by regulating the interaction effect between RE,which provides the precursor with excellent properties for the subsequent preparation of densified magnets.The results show that the intrinsic magnetic properties and GB distribution of the quinary（Ce,La,Y）-Fe-B alloy can be improved by changing the Ce/La/Y ratio.The（Ce,La,Y）-Fe-B alloy shows higher comprehensive magnetic properties than those of the ternary alloy.The inhibition mechanism of La element in Ce Fe₂ phase was explained,La atom enters the Ce Fe₂ lattice and causes the lattice expansion,which in turn causes phase instability.In addition,the chemical segregation of RE elements in（Ce,La,Y）-Fe-B alloys was discussed.La prefers to enter into GB than Ce,while Y tends to remain in the 2:14:1 main phase.Partial substitution of La for Ce in the 2:14:1 main phase can improve its intrinsic properties,and the retention of Y in the main phase can enhance thermal stability.Therefore,the RE segregation is beneficial to the room temperature magnetic properties and thermal stability of the alloys.Thirdly,the isotropic hot-pressed and anisotropic hot-deformed Ce-Fe-B,（Ce,La）-Fe-B and（Ce,La,Y）-Fe-B magnets were prepared based on the understanding of the above melt-spun alloys.The anisotropy of the hot-deformed magnets was enhanced by structural optimization without Nd and Pr addition.The results show that no texture can be found in Ce-Fe-B ternary hot-deformed magnet due to the lack of RE-rich phase with low melting point.However,La substitution for Ce can reduce the amount of Ce Fe₂ phase,allowing the formation of the RE-rich phase and facilitating the diffusion-creep process during hot-deformation.In addition,the formation of RE-rich GB phase is beneficial to decoupling the main grains and enhancing the coercivity.The addition of Y element improves the thermal stability of the magnets while maintaining the texture.The[(Ce_0.9La_0.1)_0.8Y_0.2]₁₇Fe₇₈B₆ hot-deformed magnet exhibits an intrinsic coercivity H_cj of 124 k A/m,a remanence J_r of 0.63 T and a maximum energy product（BH）_max of 29.5 k J/m³,together with low temperature coefficients of remanence（α=-0.261%/K）and coercivity（β=-0.171%/K）in 300-400 K.Finally,a two-step grain boundary diffusion process was proposed to construct the GB and optimize the interface of the hot-deformed[(Ce_0.9La_0.1)_0.8Y_0.2]₁₇Fe₇₈B₆ magnet lacking RE-rich phase.The 1^st-step involves doping Nd₇₀Cu₃₀ powder into（Ce,La,Y）-Fe-B powder before hot pressing,followed by the conventional 2^nd-step diffusion process,in which Nd₇₀Cu₃₀ resource infiltrates into the hot-deformed magnet from the surface after hot deformation.It is established that 1^st-step diffusion leads to the formation of RE-rich GB layer,which act as the diffusion channel for increasing the efficiency of the 2^nd-step diffusion.The microstructure and ⁵⁷Fe M?ssbauer spectroscopy results show that the two-step diffusion not only improves the volume fraction and distribution but also weakens the ferromagnetism of the GB.Lorentz transmission electron microscopy（LTEM）confirmed the formation of discontinuous domain walls across the thick non-magnetic GB layer,and micromagnetic simulations further verified that the weakening of the ferromagnetism is beneficial to high coercivity.By two-step diffusion,the H_cj,J_r and（BH）_max of the magnet were greatly improved to 501 k A/m,0.68 T and 72.4 k J/m³,respectively.In summary,the nanocrystalline（Ca,La,Y）-Fe-B alloys and magnets with excellent comprehensive hard magnetic properties were developed,and the interaction effect of these high abundance RE and the orientation mechanism of Ce-based magnets during hot deformation was clarified.In addition,the influence of grain boundary magnetism and distribution on the coercivity was discussed.This work provides significant guidance for the further development of RE-Fe-B permanent magnets based on Ce,La,and Y elements.