Study On Structure,Magnetic Properties And Magnetic Hardening Mechanism Of Sintered Permanent Magnets Based On Misch Metal

With the increasing demands of traditional rare earth magnets,the consumption of the rare earth（e.g.,Pr,Nd,Dy and Tb）is gradually increasing.A relatively high number of rare earth La/Ce remains.Meanwhile,the division of rare earth elements is harmful to our environment.So far,it has been more attractive that rare earth permanent magnets are made from the high-content rare earth(especially,the raw ore or partially separated misch metal（MM）substitutions for Nd or Nd₈₀Pr₂₀),which can realize the national strategic goal.In this article,we systematically study the content,intrinsic magnetic properties and microstructures of the MM-Fe-B ingot and（Nd,MM）-Fe-B casting strips.We have prepared（Nd,MM）-Fe-B sintered magnets.The effect of the variation（x）on magnetic properties and microstructures of(Nd_xMM_1-x)-Fe-B magnets has been studied.Their magnetic hardening mechanism and the thermal stability are discussed.This research aims to provide a foundation of(Nd_xMM_1-x)-Fe-B sintered magnets for the initial engineering application.The composition,melting points and magnetic properties of the MM raw material and the MM-Fe-B ingot are detected by XRF,XRD,DSC,SEM and VSM.The results of DSC show that the melting point of MM is different from that of the mixture composed of pure alloys（La,Ce,Nd and Pr）,which provides the alternative reference to analyze the melting point of the rich rare earth（RE）phases in the final sintered magnets.XRD shows that the MM solid solution originally consists of four RE elements.The main phase of the MM-Fe-B ingot belongs to the 2:14:1 tetragonal structure and demonstrates excellent intrinsic magnetic properties,e.g.,the Curie temperature T_c=229.9℃,the saturation magnetization M_s=11.3 kG and magneto-crystalline anisotropy field H_A=48.4 kOe.SEM and EDX results can illustrate that La and Ce prefer to concentrate in the RE-rich region and Pr and Nd aggregate into the main phases,which contributes to the good performance of magnetic properties of final sintered magnets.Several kilograms of（Nd,MM）-Fe-B magnetic casting-strips are prepared.Their microstructure,composition and intrinsic magnetic properties are analyzed.When x equals to 0,0.4,0.7 and 1,the melting points of(Nd_xMM_1-x)-Fe-B casting strips are 1103℃,1141℃,1166℃,1185℃,spin reorientation phase transition temperatures are-199℃,-160℃,-149℃,-138℃,magneto-crystalline anisotropy fields are 48 kOe,55.2 kOe,64.4 kOe,70.1 kOe,the magnetizations(M_{90 kOe})are 12.9 kG,13.6 kG,14.3 kG,14.5 kG and Curie temperatures are 220.2℃,265.4℃,286.7℃and 309.3℃,respectively.(Nd_xMM_1-x)-Fe-B sintered magnets with the optimal magnetic properties are prepared.For Nd-Fe-B sintered magnets,the optimal sintering temperature is 1050℃and the best magnetic properties with B_r=14.71 kG,H_k=11.72 kOe,H_cj=12.24 kOe,H_k/H_cj=0.95 and（BH）_max=52.64 MGOe are obtained.For(Nd_0.7MM_0.3)-Fe-B sintered magnets,the optimal sintering temperature is 1055℃and the best magnetic properties with B_r=13.39 kG,H_k=5.73 kOe,H_cj=6.15 kOe,H_k/H_cj=0.93 and（BH）_max=39.90 MGOe are obtained.For(Nd_0.4MM_0.6)-Fe-B sintered magnets,the optimal sintering temperature is 1050℃and the best magnetic properties with B_r=12.96 kG,H_k=2.41 kOe,H_cj=2.71 kOe,H_k/H_cj=0.88 and（BH）_max=22.82 MGOe are obtained.For MM-Fe-B sintered magnets,the optimal sintering temperature is1050℃and the best magnetic properties with B_r=11.69 kG,H_k=0.23 kOe,H_cj=0.53kOe,H_k/H_cj=0.42 and（BH）_max=2.71 MGOe are obtained.In contrast with[Nd_0.7(La_0.27Ce_0.53Pr_0.03Nd_0.17)_0.3]-Fe-B sintered magnets,the(Nd_0.7MM_0.3)-Fe-B sintered magnets exhibit better（BH）_max（maximum magnetic energy product）and higher H_cj（coercivity）.Besides,the enhanced coercivity is related to the different contents between both magnets,(Nd_0.7MM_0.3)-Fe-B sintered magnets are mainly filled with more（La,Ce）-lean and Nd-rich main phases and less（La,Ce）-rich and Nd-lean main phases compared with the other sintered magnet.Thereby,the magneto-crystalline anisotropy field of(Nd_0.7MM_0.3)-Fe-B sintered magnets is bigger so that its coercivity is ought to be higher.What is more,the magnetic properties varied with their microstructures.The diffusion phenomenon can be detected by EPMA observation,which Ce mainly lives in the RE-rich region of(Nd_0.7MM_0.3)-Fe-B sintered magnets.When x decreases,more Ce atoms are in the main phases of MM-Fe-B sintered magnets,which leads to the worse magnetic properties.Furthermore,XRD analysis demonstrates that the average grain alignment degree of the MM-Fe-B magnet weakens rather than the(Nd_0.7MM_0.3)-Fe-B magnet.The magnetic hardening methods and the temperature dependence of(Nd_xMM_1-x)-Fe-B sintered magnets are explored.With the Tb-H diffusion process,the coercivity of(Nd_0.4MM_0.6)-Fe-B sintered magnets increases by 3.95 and that of MM-Fe-B sintered magnets increases by 2.91.On the other hand,that of(Nd_0.4MM_0.6)-Fe-B sintered magnets prepared by the double main phase alloy method（DMPAM）also increases.They may be ascribed to the distribution phenomenon of different RE elements.In addition,(Nd_0.4MM_0.6)-Fe-B sintered magnets prepared by DMPAM own the bigger alignment distribution coefficient and bigger misaligned angle.It is the benefit for the enhancement of the coercivity as well.Besides,the parameters(α,N_eff)of the coercivity are discussed for(Nd_0.7MM_0.3)-Fe-B sintered magnets prepared by the single alloying method（SAM）and by DMPAM.The differences of microstructural parameters result in the different structures of both magnets.In the end,temperature coefficient of remanence and temperature coefficient of coercivity are measured in(Nd_0.7MM_0.3)-Fe-B and(Nd_0.4MM_0.6)-Fe-B sintered magnets prepared by SAM.In the range from 27℃to 127℃,the temperature coefficient of remanence in(Nd_0.4MM_0.6)-Fe-B magnets is worse,while the temperature coefficient of coercivity in(Nd_0.4MM_0.6)-Fe-B magnets becomes better.