Coercivity Promotion Of Nanocrystalline(Nd,Ce)-Fe-B Permanent Magnetic Materials And Its Mechanism

In recent years,with the purpose of balancing utilization of rare earth resources and reducing the cost of Nd-Fe-B permanent magnetic materials,the substitution of Ce for R in R-Fe-B(R is rare earth elements)permanent magnets has gradually become a hot topic in research of this area.However,with the increase of Ce content in(Ce,R)-Fe-B materials,the decrease rate of coercivity is much faster than that of magnetocrystalline anisotropic field.Therefore,how to improve the coercivity of(Ce,R)-Fe-B materials has become a key factor for wide application of Ce.In this thesis,the structure and magnetic properties of ternary Ce-Fe-B nanocrystalline alloys prepared by melt-spinning were studied firstly.The influence of the phase composition and microstructure evolution on the magnetic properties during the process of rapid quenching and after crystallization was clarified.Secondly,the microstructure of nanocrystalline(Nd,Ce)-Fe-B powder material was tuned by adding Ti element to obtain a high coercivity.The magnetic hardening mechanism and magnetization reversal process of the Ti containing material were analyzed by micromagnetic simulation and coercivity mechanism model.Finally,the microstructure and magnetic properties of nanocrystalline(Nd,Ce)-Fe-B bulk materials were studied by hot pressing and hot deformation technology(HD).To improve the coercivity of the HD magnets,the low melting point Pr68Cu32 alloy was added into the magnets by means of additives method and grain boundary diffusion process(GBDP).Firstly,the effects of production process on the microstructure and magnetic properties of Ce12Fe82B6 melt-spun alloy were studied.It is found that the phase composition of Ce12Fe82B6 alloy is significantly affected by the melt-spun wheel speed.At low wheel speed of 15 m/s,the Ce12Fe82B6 melt-spun alloy is composed of 2:14:1 main phase and a small amount of a-Fe phase.With the wheel speed increase,the phase structure gradually changes to TbCu7-type metastable phase,2:14:1 phase and a little a-Fe phase.While at wheel speed of more than 35 m/s,a completely amorphous phase is achieved.The main reason for the low properties of Ce12Fe82B6 melt-spun ribbons lies in the complex phase composition,many heterophases,and inhomogeneous microstructures.The magnetic properties of Ce12Fe82B6 alloy can be improved by increasing the Ce ratio.Two reasons respond for the improvation:first,the formation of TbCu7-type metastable phase is restrained by the upping Ce ratio which avoided the formation of inhomogeneous microstructures after the metastable phase decomposition;second,the formation of the room temperature paramagnetism CeFe2 phase is promoted,the grains are refined and the microstructure is optimized.With the increase of Ce ratio,the domination of coercivity changes from nucleation machenism to pinning macheninsm.As a result,the coercivity increases from 2.51 kOe to 5.79 kOe.Secondly,the effect of Ti addition on the microstructure and magnetic properties of Nd-Ce-Fe-B melt-spun alloy was studied.In(Nd0.8Ce0.2)12Fe82-xTixB6 samples,the addition of Ti can improve the amorphous forming ability and crystallization temperature of the alloys.Some Ti atoms enter the lattice of the main phase,resulting in the decrease of lattice constant and Curie temperature.With the increase of Ti content,the remanence and magnetic energy product of(Nd0.gCe0.2)12Fe82-xTixB6 alloy decreases gradually,while the coercivity increases.The coercivity of the Ti-free sample is enhanced from 9.48 kOe to 19.5 kOe of the Ti-7at.%sample.The various types and sizes of the grain boundary phases are responsible for the promotion of the coercivity.The alloy with low Ti content(x<5)contains a Ti-rich grain boundary phase with a size of about several nanometers,while for higher Ti containing samples,the grain boundary phase is changed to be Fe2Ti with a size of more than ten nanometers.The coercivity is doninated by typical pinning mechanism which is proved by the initial magnetization curve and the analysis of the coercivity mechanism model.The simulation results of magnetization reversal process by OOMMF software show that the larger is the grain boundary phase size,the stronger the hindrance to the formation and expansion of nuclei of reversed domain.The study of(Nd1-xCex)12Fe77Ti5B6 series alloys shows that with the increase of Ce content,the amorphous forming ability and crystallization temperature of the alloy decrease gradually.When the Ce content reaches 60%or more,the TbCu7-type metastable phase precipitates during the crystallization process of the alloy.The TbCu7-type phase containing Ti is so stable that it is difficult to decompose completely,resulting in the deterioration of the structure and the magnetic properties.Finally,the study of(Nd1-xCex)13.8Fe76.1CO4Ga0.5B5.6 hot pressed and hot deformed(HD)nanocrystalline magnets showed that adding of Ce was beneficial to improve the plasticity of the magnets.With the Ce content going up,the c-axis orientation of the hot deformed magnet increases firstly and then decreases.The x=0.3 magnets obtaines the best orientation and has a good comprehensive magnetic properties of Br=13.00 kGs,Hcj-=10.12 kOe,(BH)max=38.42 MGOe.Two ways of powder mixing(PM)and grain boundary diffusion process(GBDP)were employed to add Pr68Cu32 alloy to the HD-magnets.After addition Pr68Cu32 by PM method,the coercivity of the magnets is enhanced by 20.7%,while the remanence and magnetic energy product are reduced by 8.8%and 37.6%respectively.But for GBDP magnets,the coercivity is enhanced by 54.6%,while the remanence and magnetic energy product are only reduced by 1%and 2.4%respectively.The properties of the GBDP magnet are Br=12.87 kGs,Hc=15.65 kOe,(BH)max=37.48 MGOe.The microstructure analyses of the magnets clearly show that the diffusion of the GBDP method is more sufficient and the structure of the magnets is more uniform than that of the PM one.The results of composition distribution and Curie temperature test show that Pr diffuses into the main phase grains,which not only improves the Curie temperature of the shell of the grains,but also improves the magnetic performance of the magnets.Besides,the temperature coefficient of coercivity increases because of the addition of Pr68Cu32 alloy,but the temperature coefficient of remanence has almost unchanged.The analysis of the coercivity mechanism demonstrates that the coercivity of the PrCu free HD-magnets and the PM magnets are determined by the nucleation mechanism,while the coercivity of the GBDP magnets is dominated by the interaction of nucleation mechanism and pinning mechanism.