Research On The Microstructure And Magnetic Properties For Nanocrystalline Nd(Ce)-Fe-B Alloy

Nd-Fe-B magnets have been widely used in various fields such as wind turbine generator,medical equipment,vehicle industry,and energy-saving household appliances,due to its excellent magnetic properties.For RE-Fe-B magnets,the excessive consumption of rare earth elements is mainly concentrated in Nd and Pr,however,the high abundance rare earth Ce was only used as a by-product of Nd and Pr.Improving the utilization rate of Ce in the rare earth permanent magnet is the research focus.In this thesis,Nd?Ce?-Fe-B ribbons were prepared by melt spinning technique,and then sintered magnets were synthesized by spark plasma sintering?SPS?technique.Based on the SPS sintered magnet,the anisotropic nanocrystalline Nd?Ce?-Fe-B magnet was successfully produced by hot deformation technique.At last,hot deformed nanocrystalline magnets with high coercivity and good temperature stability were obtained through the grain boundary diffusion processes?GBDP?,followed by the secondary hot deformation process.(Nd_0.8Ce_0.2)_13.5Fe_73.5Co_6.7Ga_0.5B_5.6 ribbons were prepared by melt spining technique,and the optimized melt spinning processes were obtained.It was found that the wheel velocity had vital effects on the magnetic properties and the squareness of demagnetization curves.The optimum magnetic properties can be obtained under the wheel velocity of 30 m/s.With increasing Ce content,overall,the coercivity and remanence of the ribbons decrease.However,when x is in the range of 0.2⁰.4,the coercivity increases first,and then decreases.Abnormal increase in coercivity should be attributed to the mixed valence states of Ce and phase separation phenomenon.For Ce-doped Nd?Ce?-Fe-B spark plasma sintered magnets,it was found that,with increasing Ce content,the volume of the coarse grain area increases gradually.Meanwhile,with the increase of Ce content,the coercivity and remanence decrease monotonously from 1302kA/m and 0.77T to 773kA/m and 0.68T,respectively,which is related to the reduced intrinsic magnetic properties and microstructure evolution.The studies about open recoils suggested that,the inhomogeneity of microstructure should be responsible to the increasing openness of recoil loops with enhancing the Ce content.Using SPS magnets as precursors,anisotropic nanocrystalline magnets were prepared by hot deformation.It was found that with the increase of Ce content,both the orientation of c axis and maximum energy product?BH?_maxax of hot deformed magnets decrease.From the studies about grain boundary diffusion of PrCu alloy,it is suggested that the coercivity and temperature stability are significantly improved,and the maximum growth in the coercivity and temperature coefficient of coercivity,up to266%and 20%,can be obtained.In addition,uniform microstructure contributes to the closed recoil loops,which is beneficial to the utilization of magnet.After the grain boundary diffusion,when Ce doping amount of 30 wt.%was employed,the optimal magnetic properties were achieved:J_r=0.81T,H_ci=1925kA/m and?BH?_max=126kJ/m³.Based on SPSed magnets with the addition of PrCu alloy,effects of different hot deformation processes on the microstructure and magnetic properties were investigated in detail.Under the hot deformation condition of 700?/30min/73%,the highest magnetic properties including coercivity,remanence,and maximum energy product can be obtained,which are H_ci=1168kA/m?J_r=1.22T and?BH?_max=277kJ/m³,respectively.After the grain boundary diffusion of PrCu alloy,the coercivity increases by 81%,from 1168kA/m to 2117kA/m.Based on this magnet,anisotropic magnets were prepared by the secondary hot deformation process.The coercivity and remanence increase from 2117kA/m to 2242kA/m and from 0.73T to 0.88T,respectively.The reduction in demagnetization induced by the improved microstructure uniformity and the increase of c axis orientation should be responsible to the increase of the coercivity and the remanence,simultaneously.