Study On Grain Size Refinement And Grain Boundary Modification Of Sintered Nd-Fe-B Magnets

With the rapid development of emerging green industry such as wind generators,energy-efficient appliances and hybrid vehicles,high coercivity Nd-Fe-B magnets are widely demanded to meet the requirement in high temperature applications.Solving the strong dependency of heavy rare earth(RE)in high-coercivity Nd-Fe-B sintered magnets and realizing the high quality utilization of rare earth resource have become a significant issue recently.Generally,it is an effective approach to enhance the coercivity by optimizing microstructure of Nd-Fe-B magnet,including grain size refinement and grain boundary modification.In this thesis,HDDR was introduced to refine the microstructure of initial striping casting(SC)alloys and ultrafine grained magnets with grain size of 1 ?m was successfully prepared.The microstructure,domain structure and magnetic behavior were studied systematically.Moreover,The relationship between microstructure and thermal stability was clarified.In the aspect of grain boundary(GB)modification,a novel approach was proposed to fabricate high-performance Dy-free sintered Nd-Fe-B magnets by the magnetism regulation and structure optimization for grain boundaries.Efficient grain boundary engineering is carried out by controlling the phase constitution in antecedent alloys.A high coercivity of 20 kOe is obtained for Dy-free sintered Nd-Fe-B magnets with grain size of about 3 ?m.the major research and results are shown as following:Firstly,the key preparation methods of ultrafine grained Nd-Fe-B magnets and its microstructure evolution were investigated,Ultrafine jet-milled powders were obtained by taking advantage of the merit of refined initial structure after HDDR treatment,which possess strong uniaxial anisotropy in magnetocrystalline.The effect of RE content and sintering schedule on the densification behavior was analyzed systematically.RE content of above 31.5.wt%and sintering temperature below 950?are preferred to fabricate the ultrafine grained magnets,with the coercivity of 18.2 kOe.Comparing the domain structure and magnetic behavior in the ultrafine grained sintered magnets and conventional sintered magnets,proportion of grains with multi-domain state is less in ultrafine grained magnets,accompanying with large single-domain grains.The grains in single domain state require higher magnetic field to magnetize,leading to a two-step magnetization process in ultrafine grained magnets.However,masses of imperfect GB structure was detected in the ultrafine grained magnets by microstructure analysis,including thin GB phase with about 1.5 nm,lacking of GB or even part grain growth at some regions.Consequently,defects at the interface of matrix phase decrease the anisotropy field and lead to an in homogenous demagnetization process and deterioration of coercivity.Although the low coercivity at room temperature due to imperfect GB boundary in the ultrafine grained magnets,a better thermal stability can be achieved on account of the optimized microstructure factor Neff.In addition,the finer grain structure and thin GB channel effectively suppressed the Intergranular corrosion process.Then sintered Nd-Fe-B magnet with a composition design of lower B content and Ga-doping was carried out to study the law of the evolution of GB magnetism and its effect on coercivity.It was found that of optimized columnar grain structure can be obtained in SC alloys with low B content.Further doping Ga element,a good isolation of 2:14:1 grains by Nd-rich intergranular phase is obtained accompanied by the formation of intermetallic Nd6Fe13Ga phase without ferromagnetism.Detailedly,the evolution of composition segregation and phase constitution of Nd-rich phase in initial alloys and final magnets were investigated systematically by adjusting dehydrogenation temperature.When dehydrogenating above 500?,masses of Fe-rich GB phases were formed in the strip casting alloys and the homologous magnet suffered an unanticipated degradation of coercivity.When dehydrogenating above 500?,no new phases were formed with a relative uniform GB phase in initial alloys.Magnetic domain observation and thermal magnetic analysis imply a pronounced difference in the magnetism of GBs,which closely associated with the composition homogeneity of Nd-rich phases in antecedent alloys and element migration behavior in annealing process.The mechanism of the coercivity variation in the sintered magnets with different GBs magnetism is discussed based on magnetic behavior analysis.It was demonstrated that inhomogenous magnetism of Nd-rich phase in H620 possessed stronger exchange interaction and wider switching field distribution.In contrast,effective magnetic isolation and pinning effect from the feebly magnetic GB phase were realized in H320,leading to a remarkable coercivity enhancement(Br=13.15kGs,Hcj=19.78kOe,BHmax=42.47MGOe).Moreover,thermal stability analysis indicated that the smoothing effect of the exchange interaction at the ferromagnetic GB contributed to a decreased ?ex and optimized Neff simultaneously,which is responsible for the lower coercivity at room temperature and excellent thermal stability in H620 magnets.