| Due to excellent properties,sintered Nd2Fe14 B magnet was widely used in the new energy vehicles,home appliances,wind power generation,consumer electronics and other application.However,its coercivity deteriorated with the elevating temperature.Developing the magnet with high coercivity and mechanical properties had become significance recently.Grain Boundary Diffusion(GBD)was supposed to be one of the industrial and efficiently for preparing high performance magnets.On the basis of the Tb grain boundary diffusion magnet modified by low melting point Nd70Al30 and Nd70Cu30 alloy,the microstructure and Tb distribution,the formation of Tb-rich shell and coercivity mechanism had been investigated.The coercivity anisotropy of modified GBD-magnet was studied.The compound diffusion process has been developed and then prepared high coercivity permanent magnets with lower heavy rare earth.The mechanical properties,thermal stability and their anisotropy were systematically studied.First of all,the effects of Nd70Cu30 and Nd70Al30 modification on the magnetic properties,Tb element diffusion behavior and distribution,formation of Tb rich shell,and magnetic hardening mechanism of GBD-magnets were systematically studied.The coercivity is 1810 k A/m when the addition of Nd70Cu30 is 1% and that is 1855 k A/m when the amount of Nd70Al30 is 1%.The coercivity temperature coefficient β of GBD-magnet by Nd70Al30 modified improve from-0.722 %/K to-0.663 %/K in the temperature range 293 ~ 413 K,and from-0.722 %/K increase to-0.614 %/K by Nd70Cu30 modified.The grain boundary modification improves the coercivity and magnetic properties at high temperature.The Tb rich shells were formed on the surface of Nd2Fe14 B grains in unmodified,Nd70Cu30 and Nd70Al30 modified GBD-magnets.The Tb element of the unmodified GBD-magnet is mainly enriched near the surface with violent lattice diffusion.However,the diffusion coefficient of Nd rich phase is improved by Nd70Al30 and Nd70Cu30 modification,and the concentration of Tb in Nd rich phase is higher.TEM analysis shows that thicker Tb rich shell with content of 19.8% is formed in the Nd70Al30 modified GBD-magnet and has smooth and straight interface with the main phase,which reduce the defects of anti magnetic domain nucleation.The fcc-Nd O2 with better wettability is fabricated in the GBD-magnet modified by Nd70Cu30,which enhance the demagnetization coupling ability of Nd rich phase.Tb mainly diffuses along the grain boundary,and Tb rich shell adjoined the main phase grains is formed during cooling and solidification.The Mrev of GBD-magnet without modification has an obvious peak near the lower demagnetizing field H = 796 k A/m,The Mirr starts to increase at lower demagnetizing field H = 637 k A/m,the resistance to demagnetization is weakened.The magnetic hardening of the main phase and the enhancement of demagnetization coupling ability between the main grains are the main reasons for coercivity increasing.Secondly,the anisotropy of magnetic properties,microstructure and coercivity mechanism of the Nd70Cu30 and Nd70Al30 modified GBD-magnet were revealed with parallel and perpendicular to the c-axis diffusion.Coercivity of with parallel and perpendicular to the c-axis diffusion reaches about 1800 k A/m when the Nd70Al30 addition reaches 1%.In the range of 293 ~ 413 K,the temperature coefficient of the coercivity of the modified GBD-permanent magnet increases from-0.60%/K to-0.496%/K wiht parallel to c-axis diffusion.The temperature coefficient of coercivity of modified GBD-magnet increases from-0.61%/K to-0.497%/K wiht perpendicular to c-axis diffusion,the anisotropy of coercivity and its temperature coefficient almost disappears.Compared to the GBD-magnet without modification,the concentration of Tb is higher,close to 40% in the Nd rich phase,which promotes its deeply diffusion.Continuous and uniformly Tb rich shell is formed in the magnet.However,the concentration of Fe of the GBD-magnet is higher without modification,and the ability of demagnetization coupling become weak.TEM observation shows that the interface of Nd rich phase between main phase grains is straight,thin and continuous.After Simulation calculation of coercivity mechanism,Microstructure factor α enhances from 0.42 to 0.48,The demagnetization factor Neff decreases from 1.71 to 1.67,the coercivity of GBD-magnets along the vertical c-axis increases significantly attribute to the optimization of microstructure factor.On account of the coercivity improving of modified magnet with perpendicular to the c-axis diffusion,the coercivity of the GBD-magnet is further improved by the compound diffusion process.Finally,the effects of modification on the mechanical properties,irreversible loss and their anisotropy of GBD-magnets are revealed.Vickers hardness is decreased of the modified GBDmagnet,The brittleness of permanent magnet is reduced,which is improving the qualification rate during machining process.The compressive strength increased from 811.9 MPa to 1032.3 MPa of the modified GBD-magnet with parallel to the c-axis diffusion,and that increased from 702.4 MPa to 925.3 MPa along perpendicular to the c-axis diffusion.There are radial tearing traces on the main phase grains since transgranular fracture occurs,the energy of consumption for crack propagation increases,and the strength of the diffusion magnet improves.The irreversible loss is only 1.4% when the modified magnet parallel to the c-axis diffusion,while without modification is 26.5%.The irreversible loss is only 2.2% with the perpendicular to the c-axis diffusion,while without modification is 30.4%,Magnetic stability at high temperature of modified GBD-magnet is improved.To conclude,this paper makes a deeply and systematicly study on the preparation,microstructure,properties,element distribution and coercivity mechanism and comprehensive properties of the modified GBD-magnet which has important academic value and practical significance for sintered Nd2Fe14 B permanent magnets with high coercivity and fine mechanical properties. |
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