Grain Boundary Diffusion Behavior And Magnetic Hardening Mechanism Of Sintered Neodymium-cerium-iron-boron Magnets

NdFeB magnet has been attracted the attention of scholars due to its high magnetic properties.Now it has been widely used in the key components of high-tech fields.But the excessive use of rare earth resources such as Nd and Pr restricts the development of NdFeB magnets.Therefore,the dual-main-phase Ce-containing magnet has gradually become the development focus of permanent magnetic materials.However,its defects of low coercivity and poor corrosion resistance severely limit the application of Ce-containing magnets.Therefore,the method regulating microstructure and properties of magnet by grain boundary modification has been explored in this thesis.This paper strives to make new breakthroughs in the basic theory and process control of dual main phase magnets such as interface control and magnetic mechanism,providing important theoretical guidance for the improvement of comprehensive properties and application of dual-main-phase Nd-Ce-Fe-B magnet.This paper focuses on the investigation of grain boundary modification of dual-phase Nd-Ce-Fe-B magnets.According to the thermodynamic theory calculation,the reaction rule of grain boundary modification of dual-main-phase Nd-Ce-Fe-B magnets was explored.Meanwhile,the modification rules of microstructure and properties of the dual-main-phase Nd-Ce-Fe-B magnets via intergranular addition of PrCu alloy and DyF₃ and Cu powders were systematically investigated.The variation rules of microstructure and composition of the magnets were revealed,and the enhancement mechanisms of the magnet properties were illustrated.The intergranular addition of PrCu alloy powders can effectively improve the microstructure of the dual-phase Nd-Ce-Fe-B magnet and enhance the magnetic properties and corrosion resistance of the magnet.Thermodynamic calculation results show that the substitution of Pr for Nd or Ce reduces the formation enthalpy of the main phase,increasing the formation ability of（Nd,Pr）₂Fe14B or（Ce,Pr）₂Fe14B phase.However,Cu substitution reduces the formation ability of Nd₂（Fe,Cu）₁₄B and Ce₂（Fe,Cu）₁₄B phase.This indicates that Pr tends to diffuse into the main phase,while Cu is mainly present in the grain boundary phase.The addition of PrCu alloy increases the intrinsic coercivity of magnet only with slight reduction of remanence.When the addition amount of PrCu alloy is 2 wt%,the coercivity of magnet reaches1130 kA/m,improved by 11.1%compared to that of the magnet without PrCu alloy.While the remanence is 0.696 T,which only reduced by 1.99%.This is because Pr replaces Nd or Ce in the main phase grains to form（Nd,Pr）₂Fe₁₄B or（Ce,Pr）₂Fe₁₄B phase with a high anisotropy field,increasing the nucleation field,and the RE-rich phases is increased,thereby weakening the magnetic coupling between adjacent main phase grains.In addition,the absolute value of the reversible temperature coefficient of the coercivity is reduced by the addition of PrCu alloy,and the thermal stability of magnet is improved.Meanwhile,the corrosion potential of magnet is increased and the corrosion current is decreased,and thus the corrosion resistance of magnet is improved.The intergranular addition of DyF₃ and Cu powders improves the properties and microstructure of the dual-main-phase Nd-Ce-Fe-B magnet.On the basis of the binary alloy phase diagram analysis,the mixing ratio of DyF₃ and Cu powders was designed.It was found that the intergranular addition of DyF₃ and Cu powders significantly increases the intrinsic coercivity H_cj of the magnet.When the addition amount is 2wt%,the H_cj of magnet attains976.7kA/m,improved by 63.33%in comparison with 598kA/m of magnet without addition.The remanence B_r of magnet decreases slightly,which is reduced by 4.53%compared to the magnet without addition.However,the maximum magnetic energy of magnet is increased by 2.66%.The Dy element introduced by grain boundary modification mainly exists in the main phase flakes,which is consistent with the thermodynamic calculation results.This indicates that Dy easily diffuses into the flakes to replace Nd or Ce to form（Nd,Dy）₂Fe₁₄B or（Ce,Dy）₂Fe₁₄B phase,enhancing the anisotropy field of main phase grains and then suppressing the magnetic reverse domain nucleation,therey increasing H_cj.At the same time,the increase of RE-rich phase reduces the magnetic exchange coupling between adjacent grains and also contributes to the improvement of H_cj.However,the increase in nonmagnetic phase and the ferrimagnetic coupling between Dy and Fe are the main reasons for the decrease in remanence.