Study On Grain Boundary Diffusion Of Low-melting Alloys In Sintered Nd-Fe-B And Nd-Ce-Fe-B Magnets

Sintered Nd-Fe-B magnets are widely used in various fields of modern society due to their excellent magnetic properties,and have become indispensable basic materials in human life.With the development of modern technology and information industry,higher requirements have been placed on the magnetic properties of sintered Nd-Fe-B magnets.At the same time,how to balance the utilization of rare earth（RE）resources is also one of the current research hotspots.Although the addition of heavy rare earth（HRE）elements is an effective method to improve the coercivity of sintered Nd-Fe-B magnets,it will increase the cost of the magnet and accelerate the consumption of HREs.Therefore,it is of great significance to develop high-performance sintered Nd-Fe-B magnets that do not contain HREs and sintered Nd-Ce-Fe-B magnets that incorporate light REs while taking into account the magnetic properties.In this thesis,the grain boundary diffusion process was performed in sintered Nd-Fe-B magnets by using HRE-free alloys with the composition of Pr_81.5Ga_19.5（PG）and Pr_81.5Ga_14.5Cu₅（PGC5）（at.%）.We also tried the grain boundary diffusion in multi-main-phase sintered Nd-Ce-Fe-B magnets with 26 wt.%Ce using the above PG and PGC5 alloys.After optimizing the heat treatment process,the effects of grain boundary diffusion of the above alloys on the microstructure,magnetic properties and corrosion resistance of sintered Nd-Fe-B magnets and multi-main-phase Nd-Ce-Fe-B sintered magnets were studied.The main results are as follows.Under the heat treatment process of 800°C-4 h+450/550°C-2 h,the coercivities of sintered Nd-Fe-B magnets after grain boundary diffusing Pr-Ga and Pr-Ga-Cu alloys were increased from original 1090 k A/m to 1494 and 1505 k A/m,respectively.The remanence of PG diffused magnet decreased from 1.26 T to 1.15 T,and the remanence of PGC5 diffused magnet did not change.In the temperature range of 300～400 K,the temperature coefficient of coercivity（β）was improved from-0.660%/K for the original magnet to-0.638%/K for the PG diffused magnet.The temperature coefficient of remanence（α）changed from-0.113%/K to-0.115%/K.Microstructure observation revealed that continuous grain boundaries were formed in the near surface and central area of the two diffused magnets,but the continuous grain boundary morphology near the surface of PG diffused magnet was more continuous and wider than that of PGC5 diffused magnet.Element mapping and line scan results showed that Pr atoms substituted the Nd atoms on the surface of the 2:14:1 grains to form a Pr-rich shell.Due to different annealing temperatures,the distribution of Ga on the continuous grain boundaries of these two diffusion magnets was not exactly the same.In the PG diffused magnet,Ga atoms not only existed in the grain boundary,but also substituted the Fe atoms on the surface of the2:14:1 grains to form Nd₂（Fe,Ga）₁₄B,which reduced the remanence.In the PGC5 diffused magnet,Ga and Cu distributed at the grain boundary could not only improve the wettability of grain boundary phase but also form a non-ferromagnetic grain boundary phase.The coercivity enhancement of PG diffused magnet was attributed to the formation of rich continuous grain boundaries and Pr-rich shell;while the coercivity enhancement of PGC5 diffused magnet was attributed to the formation of non-ferromagnetic grain boundary phase and Pr-rich shell.The distribution of Ga and Cu at the grain boundary increased the electrode potential of the grain boundary phase,thereby improving the corrosion resistance of the sintered magnets.Under the heat treatment process of 800°C-5 h+450°C-3 h,the coercivities of multi-main-phase sintered Nd-Ce-Fe-B magnets after grain boundary diffusing PG and PGC5alloys were increased from original 928 k A/m to 1036 and 1171 k A/m,respectively.The remanences did not change.In the temperature range of 300～400 K,the temperature coefficient of coercivity（β）was improved from-0.666%/K for the original magnet to-0.632%/K for the PGC5 diffused magnet.The temperature coefficients of remanence（α）were calculated to be-0.136%/K and-0.144%/K,respectively.Microstructure observation revealed that there were two kinds of continuous grain boundaries with different contrasts in the PG and PGC5 diffused magnets.The line scan and quantitative analysis results attested that these two types of continuous grain boundaries with different contrast originated from the REFe₂ phase and the Nd-rich phase,respectively.Based on SEM and element mapping characterization,it was found that Ga atoms were heterogeneously distributed in the intergranular phases.Most of the Ga atoms combined with a large amount of Pr atoms to form a non-ferromagnetic intergranular phase.The remaining Ga atoms not only diffused into the Nd-rich phase but also entered the REFe₂ phase to replace part of Fe atoms to lower the melting point of the REFe₂ phase and improve the wettability to form continuous grain boundaries.Cu atoms further lowered the melting point of the Nd-rich phase,but hardly entered the REFe₂ phase.Compared with the uniform distribution of Cu in the intergranular phase,due to the uneven distribution of Ga,intergranular phases with different wetting ability could not form uniform continuous grain boundaries around each2:14:1 grain,thus the coercivity enhancement effect was not as good as that of the Pr-Cu diffused magnet.In addition,heat treatment would lead to the homogenization of Ce in the2:14:1 grains of the original magnet,destroying the long-range magnetostatic interaction and thus counteracting partially the enhanced coercivity.The corrosion resistance of the magnets was also improved after the grain boundary diffusion process due to the fact that the Ga and Cu elements have a more positive electrode potential at the grain boundary.