| Nd-Fe-B sintered magnet is currently the most widely used permanent magnetic material because of its strong magnetic properties.It is widely used in transportation,information,national defense,energy and other important fields.The experimental value of Br has reached about 97% of the theoretical value,and(BH)max has also reached about 93% of the theoretical value.However,the experimental value of Hcj is still far from its theoretical value,and currently it can only reach about 1/3 of the theoretical value.The diffusion of heavy rare earth elements is often used to improve the coercivity.However,given the scarcity and high cost of heavy rare earths,this method will lead to an unbalanced utilization of rare earth resources and increase the cost of magnets.Therefore,how to increase the use of abundant rare earth elements La/Ce or reduce the use of heavy rare earth elements Dy/Tb to effectively improve Hcj by grain boundary diffusion remains an important subject of strategic significance.In view of this problem,three representative sintered magnets,LaCe40,N48 M without heavy rare earth,and N50 HC with heavy rare earth Tb,are selected as base magnets,low-melting-point Pr80Al20 and Pr40Dy40Al20 alloy powders are used as diffusion sources.The main results are as follows:Through Pr80Al20 and Pr40Dy40Al20 diffusion,coercivity approaching 15 kOe is acquired for LaCe40 sintered magnets(La-Ce/TRE = 40 wt.%,TRE: total rare earth).However,anomalies including similar coercivity increment level together with drastic remanence decrement from 12.6 to 12.2 kG by PrDyAl diffusion are beyond expectations.Microstructural analysis reveals thick Dy/Fe-rich surface layer,and below that Dy/Al infiltration into the interior of 2:14:1 grain,both of which consume large amounts of expensive Dy and are detrimental to the remanence.Above Dy agglomeration can be well interpreted by the negative formation enthalpy between Dy-Fe,as evidenced by the calculation based on Miedema's model.Consequently,the positive role of Dy in strengthening the local shell region of 2:14:1 phase and enhancing the coercivity is weakened,while its negative role in deteriorating the remanence is exacerbated.Comparably,for PrAl diffusion,Pr infiltration to form magnetically hardening shell can well compensate the magnetic dilution of Al in 2:14:1 lattice,paving PrAl diffusion as a versatile route for significant coercivity enhancement of Nd-La-Ce-Fe-B without sacrificing remanence.The cost performance,[Hcj+(BH)max]/cost,has been raised by 21.6% after PrAl diffusion,compared to the LaCe-0 commercial grade sintered magnet,further demonstrating the high prospect of low-cost Nd-La-Ce-Fe-B for practical application.For N48 M sintered magnets without heavy rare earth and N50 HC sintered magnets with heavy rare earth element Tb,Hcj of PrDyAl diffusion magnet is better than that of PrAl diffusion magnet,but Br is lower than that of PrAl diffusion magnets.For N48 M and N50 HC PrAl GBDP magnets,the increase of Hcj mainly comes from the formation of continuous grain boundary phase,and the large amount of Al element entering the main phase,which causes the magnetic dilution effect to lead to a significant reduction in Br.For N48 M and N50 HC PrDyAl GBDP magnets,the increase in Hcj mainly comes from two aspects.The first is the formation of a continuous grain boundary phase,and the second is the generation of(Nd,Dy)2Fe14B phase with higher magnetic crystal anisotropy field at the grain edge of the main phase,which leads to a further increase in Hcj and is significantly higher than that of PrAl diffusion magnets.However,the saturation magnetic polarization of the(Nd,Dy)2Fe14B phase is also smaller and there are REFe phase enrichments on the surface of the PrDyAl magnet,which makes Br of the PrDyAl diffusion magnet lower than that of the PrAl diffusion magnet.Therefore,for N48 M and N50 HC magnets,the use of a PrDyAl diffusion source containing heavy rare earth is more helpful for improving the coercivity. |
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