Chemical Composition,Element Distribution And Magnetic Properties Of Sintered Nd-Fe-B/Tb-Fe-B Magnets

Since Nd-Fe-B permanent magnet was invented in 1983,it has been widely used in various fields due to its high remanence and high magnetic energy product.Although the remanence and maximum magnetic energy of the magnet are close to the theoretical values,the coercivity is less than one third of the theoretical value.Improving the coercivity of Nd-Fe-B permanent magnet can extend its application and enhance its value.Tb-Fe-B has ultra-high anisotropic magnetic crystal field and high coercivity.Tb element is directly added into Nd-Fe-B prepare magnets by traditional method,but the increase of the coercivity is limited in conventional?Nd,Tb?-Fe-B.Nd-Fe-B and Tb-Fe-B powders are mixed to prepare the sintered magnets,and the coercivity increases more largely in the dual-main-phase magnets.This paper aims to investigate the relationship of between the element distribution,microstructure and magnetic properties of dual-main-phase magnets by adjusting the composition of Nd-Fe-B and Tb-Fe-B alloys,which can provide theoretical basis and experimental basis for further improving the magnetic properties of magnets.Nb element was added to prepare Nd_12.2Fe_81.8-xNb_xB₆?x=0,0.1,0.3,0.5,0.7?sintered magnets.The coercivity of Nd_12.2Fe_81.8B₆ sintered magnet is 7.20 kOe without Nb element addition.For the addition of 0.3 at.%Nb,the coercivity of the magnet is 9.17 kOe,which increases by about 2 kOe.With more amount addition of Nb element,the coercivity doesn't increase obviously,and the remanence and magnetic energy product decrease largely.According to the distribution of magnetic elements,Nb element is mainly concentrated in the intergranular phase,which may optimize the structure at the grain boundary of the main phase and improve the coercivity of the magnet.However,the magnetic properties of the magnet would decrease if more amount of Nb is added.Nd_12.2Fe_81.8-xNb_xB₆ and Tb₁₇Fe₇₅B₈are mixed to prepare dual-main-phase sintered magnet,and Tb content is 2 wt.%of the total mass of the magnet.The coercivity of the magnet is 15.89 kOe without Nb addition.For Nb addition of 0.3at.%,the coercivity increase is 3.5 kOe in the dual-main-phase magnet,and the remanence and magnetic energy product don't basically decrease.For Nb addition of 0.5 at.%,the magnetic energy product decreases from 37.76 MGOe to 34.89 MGOe in the sintered magnet.Increasing continuously Nb element the magnetic properties of the sintered dual-main-phase magnet become worse.The effects of Nb on the magnetic properties of traditional Nd-Fe-B magnets and Nd-Fe-B/Tb-Fe-B dual-main-phase magnets are basically the same.The coercivity is improved with an addition of 0.3 at.%Nb element,and the remanence and magnetic energy product are nearly unchanged in the magnets.The distribution of Tb element affects the magnetic properties of Nd-Fe-Nb-B/Tb-Fe-B dual-main-phase sintered magnets.Sintered magnets?Tb content is 2 wt.%of the total mass of magnets?were prepared by mixing Nd_12.2Fe_81.5Nb_0.3B₆ with Tb_13+xFe_79-xB₈?x=0,2,4,6,8,10?,and the Tb content of Tb-Fe-B alloy was adjusted to optimize the magnetic properties.The coercivity is 14.37 kOe in Nd_12.2Fe_81.5Nb_0.3B₆/Tb₁₃Fe₇₉B₈ magnet.Increasing Tb atomic percent from 13 at.%to 17 at.%,the coercivity is 19.46 kOe in Nd_12.2Fe_81.5Nb_0.3B₆/Tb₁₇Fe₇₅B₈,and the increase of coercivity is 5.09 kOe.Increasing more largely the content of Tb in Tb-Fe-B alloy,the coercivity doesn't basically increase,and both the remanence and magnetic energy product decrease obviously.The magnetic energy product of the magnet decreases from 37.76 GMOe to 33.27 MGOe in Nd_12.2Fe_81.5Nb_0.3B₆/Tb₂₃Fe₆₉B₈.According to distribution of magnet elements,increasing the Tb content in Tb-Fe-B alloy,excess Tb atoms in Tb-Fe-B enter the intergranular phase.Some of Tb atoms enter the Nd-Fe-B main phase,forming?Nd,Tb?₂Fe₁₄B shell at the Nd-Fe-B main phase boundary,which enhances the magnetocrystalline anisotropy of grain boundary,improves the nucleation field of the dual-main-phase magnet in magnetization reversal and enhances the coercivity.However,for high content Tb in Tb-Fe-B alloy,the rare earth content would increase in the magnet,resulting in the decrease of remanence and magnetic energy product.To further investigate the effect of Tb-Fe-B addition on the coercivity of magnets,2.6wt.%,5.2 wt.%,7.8 wt.%,13 wt.%,18.2 wt.%Tb₁₇Fe₇₅B₈ are mixed with Nd_12.2Fe_81.5Nb_0.3B₆powders to prepare the sintered magnets.For the addition of 2.6 wt.%Tb₁₇Fe₇₅B_8,and the coercivity of the magnet increases from 9.17 kOe to 15.49 kOe.For the addition of 5.2 wt.%Tb₁₇Fe₇₅B_8,the coercivity increases to 19.46 kOe in the dual-main phase magnets.The coerciveity of dual-main phase magnets increases almost linearly with the increases of addition amount of Tb-Fe-B.Compared with that in the sintered magnets prepared by traditional method,the coercivity increases more greatly in dual-main-phase magnet.According to the investigation of elemental distribution,there exists Tb₂Fe₁₄B main phase besides?Nd,Tb?₂Fe₁₄B grain boundary phase.With the increase of Tb-Fe-B addition amount,the amount of Tb-Fe-B phase increases in the magnets.Tb-Fe-B phase could prevent from magnetization revesal in Nd-Fe-B phase,and so the increase of coercivity is more significant in the dual-main phase magnet.