Study Of Grain Boundary Diffusion Processed Nd-Fe-B Sintered Magnet Using Low Melting-point Alloys

High-coercivity Nd-Fe-B sintered magnet has been widely used in many functional devices.A large amount of heavy rare earth elements Dy/Tb were required to prepare high-coercivity Nd-Fe-B magnet by conventional methods,which significantly reduced the remanence Br and maximum energy product(BH)max.Recently,it has been confirmed that coercivity of Nd-Fe-B magnet can be greatly enhanced by grain boundary diffusion process(GBDP)with little reduction of Br and(BH)max,which has attracted extensive attention.In this study,low-melting-point rare earth alloys were designed as diffusion source using Tb,Pr,Cu and Al elements,and the researches were mainly focused on:high-temperature wetting behavior,interfacial metallurgy reaction,diffusion behavior and boundary structure,Tb-rich shell and magnetic hardening,coercivity strengthening mechanism,etc..Low heavy rare earth high-coercivity Nd-Fe-B magnets were obtained.A series of low-melting-point rare earth alloys with atomic compositions of Tb70Cu30,Pr52 5Tb17.5Cu30,Pr60Tb20Al20 and Pr60Tb10Cui5Ali5 were designed as diffusion source,and N50 grade Nd-Fe-B magnet thick in 5 mm was used as original magnet.After grain boundary diffusion,the coercivities of magnets increased to 20.4 kOe,21.9 kOe,21.9 kOe and 23.2 kOe,respectively,from initial 11.9 kOe.Microstructural analyses indicated the formation of Tb-rich core-shell structure on the surface of Nd2Fe14B grains for all diffused magnets.Especially,the PTCA magnet showed deeper diffusion length and more uniform thin Tb-rich core-shell.Meanwhile,continuous thin Ia3-Nd2O3 grain boundary phase and smooth interfaces were observed.The coercivity enhancement of the diffused magnet was attributed to Tb-rich shell magnetic hardening and grain boundary phase exchange-decoupling strengthening.In order to better understand diffusion behavior of low melting-point alloys,the wettability of the alloys to N50 substrates at 900? was investigated.The results showed that wettability of Tb70Cu30 alloy ingot on Nd-Fe-B substrate was the lowest.Compared with Pr52.5Tb17.5Cu30 alloy,wetting and spreading abilities of Pr60Tb20Al20 and Pr60Tbi0Cu15Al15 alloys with Al element were obviously enhanced,which was beneficial for rapid and efficient diffusion.The higher wettability of Pr60Tb10Cu15Al15 alloy caused uniform microstructure along depth and high coercivity.The interfacial metallurgical reactions between the alloy ribbons and N50 magnet at different temperatures were researched.It revealed that grain boundary diffusion process of Tb element in Pr52.5Tb17.5Cu30 alloy occurred at 700 0C.and about 30?m thick near-Tb2Fe14B diffusion layer was formed at the interface with increasing the temperature to 800?,and a layer of abnormal near-Tb2Fe14B grains was formed on the diffusion layer with increasing the temperature further to 900?.This kind of Tb enrichment in the surface would suppress grain boundary diffusion of Tb element to inner of the magnet.In comparison,a transition layer of Tb(Fe,Al)2 compound formed at the interface of Pr60Tb20Al20 alloy and N50 magnet at 700?,which was further thickened at 800?.With increasing the temperature to 900?,the Tb(Fe,Al)2 layer was partially melted and grain boundary diffusion process of Tb element was promoted.For Pr60Tb10Cui5Ali5 alloy,Tb element obviously diffused along grain boundaries into the inside of magnet at 700?.A layer of?3?m thick near-Tb2Fe14B grains formed at the interface with increasing the temperature to 800? and 900?.Based on these results,a two-step grain boundary diffusion process was carried out,which consisted in low-temperature grain boundary diffusion along grain boundaries into the inside of Nd-Fe-B magnet,following subsequent high-temperature aging processes to develop continuous and uniform Tb-rich shell structure.The coercivity increment of 13 kOe was obtained.The results also indicated that diffusion behavior of low melting-point alloys parallel to magnetic alignment direction of Nd-Fe-B magnet was quite different from vertical direction.The grain boundary diffusion along parallel direction was much more sufficient,which resulted in continuous and uniform distribution of Tb-rich shells.Further study of local elemental distribution at boundaries showed that Tb distribution in intergranular regions was quite different from triple junction regions in the diffused magnet by Pr60Tb10Cui5Al15 alloy.The Tb distribution in intergranular Tb-rich shell was relatively uniform,which was inferred to interfacial metallurgical reaction.However,the Tb distribution of Tb-rich shell near triple junction regions exhibited obvious gradient,which was formed by solid state diffusion.