Study On Microstructure And Properties Of Sintered NdFeB Magnets With Strong Exchange-decoupling Boundary Phases

Sintered Nd-Fe-B rare earth permanent magnets(REPM)with brilliant properties are widely used in the new energy field such as hybrid vehicles and wind generators.The intrinsic coercivity(Hcj)which represents the anti-magnetic ability was an important index.To enhance Hcj,the heavy rare earth elements(HREs)such as Tb and Dy were introduced to increase the magnetic anisotropy of main phases.However,there is a restriction due to the high production postsof HREs.In this work,continuous and strong exchange-decoupling boundary phases were built by the light rare-earth element(LRE)based alloys.High coercivity sintered magnets with non HREs were obtained.The low-melting-point eutectic Pr-Cu alloy was chosen as the minor alloy for the replacement of traditional RE-Fe boundary phases.Meanwhile,the nearly(Nd,Pr)2Fe14B alloy was chosen as major alloy.New series of hydrogen decrepitation,powder making and sintering processes were obtained by investigating the hydrogen absorption and dehydrogenate behaviors of minor alloy.The relationship among the Hcj,microstructure and annealing behaviors were systematically studied.The best annealing temperature(480℃)for Pr-Cu processed magnets was slightly higher than the lowest eutectic temperature(476℃).The discussion for the annealing mechanism showed the dominant annealing mechanism was the formation of strong exchange-decoupling grain boundary phases(GBPs)which were originated from the eutectic liquid segregation.This liquid segregation showed no influence on the structure transition of boundary phases but changed their distribution patterns.The repeated heating followed by quenching/furnace cooling for optimal annealed magnet demonstrated reversible eutectic liquid segregation.Annealing under the lowest eutectic temperature(460℃)was harmful which indicated that the atoms solid diffusion mechanism was not accessible.Meanwhile,Pr-Cu grain boundary reconstruction can effectively enhance the bending strength.Due to the directional liquid segregation,we finally obtained a kind of abnormal magnets with higher bending strength perpendicular(404 MPa)to c-axis than parallel direction(345 MPa).The grain boundary diffusion process(GBDP)was applied to the 38M commercial sintered Nd-Fe-B magnets(5mm)using Pr-Cu eutectic ribbons as direct diffusion source.The Hcj increased from 14.78 to 20.35 kOe,which was superior to the Dy-Cu diffused sample(17.19 kOe).Different from the volume diffusion mechanism of Dy-Cu alloy,the grain boundary diffusion mechanism is dominant for Pr-Cu alloy.For Dy-Cu sample,the magnetic hardening structure was formed in the high-temperature diffusion process and the diffusion depth is about 500μm.For Pr-Cu sample,the strong exchange-decoupling boundary phases were formed in the low-temperature annealing process and the diffusion depth is about 4mm.The APT results exhibited larger thickness(3→11nm)and lower Fe atom concentration(65→40 at.%)during the generation of strong exchange-decoupling boundary phases.The Pr-Cu diffusion was deeply affected by gravity which brought low thickness in lower surface.A further flip treatment was conducted and we finally obtain a kind of none-HRE sintered magnet(the average grain size is 6.87μm)with total beneficial microstructure and 21.95 kOe coercivity.The mutual interactions between Pr-Cu/Pr-Al alloys and original boundary phases in the magnets with various RE and A1 contents were investigated.Different from Pr-Cu alloy,Pr-Al alloy was universal for coercivity enhancement and Al atom which showed a negative mixture enthalpy with matrix phase played an important role on the liquid segregation into GBPs.For Pr-Cu samples,the Hcj enhancements were mainly found in the low-temperature annealing process and the Pr-Al samples were in the high-temperature diffusion process.The key factor of the generation of strong exchange-decoupling boundary phases was related to one kind of RE-Fe-Al(Al=4～6 at.%)junction area,which exhibited superior wetting ability with matrix grains.The diffusion depth of 29 and 31.5 wt.%RE magnets are similar(<1.5 mm).The original thin and continuous GBPs in high-Al(Al=0.74 wt.%)magnet may acted as the channels of the 4 mm-depth liquid diffusion.The difference of Pr70Cu30 and Pr80Al20(strip casts,SC)hydrogen behaviors were studied.The results showed that Pr70Cu30 SC had stronger hydrogen absorption capacity and ternary dehydrogenation behavior were identified.The Pr80Al20 SC had surface passivation layer and showed weaker hydrogen absorption capacity.The dehydrogenation behavior exhibited single property.The results of GBDP showed that Pr-Cu ternary HD powder were oxide and hardly melt.Pr-Al single HD powder dehydrogenated and melted at diffusion temperature,which finally realized the "liquid to liquid" GBDP.