Study On Low Temperature Ageing Processing,Microstructures And Properties Of The Dysprosium Doped Sintered Nd-Fe-B Permanent Magnetic Alloy

The dysprosium doped sintered Nd-Fe-B permanent magnetic alloys had outstanding magnetic properties,higher Curie temperature,and better corrosion resistance.With the development of some industry fields,such as wind power,hybrid vehicle(HVs)and electric vehicles(EVs),the demand for the Dy-doped Nd-Fe-B permanent magnetic alloys with very high intrinsic coercivity increased dramatically.However,because the heavy rare-earth element Dy is very expensive and its resources has been decreasing,the development of the Dy-doped sintered Nd-Fe-B permanent magnetic alloys with low cost and high performance will be the one of the main development direction in future.At present,the two-stage ageing process consisting of high temperature ageing and low temperature one was generally adopted for the Dy-doped Nd-Fe-B permanent magnetic alloys at home and abroad.In this dissertation,the ageing process for the N35TH model of the Dy-doped sintered Nd-Fe-B permanent magnetic alloy produced by Yantai shougang magnetic materials co.,LTD was systematically optimized.The optimal two-stage ageing processing was determined at 900? for 2h in the first high temperature ageing stage and then at 470 ? for 2h in the second low temperature one.But it was found in the ageing process optimization that the Dy-doped sintered Nd-Fe-B magnets only aged at 470 ? for 2h had the similar magnetic properties with that aged by optimal two-stage ageing process.And the low temperature ageing process was stable and had some characteristics,such as short production cycle,low energy consumption,high production efficiency and significant reduction of the production cost,so it could be used for the Dy-doped Nd-Fe-B permanent magnetic alloy and had important popularization and application value.The microstructure,composition and fracture morphology of the Dy-doped sintered Nd-Fe-B permanent magnetic alloy treated by the low temperature ageing process and the optimal two-stage one were studied by optical microscope(OM),scanning electron microscope(SEM),X-ray diffraction(XRD),atomic force microscope(AFM),magnetic force microscope(MFM),electron backscattered diffraction(EBSD)and transmission electron microscope(TEM),respectively.The metallographic microstructures of the optimal two-stage aged and the low temperature aged Dy-doped Nd-Fe-B magnets indicated that the former happened recrystallization and had fine grains and more grain boundaries,while the latter had more sediment on the surface of main crystal phase Nd2Fe14B and larger surface roughness.The investigations carried out with back scattering scanning electron microscope indicated that there was the thin layer of rich rare earth phase existed along the main crystal phase Nd2Fe14B grain boundaries for two kind aged magnets and there was the mesh structure of the rich rare earth phase in the multiple main crystal phase Nd2Fe14B grains.Through the composition analysis of the triple junction,the main crystal phase Nd2Fe14B grain boundary layer and the core of the main crystal phase,it was found that Dy atoms in the two kind aged magnets distributed not only in the main crystal phase Nd2Fe,4B grain,but also in the grain boundary,and some magnetic hardening layers formed at the grain boundary.The morphologies between the low-temperature aged magnet and the optimal two-stage aged one were observed by the secondary electron SEM and compared.It showed that there were two kinds of different size particles existing on the main crystal phase Nd2Fe14B grain of the low temperature aged magnet,one kind of the particle size was about 100 nm,another kind of the tiny particle size was about 1 nm.Because of the uneven stress distribution and local energy fluctuation caused by the uneven microstructure in the low temperature aged Dy-doped Nd-Fe-B magnet,Dy atoms could effectively diffuse in the main crystal phase Nd2Fe14B and the grain boundary phases during low-temperature ageing process,which resulted in the reasonable microstructure and high intrinsic coercivity.Based on EBSD test and analysis,it showed that the main crystalline phase Nd2Fe14B grains had the<001>orientation texture characteristics.The<001>orientation grain percentage in the low-temperature aged magnet was similar to that in the optimal two-stage aged one,so the texture features of the two kind aged magnets were basically same.It showed that the magnetic domains of the Dy-doped sintered Nd-Fe-B were consisted of a large number of spike magnetic domains and corrugated ones according to the AFM and MFM observations.There was a large amount of small particles in the two kind aged magnets by AFM observation,while corrugated magnetic domain was relatively more obvious through MFM observation because it was a characte of the magnetic domain structure which can reduce the demagnetization on the magnet surface.It showed that the strip precipitated phase structure also existed in the grain boundary of the two kind aged magnets by using TEM observations.It could be explained that to improve the intrinsic coercivity the thin layer of the grain boundary phase in the magnet could be formed during the low temperature ageing process.The comparative analysis and comprehensive research of the magnetic performance,mechanical properties and corrosion resistance of the Dy-doped sintered Nd-Fe-B magnets which were respectively treated by the low-temperature ageing process and by the optimal two-stage ageing process had been carried out.The results revealed that the intrinsic coercivity,Vickers hardnes and compressive strength of the two aged magnets were basically the same,but the squareness(Hk/Hg),the thermal magnetic performances and the bending strength of the former were better than that of the latter.Vickers hardness indentation combined with acoustic emission technique was employed to test the brittleness of the Dy-doped sintered Nd-Fe-B permanent magnetic alloy.The results indicated that the brittlenesses of the two kind aged magnets were higher than that of the as-sintered magnet.But compared with the the optimal two-stage aged magnet,the brittleness of low temperature aged magnet was lower.Electrochemical corrosion experiment results showed that the corrosion resistance of the low temperature aged magnet was better than that of the optimal two-stage aged one in 3.5%NaCl solution.The experiment of accelerating corrosion weightlessness under high temperature and high pressure results showed that the corrosion resistance of the low temperature aged magnet was obviously better than that of the optimal two-stage aged one.