Research Of Composition,Structure And Magnetic Hardening Mechanism Of 2:17 Type SmCo Permanent Magnets With High Magnetic Properties

As the second generation of rare-earth permanent magnet,2:17-type SmCo permanent magnets have excellent magnetic properties,high Curie temperature,good thermal stability,strong corrosion resistance and oxidation resistance.It can be used in many fields,especially for the high temperature applications,such as aviation,aerospace and military,due to its maximum operature temperature of 550?.Recently,2:17-type SmCo permanent magnets draw new attentions.The present work had been done to address the problems about the complex preparation process of 2:17-type SmCo permanent magnets with high Fe content.The magnets were prepared by powder metallurgic method,correlationship among composition,structure,and magnetic properties induced by changing of element content and modification method were investigated,influence of element distribution on magnetic properties and related mechanim were observed,and doping technology was performed to modify the microstructure of grain boundaries in Sm?CoCuFeZr?_z magnets.We analyzed the problem with a new perspective and enhanced the magnetic properties with new method.Six alloys with different nominal compositions of Sm(Co_balFe_xCu_0.06Zr_0.025)_z magnets?where x=0.20,0.25,0.28,and z=7,9.5,respectively?were prepared and divided into three groups by Fe content(Such as Fe_0.20,Fe_0.25 and Fe_0.28).The correlationship of Fe content and z value with the coercivity was established to obtain the high coercivity.In detail,with the increasing of Fe content,corresponding z value of magnet should be reduced simultaneously to obtain the coercivity as high as possible.Combined with the magnetic properties and microstructure evolution,based on the composition of Sm(Co_0.715Fe_0.20Cu_0.06Zr_0.025)_7.86 magnet(H_cj=30.38 kOe),it is found that the cellular phase grew up with increasing Fe content,thus the cell boundary phase could hardly form without enough Sm if the Sm content keep invariant in magnet,resulting in the decreasing of coercivity.After supplyment of Sm content,more cell boundary phase formed and became complete,coercivity enhanced remarkably due to the large concentration gradient between the 1:5 cell boundary phase and 2:17 cellular phase.As a result,the Sm(Co_0.635Fe_0.28Cu_0.06Zr_0.025)₇ magnet with better magnetic properties of?BH?_max=27.50 MGOe and H_cj=14.77 kOe was obtained.Nevertheless,coercivity of magnet with high Fe content could not reach to the level of the magnet with low Fe content.On one hand,cellular size increased from120 nm to 180 nm with increasing Fe content,leading to the decreasing of volume fraction of cell boundary phase,one the other hand,difference between average thickness of cell boundary phase and domain wall thickness enlarged,which undermined the restraint of the domain wall moving.Cu distribution of grains in Sm(Co_0.665Fe_0.25Cu_0.06Zr_0.025)₇ magnets were observed and the effect of sintering temperature on the structural and magnetic properties of magnets were investigated by EPMA.Cu-lean grain boundaries in magnets were found after aging,and the average width of Cu-lean region is close to0.75?m.When the sintering temperature increased to 1195?,the density of magnets have achieved to the theory value of 8.40 g/cm³ and kept invariant.Although the magnets has high coercivity of 22²5 kOe,but an obvious kink were observed on demagnetization curve at reverse magnetic field of-2 kOe,result in the poor squareness,and?BH?_max is only about 22 MGOe.As the sintering temperature increases to around 1220?,S_r was remarkably improved due to the decreasing volume fraction of Cu-lean grain boundary regions,leading to an enhanced?BH?_max of about 25²7 MGOe.However,the coercivity decreases to 16 kOe,corresponding optimal solid solution temperature also decreased.MOKE observation shows the magnetic domains were easily reversed at grain boundaries during demagnetization process,this problem can not be addressed by tunning solid solution temperature.Increasing sintering temperature promoted the amalgamation of small grains into big ones,resulting in an enlargement of average grain size from 10 to 25?m and diffusion of Cu from inner grains to Cu-lean grain boundaries.The volume ratio of Cu-lean regions in magnets dcreased from 19.5%to 8.5%based on estimation,leading to an enhancement of S_r and increasing energy product.Moreover,increase of grain size induces diffusion of Cu from inner grains to grain boundaries,leading to the deceasing of Cu concentration in bigger grain,and this Cu dilution phenomenon is responsible for decreasing of coercivity.Sm(Co_0.665Fe_0.25Cu_0.06Zr_0.025)₇ magnets with Cu particles were investigated systematically,it was found that distributon of every element at grain boundary regions are similar with inner grain after solid solution treatment.While,only the Cu-lean phenomenon was found at grain boundary regions after aging.To address this problem,Cu particles with an average size of 2?m were adulterated into the Sm(Co_0.665Fe_0.25Cu_0.06Zr_0.025)₇ ball milled powders with a content of 1.0 wt.%,and then the magnets were prepare by powder metallurgic method.The density of magnets before and after doping kept invatiant,giant enhancement of coercivity,from 11.94kOe to 21.21 kOe,was achieved in the Cu doped magnets regardless the remanence droped slightly.Doped magnet has an energy product of 25.02 MGOe higher than that of un-doped magnet.EPMA result shows that the popular Cu-lean regions along grain boundaries in the original magnet reduced substantially and Cu content in cell boundary phase is increased after doping.It is therefore suggested that capillary effect promoted the Cu to cover the surface of powders in liquid-state evenly and diffuse into inner grains of the magnets during sintering process.TEM observation shows the Cu-lean grain boundary is surrounded by some incomplete cellular structure,while fine and uniform cellular structure is well established after doping.Meanwhile,Cu content is higher in cell boundary phase of doped magnets.Furthermore,Cu nanoparticles and microparticles with different size and content have been used to prepare magnets.Coercivity of doped magnets increased in a range of 10¹5 kOe as well as the energy products are around about 25 MGOe.However,some undiffused grain boundary regions were found in magnets doped with nanoparticles,and inhomogeneous concentration gradient of Cu,results in the obvious kink shown in demagnetization curves.On contrary,uniform Cu distribution was observed in magnets doped with microparticles,it is therefore concluded that doping Cu microparticles is more effective in enhancement of magnetic properties.