Preparation,Structure And Magnetism Of Sm-Co Based Multiphase Heterogeneous Nano Permanent Magnetic Materials

Sm-Co based permanent magnetic materials are widely used in defense,sophisticated weapons,aerospace and other fields for its good high temperature magnetic characteristics.However,the development of Sm-Co permanents is severely limited because of that the magnetic energy products of traditional Sm-Co permanents are close to their theoretical values,and the Sm and Co elements are scarce and expensive.Sm-Co based nanocomposite permanent magnet materials have a potential of high theoretical energy product and need less rare earth.However,the development of bulk nanocomposite permanent magnet materials faces two serious scientific problems:1)How to optimize the size,content and distribution of soft magnetic nanocrystals while obtaining the texture of permanent magnetic nanocrystals.2)How to reduce conflict between the improvement of M_sand the significant reduction of coercivity.In this paper,a new idea by using hetero multi-phase“team-form”group to prepare bulk Sm-Co based nanocomposite permanent magnet materials was put forward aiming to solve the two key problems mentioned above.Combined with the high-pressure hot compression technique developed by our group,anisotropic bulk nanocrystalline Sm Co₃+Sm Co₇/α-Fe（Co）permanent magnets had been prepared for the first time,which had obvious higher maximum energy product than the single phase Sm Co₃,Sm Co₇magnets and the two-phase Sm Co₃/α-Fe（Co）,Sm Co₇/α-Fe（Co）nanocrystalline magnets.The effects of the technique parameters,the hard phase content and the morphology of initial Fe powders on the microstructures and magnetic properties are investigated.The details are as following:Sm-Co based amorphous alloy precursors containingα-Fe（Co）soft magnetic nanocrystals was prepared by mechanical alloying with Sm-Co powder（weight ratio between Sm and Co is 33:67）and Fe powder in the weight ratio of 74:26.The bulk Sm Co₃+Sm Co₇/α-Fe（Co）multiphase nanocomposite permanent magnet materials were prepared by high-pressure hot compression deformation with temperature of 630 ^oC,strain of 78%and pressure of 1GPa.The contents of soft magneticα-Fe（Co）phase,permanent magnetic Sm Co₃phase and permanent magnetic Sm Co₇phase were 26 wt.%,20 wt.%and54 wt.%,respectively.The average grain size of the soft magnetic phase is～13 nm.The hard magnetic Sm Co₇phase has a preferred orientation in the（002）direction and the hard magnetic Sm Co₃phase has a preferred orientation in the（0012）direction.The magnet showed obvious magnetic anisotropy with a coercivity of 5.1 k Oe and a maximum enrgy product of 26 MGOe,which was higher than that bulk single phase Sm Co₃（4 MGOe）,Sm Co₇（18.4 MGOe）magnets and the two-phase nanocomposite Sm Co₇/α-Fe（Co）（20.6 MGOe）magnets.We found that the high pressure deformation parameters of height reduction and deformation temperature had significant effects on the crystallographic texture strength and the magnetic properties of the prepared Sm Co₃+Sm Co₇/α-Fe（Co）multi-phase magnets.With the height reduction increase from 0%to 80%,the texture strength and the magnetic anisotropy of the Sm Co₃and Sm Co₇hard phases enhanced obviously.The maximum energy products of the magnets increased from 14.4 MGOe to 26.6 MGOe.With the deformation temperature increase from 550 ^oC to 700 ^oC,the grain morphology changed from the equiaxed crystals with a grain size of～9 nm to the platelet-shaped crystals 25-35 nm in thickness and 50-80 nm in length.The phase compositions changed from 36 wt.%Sm Co₃+25 wt.%Sm Co₇+29 wt.%α-Fe（Co）+10 wt.%amorphous to90 wt.%Sm Co₇+10 wt.%α-Fe（Co）.The maximum energy product and coercivity increased firstly and then decreased,the pesk values of 26 MGOe and 5.1 k Oe were obtained respectively at 630 ^oC.Controlled the content of theα-Fe（Co）at 28 wt.%,with the Sm Co₃phase content of the multi-phase nanocrystalline magnets increased from 0%to 66%,the magnetic anisotropy and the coercivity enhanced gradually,while the saturation magnetization decreased,and the maximum energy product increased firstly and then decreased.The highest maximum energy product of 25.5 MGOe was obtained with the Sm Co₃and Sm Co₇phase contents of 27 wt.%and 45 wt.%.This phenomenon originates from that the（0012）texture of the Sm Co₃phase generates more easily than the（002）texture of the Sm Co₇phase under the above preparation conditions.The initial morphology of the Fe powder has important influences on the microstructures and magnetic properties of the multi-phase nanocomposite magnets.Compared with the spherical nanocrystalline Fe powder,the branch-like coarse-grained Fe powder facilitated the amorphization more effectively.Under the same deformation conditions（deformation temperature of 550 ^oC and height reduction of 75%）,the magnets prepared from the precursors contained branch-like coarse-grained Fe powder was composed ofα-Fe（Co）,1:7 phase and 1:3 phase,while the magnets prepared from the precursors contained branch-like coarse-grained Fe powder was only composed ofα-Fe（Co）phase and 1:7 phase.The former exhibited smaller grain size,higher soft phase content and stronger crystallographic texture.A high coercivity of 4.6 k Oe and a maximum energy product of 25.5 MGOe were obtained.