Research On Process Of Plastic Deformation And Mechanism Of The Formation Of Texture For Nanocrystalline Rare Earth-cobalt Based Permanent Magnet Prepared By Hot Deformation

As the development of aerospace and electric automobile technique,the request for high temperature permanent magnets becomes stricter.Rare earth-cobalt?Re-Co?based nanocrystalline magnets are one of the most potential magnets which could be used under high temperature due to their high coercivity,Curie temperature and uniaxial magnetic anisotropy.The Re-Co based nanocrystalline magnets could gain strong texture and anisotropy through hot deformation technology,which is very significant for the application of Re-Co based nanocrystalline magnets.However,the process and impact factors of the deformation,as well as the mechanism of the texture formation are still unclear,which may restrict the further optimization of their magnetic properties.In this work,the preparation and magnetic properties of hot deformed Re-Co based nanocrystalline magnets were studied from the aspects of component,phase composition,microstructure and thermal-mechanical coupling condition.The main results based on a thorough discussion about the above issues are shown below.The SmCo₅,Sm₂Co₇ and Sm₂Co₁₇7 nanocrystalline hot deformed magnets with high crystallographic texture and magnetic anisotropy were prepared successfully.The coercivity and the ratio of M_r/M_s of SmCo₅ hot pressed magnet reach 36.11 kOe and 0.828,respectively.After 70%of deformation,the coercivity of the SmCo₅magnet reaches a higher value of 38.01 kOe.For Sm₂Co₇ hot deformed magnet,the coercivity can be as high as 50.6 kOe after 70%of deformation.With further increase of the deformation degree up to 90%,the intensity of the texture become 7.6 times of that in the hot pressed Sm₂Co₇ magnet,and the maximum magnetic energy product is of 8.21 MGOe.As for the Sm₂Co₁₇7 magnets,the appearing of Sm₂Co₁₇7 rhombohedral phase will hinder the texture formation of the Sm₂Co₁₇7 hexagonal phase,thus the magnetic properties cannot be improved.In addition,the hot deformed temperature is determined to be the key to control the deformation of Sm-Co magnets.The effect of phase composition and microstructure on texture and magnetic performance of SmCo₅ hot deformed nanocrystalline magnets were investigated systematically.The hot deformed magnets with different phase composition were prepared by adjusting the Sm content.It is found that Sm₂Co₁₇7 hexagonal phase will formed in the hot pressed magnet with lower Sm content,which will transform to rhombohedral phase during the process of hot deformation.While Sm₂Co₇ phase will show up in hot pressed magnets with higher Sm content,which will form its own texture and competes with the SmCo₅ phase during the process of hot deformation.Since Sm₂Co₇ phase is easier to form texture than SmCo₅ phase,with the increase of Sm₂Co₇ phase,the texture of SmCo₅ phase becomes less evident.Only with the appropriate Sm content,the hot deformed magnet shows pure SmCo₅ phase and possesses the strongest crystallographic texture and magnetic anisotropy.As for the grain size,the fine grain magnets are easier to get better texture than the coarse grain magnets.And the coercivity and M_r/M_s of fine grain magnets are 4.48 times and 1.37times higher than that of the coarse grain magnets,respectively.The effect of above factors on the texture formation was considered by using schmind factor and coincidence site lattice grain boundary of different slip systems.When both?10-10?<-12-10>and?1-101?<11-20>slip systems are activated,the texture of grains will correspond with schmind factor and establish a self-consistent relationship between them,which is responsible for the formation of texture.In addition,there is a dependence between the frequency of the occurrence of?13a twist boundaries like 27.796°/[0001]and the texture of SmCo₅ magnets.The frequency of?13a type of coincidence site lattice grain boundary in fine grain magnet is much higher than that of coarse grain magnet,which indicates the activation of grain rotation during the process of hot deformation is more sufficient in fine grain magnet,and thus form stronger texture than the coarse grain magnet.Finally,the magnetic harding mechanism of hot deformed SmCo₅ magnets was studied by using the mini loops,recoil loops and henkel loops.The two phases structure resulted from the variation of Sm content lead to the weakening of pinning effect and hence the decrease of coercivity.The irreversible part of demagnetization process and the reversible part of magnetization process are all showing higher de-pinning field in fine grain magnet than that in coarse grain magnet,which results in higher coercivity in the former one.In addition,the evolution of magnetic domains in SmCo₅ hot deformed magnets was observed with magneto-optical Kerr microscope.It is found that the poor texture in coarse grain magnet would cause more strip domains which will demagnetized easily and leads to the decrease of coercivity.For fine grain magnets with exchange-coupled domains,the magnetic anisotropy would become obvious when a strong texture forms,showing a consistent twirl of the magnetic domains.In this work,the nanocrystalline Sm-Co anisotropic magnets with high magnetic properties were developed.The effect of various factors on the formation of texture has been revealed.Moreover,a methodology for analyzing the texture of nanocrystalline hot deformed magnets was presented.This provides a significant reference for the understanding of the formation of texture and the process of hot deformation in Sm-Co magnets.It also lays the foundation for developing high performance rare-earth-based magnets for elevated temperature application.