Structure And Properties Of Hot-pressed/Hot-deformed RE- Fe-B Magnets Based On Intergranular Addition

Nd-Fe-B based permanent magnets are applied in various traditional and emerging fields,such as alternative fuel vehicles,wind powder and maglev track due to their excellent magnetic properties.However,these magnets are not suitable for high temperature applications owing to their poor thermal stability.The partial substitution of Nd by heavy rare earth Dy can improve the thermal stability through increasing the magnetic anisotropy field of Nd-Fe-B magnets.The Dy addition increases the raw material cost,however,the Nd-Pr-Fe-B based and Nd-Ce-Fe-B based magnets are fabricated by the inter-granular doping technology to optimize the grain boundary,in order to further reduce the content of heavy rare earth Dy.In this thesis,isotropic Nd-Pr-Fe-B based magnets were prepared by spark plasma sintering（SPS）techniques using commercial Nd-Pr-Fe-B magnetic powder,the relationship between sintering temperature and the magnetic properties of magnets was clarified.Using isotropic SPSed magnets as a precursor,the anisotropic Nd-Pr-Fe-B magnets were produced using hot-deformation technology.The best magnetic properties:J_r=13.23 kG,H_cj=9.94 kOe,（BH）_max=37.59 MGOe,were obtained at 790oC/100 MPa,holding time of 3 min and holding pressure time of 1 min.1 wt.%Dy₇₀Cu₃₀,Dy₂₀Pr₅₀Cu₃₀,Dy₂₀Pr₅₀Cu₁₅Al₁₅,Dy₁₀Pr₅₀Cu₂₀Al₂₀alloys were mixed with the commercial Nd-Pr-Fe-B powders,dual alloy hot-pressed magnets and hot-deformed magnets were produced under the optimized conditions.The results showed that the coercivity of the hot-pressed magnets were improved by 1.27¹.69 kOe after the Dy-alloys doping.The temperature coefficient of remanence（α）and coercivity（β）were optimized from-0.108%/K,-0.510%/K to-0.087^-0.105%/K,-0.489^-0.495%/K,respectively in the temperature range of 300⁴00 K.The microstructure results showed that the powder boundary in Dy₇₀Cu₃₀0 added alloys is rich in Dy element.As the Dy content was decreased,Dy inclined to enter into the main phase areas,which proved to be beneficial for improving the magnetic properties.After hot-deformation,a slight shift in XRD patterns to high angle is observed in the main phase peak for the anisotropic Nd-Pr-Fe-B based magnets and the lattice constants is reduced,indicating that a small amount of Dy atoms enter into 2:14:1 main phase,forming（Nd,Pr,Dy）-Fe-B hard magnetic phase makes the coercivity increased by 3.65⁴.08 kOe.The nanocrystalline Nd-Ce-Fe-B based ribbons were prepared by melt-spinning technique.The effect of Ga addition on the magnetic properties and microstructures of the alloys were systematically investigated.The Nd-Ce-Fe-B based alloy depicted a relatively uniform microstructure and the best comprehensive magnetic properties:J_r=7.60 kG,H_cj=17.28 kOe,（BH）max=12.51 MGOe,were obtained in（Nd0.8Ce0.2）13.8Fe68.46Ga0.5Co3.31B5.74 alloy.Hot-pressed magnets were fabricated by SPS technology using the above optimized alloy components,and the magnetic properties and microstructure of magnets were explored in detail.Anisotropic Nd-Pr-Fe-B based magnets were prepared and the effects of deformation temperature,deformation pressure,holding time and holding pressure time on the magnetic properties were investigated,and the processing conditions were optimized.When the deformation temperature was 790oC,deformation pressure was 80 MPa,holding time was 5min and holding pressure time was 1 min,the best magnetic properties of the:J_r=11.59 kG,H_cj=12.32 kOe,（BH）_max=9.94 MGOe were obtained.After the addition of Dy-alloys at the grain boundary,the coercivity of the dual alloy Nd-Ce-Fe-B based magnets achieved by 0.6¹kOe,and the remanence and maximum magnetic energy product decreased slightly.Theαandβwere improved from-0.128%/K,-0.497%/K to-0.110^-0.127%/K,-0.475^-0.492%/K,respectively.The microstructure analysis indicated that RE-rich phase is formed at the powder boundary by adding Dy₇₀Cu₃₀ alloy.As the Dy content reduced,the RE-rich phase slightly decreased at the powder boundary.However,the grain alignment of the hot-deformed Nd-Ce-Fe-B based magnets is slightly reduced as a result of Dy-alloys doping,leading to the slight decrease in remanence and the coercivity was improved by 1² kOe.In summary,the coercivity of hot-pressed/hot-deformed magnets of two different compositions were increased by Dy-alloys doping.With the Dy content reduces,the magnetic properties of magnets practically remained unchanged.It showed that reducing the use of heavy rare-earth Dy,the magnetic properties of magnets can still be improved.