The Regulation Mechanism Of Microstructures And Magnetic Properties Of Boron-lean Nd-Fe-B Based Sintered Magnets

The demand for high-performance sintered Neodymium-Iron-Boron(Nd₂Fe₁₄B-based)magnets has increased tremendously since the rapid growth of wind power generators,servo motors and new energy vehicles.For high performance sintered Nd Fe B magnets,especially high coercivity sintered Nd Fe B magnet,heavy rare earth elements,such as dysprosium（Dy）or terbium（Tb）,are generally required as extra supplements.However,the proven reserves of strategic rare earth（RE）elements,particularly Dy or Tb are dwindling and expensive.From the perspective of supply safety and cost,it is imperative to find a good approach to prepare sintered Nd Fe B magnets without heavy rare earths or with low content heavy rare earths.In this thesis sintered Nd-Fe-B magnets were prepared by traditional powder metallurgy and double alloy technology.The effects and regulation mechanism of B content on the magnetic properties,microstructures,mechanical and corrosion properties of the designated sintered Nd Fe B magnets are systematically studied.The influences and tailoring mechanism of Pr Nd,Ga,Cu,Ti and Zr content on the comprehensive properties and microstructures of the B-lean sintered Nd Fe B magnets（0.90 wt.%B）are investigated in depth.Furthermore,revealed and clarified are the effect and mechanism of Cu and Ga content on low Tb efficiency in enhancing the properties of the B-lean sintered Nd Fe B magnets made by traditional powder metallurgy and the mechanism of abnormal"double enhancement/simultaneous boosting"of coercivity and remanence after post sinter annealing of the B-lean sintered Nd Fe B magnets made by double alloy technology and two-steps annealing method.Firstly,sintered Nd Fe B magnets with different B content were fabricated by traditional powder metallurgy.It is shown that with the decrease of B content the coercivity increases rapidly and then decreases slowly while the remanence decreases monotonously.As B content decreases,RE₆（Fe,M）₁₄ phase with antiferromagnetic characteristics is formed on the triple junction areas of the magnet,acting as the pinning center during demagnetization and reducing the Fe content in the grain boundary phase,which results in weakening the exchange coupling between the neighboring matrix phase grains to improve the coercivity of the magnet.A further decrease of B content from0.90 wt.%,the volume percentage of 2:14:1 matrix phase will continue to decrease,and the excess Fe atoms are distributed in the triple junction areas and the grain boundary phases,which leads to the reduction of coercivity resulted from the enhancement of the exchange coupling between the neighboring matrix phase grains of the magnet.In general,with the increase of Pr Nd content,remanence decreases while coercivity increases in B-lean sintered Nd Fe B magnets.Particularly,in the range of 30.5-32.2 wt.%Pr Nd,it is calculated by linear fitting method that for every 1 wt.%increase of rare earth Pr Nd,the remanence decreases by about 0.289 k Gs and the coercivity increases by about 0.839 k Oe.Secondly,B-lean sintered Nd Fe B magnets with different Ga and Cu contents（0.90 wt.%B）were prepared by the same method.The effects and regulation mechanism of Ga and Cu contents on the microstructures,magnetic properties,and mechanical and corrosion properties of the samples are systematically studied.The results indicate that in the range of 0.0-0.8 wt.%Ga or Cu,the coercivity first increases rapidly and then slowly while the remanence decreases monotonously.The main reason for the increase of coercivity is attributed to the reduction of the exchange coupling between the neighboring matrix phase grains,resulting from a smooth,continuous and clearly visible grain boundary phase formed after primary and secondary annealing processes.Thirdly,B-lean sintered Nd Fe B magnets（0.90 wt.%B）with various Ti and Zr contents were prepared similarly.The effects of Ti and Zr contents on the magnet properties and regulation mechanism are systematically studied.The results show that with the increase of Ti or Zr content,the remanence of B-lean sintered Nd-Fe-B magnet decreases monotonously while the coercivity of magnet increases firstly and then decreases rapidly.The main reason for the increase of coercivity after adding Ti or Zr is accredited to following factors.Comprehensively,Ti or Zr forms boride precipitation which reduces B content in the RE-rich phase;and antiferromagnetic RE₆（Fe,M）₁₄phase forms on the triangular grain boundary of the magnet,which reduces the concentration of Fe in the grain boundary phase between the main phase grains.Moreover,RE,Cu,Ga and other elements enriched at the triple junction areas would diffuse into the grain boundary phase between the main phase grains,which is conducive to both the formation of continuous and smooth grain boundary phase and to reduction of the exchange coupling between the neighboring main phases grains.Finally,the Tb doped B-lean sintered Nd-Fe-B magnets（0.91 wt.%B）for new energy vehicles were manufactured by traditional single alloy powder metallurgy and B-lean sintered Nd-Fe-B magnets（0.94 wt.%B）without heavy rare earths for wind power generators were manufactured by double alloy technology,respectively.The effects of Cu and Ga contents on the efficiency of Tb substitution in enhancing coercivity and the function of double alloy process on the magnetic properties and microstructures are systematically studied.The results show that the coercivity can be increased by 2.6～2.8 k Oe for 1.0 wt.%Tb-doping in magnets with low Cu and no Ga,and 4.2～4.6 k Oe for 1.0 wt.%Tb-doping in magnets with high Ga and Cu.The remanence and coercivity are 14.49 k Gs and 17.19 k Oe respectively for the B-lean sintered Nd-Fe-B magnet without HRE by adding auxiliary alloy Pr₆₅Fe₃₀Ga₅ alloy.After primary annealing and secondary annealing process,it is found out that there is an abnormal"double enhancement/simultaneous boosting"phenomenon of the coercivity and remanence of the magnet（the coercivity increased by70%and the remanence increased by 2.5%）.It is demonstrated that after adding the auxiliary alloy Pr₆₅Fe₃₀Ga₅,the melting point of the rare earth rich phase decreases and its wettability is significantly improved.After the secondary annealing treatment,it is observed that the grain boundary phase is evenly distributed around the main phase,and Pr partially replaces Nd on the surface of the main phase to form a core-shell structure of Pr₂Fe₁₄B shell and Nd₂Fe₁₄B core.It is further confirmed that the synergistic effect of multiple elements makes the microstructure,grian boundary phase and grain distribution more uniform.All of these factors are beneficial to the improvement of both coercivity and remanence.