Research On Grain Boundary Regulation Of Microstructure And Magnetic Properties Of Multi-main-phase (Nd,Ce)-Fe-b Magnets

Nd-Fe-B magnets are widely used on account of their strongest comprehensive magnetic properties.Applying abundant and cheap Ce to Nd-Fe-B magnets to prepare（Nd,Ce）-Fe-B magnets with comprehensive magnetic properties can reduce the manufacturing cost of magnets,and promote the rational utilization of rare earth resources.However,low coercivity and thermal stability of magnets has always been a difficult problem limiting the application of（Nd,Ce）-Fe-B magnets.Generally,the coercivity and thermal stability of magnets are improved by diffusing heavy rare earth elements（Dy,Tb,etc.）to form a hard magnetic shell with high magneto-crystalline anisotropy field.But this not only exacerbates the increasing consumption of heavy rare earth resources,but also increases the cost of magnet manufacturing.In order to solve the above problems,based on micro-magnetic simulation and thermodynamic calculation,（Nd,Ce）-Fe-B magnets are prepared by multi-main-phase-process,and the microstructure of magnets is optimized by adding Pr F₃+Cu powder and Y-Gd based alloy at grain boundaries,in order to prepare（Nd,Ce）-Fe-B magnets with high coercivity and thermal stability on the basis of reducing or not using heavy rare earth elements Dy and Tb.The effects of grain boundary phase structure,main phase grain size and composition on magnetic properties of multi-main-phase（Nd,Ce）-Fe-B magnets were investigated by micro-magnetic simulation.The simulation results show that when the grain boundary is nonmagnetic,the thicker the grain boundary phase is,the better it is to improve the coercivity of the magnet;However,when the grain boundary phase is magnetic,the coercivity of magnet decreases significantly with the enhancement of the grain boundary phase magnetism.Regarding the size and composition of the main phase grain:the finer the main phase grain,the better the coercivity of the magnet;The(（Nd,Ce）_1-xPr_x)₂Fe₁₄B shells formed on the surface of the main phase grain after Pr diffusion improve the nucleation field,thus significantly improving the coercivity and thermal stability of the magnet.The Curie temperature T_cof magnet is increased by diffusion of Y and Gd,and the coercivity H_cjof magnet is significantly increased by the nonmagnetic phase generated by diffusion of Y and Gd,thus improving the high thermal stability of the magnet.Doping Pr F₃+Cu powder is beneficial to improve the magnetic properties and microstructure of multi-main-phase（Nd,Ce）-Fe-B magnets.Thermodynamic theoretical calculation shows that the diffusion of Pr into the main phase grains is beneficial to weaken the inter-diffusion behavior of Nd and Ce of magnets.Cu is easier to diffuse into grain boundary phase,which is beneficial to improve the wettability of grain boundary phase.Experimental research shows that doping Pr F₃+Cu powder can improve the H_cjof magnets on the basis of reducing B_rby a small amount.When 1wt%Pr F₃+Cu powder is added,the H_cjof the magnet reaches the maximum value of 1115.2k A/m,which is 6.41%higher than that without adding magnet,while the B_ris only 2.11%lower.The reason is that doping Pr F₃+Cu powder increases the RE-rich phase between main phase and between grains in main phase,and improves the interface structure of magnets.At the same time,Pr diffused into the main phase of magnet instead of Nd and Ce to form the(（Nd,Ce）_1-xPr_x))₂Fe₁₄B shell with high magneto-crystalline anisotropy field,which inhibited the nucleation of demagnetization domain.Meanwhile,the microstructure research found that adding Pr F₃+Cu powder not only inhibited the growth of microcrystals at the interface of main phases,but also reduced the inter-diffusion behavior of Nd and Ce in the magnets,which was an important reason for the enhancement of H_cj.Adding Y-Gd based alloy can improve the microstructure and enhance the coercivity and thermal stability of multi-main-phase（Nd,Ce）-Fe-B magnets.Thermodynamic enthalpy calculation shows that Y and Gd diffuse into the main phase grains,replacing Nd and Ce to form(Nd/Ce_1-xY/Gd_x)₂Fe₁₄B alloy phases,which is beneficial to weaken the inter-diffusion of Nd and Ce in magnets.Experimental research shows that with the increase of Y-Gd base alloy addition,the H_cjof magnet increases gradually.With the addition of 4wt%Y-Gd based alloy,the H_cjof the magnet is significantly increased to 1131k A/m,which is about 12.3%higher than that without adding magnet(H_cj=1007k A/m).The reason is that doping Y-Gd based alloy increases RE-rich phase between main phase and grains in main phase,and forms nonmagnetic phase after diffusion of Y and Gd,which improves H_cjof magnet,promotes the formation of uniform nonmagnetic grain boundary phase and enhances demagnetization coupling between main phase grains,which is consistent with the results of micro-magnetic simulation.In addition,the addition of Y-Gd-based alloy significantly improve the thermal stability of the magnet.The absolute value of remanence and coercivity reversible temperature coefficient of magnet decreases with the increase of Y-Gd-based alloy addition.When the addition amount is 4wt%,High thermal coercivity and magnetic energy product of magnet was high.The microstructure study found that the addition of Y-Gd based alloy inhibits the abnormal growth of main phase grains,and reduces the inter-diffusion behavior of Nd and Ce in magnets.