Enhancement On The Coercivity Of Sintered NdFeB With TbPr-M System Low-heavy Rare Earth Multicomponent Alloy By Grain Boundary Diffusion

Sintered Nd Fe B is widely used in high-tech fields such as energy-saving household appliances,wind power generation,and electric vehicles.However,the actual coercivity of ternary Nd Fe B magnets is less than 20%of the theoretical value,so improving the coercivity of Nd Fe B is the focus of research in this field.Heavy rare earth grain boundary diffusion technology is an effective means to improve the coercive force of Nd Fe B magnets.Yet,conventional diffusion sources and methods such as hydride and fluoride of heavy rare earth can easily lead to the enrichment of heavy rare earth in the surface layer of the magnet,resulting in low diffusion efficiency,which is not conducive to the application and development of diffusion magnets.Low melting point alloy diffusion sources have become the current development trend due to their high wettability and good diffusion effects;At the same time,it was found that non-rare earth metal elements such as Cu,Al,Ga,Co,and Zr can promote the grain boundary diffusion effect of heavy rare earths,but the specific effect is not yet clearly.Therefore,in this study,Tb-Pr-M series low heavy rare earth alloy diffusion sources were designed and prepared using heavy rare earth Tb,light rare earth Pr,Nd,and non-rare earth elements M（Cu,Al,Ga,Co,Zr）.The diffusion behavior and coercivity strengthening mechanism of low heavy rare earth alloys in Nd Fe B were further studied.The results are as follows:（1）Low weight rare earth alloy Tb₁₀Pr₆₀Al₃₀、Tb₁₀Pr₆₀Cu₃₀、Tb₁₀Pr₆₀Ga₃₀（at%）were prepared for grain boundary diffusion treatment of commercial N50 magnets.The coercivity of Tb₁₀Pr₆₀Al₃₀ diffused treated magnet increased by 10.24 k Oe compared to the original magnet.The microstructure characterization showed that Cu promoted the substitution of heavy rare earth element Tb in the diffusion surface layer for Nd in the main phase grains,and Tb was enriched in the diffusion surface layer;Al improves the wettability of the grain boundary phase,widens the grain boundary,and increases the demagnetization exchange coupling between grains;Ga enter the triangular grain boundary,the temperature stability of the magnet increases.（2）Low weight rare earth alloy Tb₁₀Pr_90-x（Cu,Al,Ga）_x（at%）was prepared to grain boundary diffusion treat the commercial N50 magnets.It was found that the coercivity of Tb₁₀Pr₆₀（Cu,Al,Ga）₃₀ alloy increased by 11.26 k Oe after diffusion and tempering treatment.Studies on the effective introduction of Tb and Pr into alloy diffusion sources have found that Tb₁₀Pr₆₀（Cu,Al,Ga）₃₀ and Tb₁₀Pr₄₀（Cu,Al,Ga）₅₀ alloys have the highest utilization rates for Tb and Pr,with 94.74%and 81.76%,respectively.In addition,the microscopic demagnetization domain evolution results show that the shell structure with a higher Tb content has a greater inhibitory effect on reverse domain expansion.（3）A multi-element alloy Tb₁₅Pr₁₅Nd₄₅Cu₅Al₅Co₅Ga₅Zr₅（at%）was prepared,and the multi-step diffusion process was designed based on its melting point characteristics to conduct grain boundary diffusion treatment on commercial N50 magnets.The study found that the coercivity of the magnet after multi-step diffusion treatment reached to23.69 k Oe.In multi-step diffusion,at low temperatures,Tb migrates to the magnet and undergoes a metallurgical reaction with Fe to form a Tb Fe alloy.At high temperatures,Tb Fe alloy melts and grain boundary diffusion occurs.Compared with conventional diffusion processes,after multi-step diffusion,the distribution of Tb elements is more uniform,the diffusion depth is greater,and the surface enrichment phenomenon is improved.