Grain Boundary Modification Of Sintered Nd-Fe-B Magnets With Low-melting-point Dy-containing Alloys

Sintered Nd-Fe-B magnets have been widely applied due to their excellent magnetic properties.However,the poor thermal stability of Nd-Fe-B limits its applications at high temperature field.Although the thermal stability can be enhanced by adding heavy rare earth elements,taking account of the rise of manufacture cost,it is of importance to improve the high temperature magnetic properties while reducing the content of heavy rare earth.In this thesis,three kinds of low-melting-point Dy alloys,Dy70Cu30,Dy69Ni31,and Dy60Ni10Al10Cu20(at.%),were designed and used as diffusion sources of the sintered Nd-Fe-B magnets.Through controlling the heat treatment process,the effects of grain boundary diffusion process(GBDP)on the microstructure and properties of sintered magnets were studied,and the enhancement mechanisms of coercivity,thermal stability and corrosion resistance were also investigated.By using the Dy70Cu30 alloy to optimizate the grain boundary,the magnets with higher coercivity and better thermal stability was obtained by the GBDP.In the range of experiments,the maximum coercivity was 1854 kA/m under the optimal heat treatment condition of 860 °C / 2 h + 500 °C / 1 h,which was 18.5% higher than that of original magnet.Compared with the original magnet during the temperature range of 20 ~ 120 °C,the absolute value of temperature coefficient of remanence decreased from 0.110 %/°C to 0.106 %/°C and the absolute value of temperature coefficient of coercivity decreased from 0.563 %/°C to 0.515 %/°C.By using the Dy69Ni31 alloy as the diffusion source,the optimized heat treatment process was obtained at 770 °C / 2 h + 500 °C / 1 h.Under this heat treatment condition,the coercivity increased from 1407.1 k A/m to 1565.8 kA/m with little reduction in remanence and maximum energy product.Compared with the original magnet during the temperature range of 27 ~ 227 °C,the absolute values of temperature coefficient of remanence and coercivity decreased from 0.145 %/°C and 0.459 %/°C to 0.127 %/°C and 0.443%/°C,respectively.The Curie temperature of magnet rase from 344 °C to 360 °C.The quaternary Dy60Ni10Al10Cu20 alloy was also used as the diffusion source.The best magnetic properties were achieved when the heat treatment process was 730 °C / 4 h + 500 °C / 1 h.Under the premise that the remanence reduced little,the coercivity,maximum energy product and Curie temperature enhanced from 1407.1 kA/m,43.06 MGOe,344 °C to 1794.4 kA/m,45.86 MGOe,377 °C,respectively.During the temperature range of 27 ~ 227 °C,the absolute value of temperature coefficient of coercivity decreased from 0.459 %/°C to 0.455 %/°C.Corrosion resistance of sintered Nd–Fe–B magnets was also significantly improved by Dy-Ni-Al-Cu grain boundary restructuring.Comparing these three kinds of diffusion sources,we found that the effects of diffusion time on microstructure were similar.As the diffusion time increased,the depth of hard-shell phase in SEM image increased first and then decreased which caused the variation of magnetic properties.The micromagnetic simulation results showed that the core-shell structure with high Dy content in shell phase and low Dy content in core phase was beneficial to the coercivity.By combining the results of microstructur and magnetic properties,the following conclusions can be drawn:(a)the heavy rare earth elements such as Dy enhance the anistropic field and thermal stability by diffusing into the matrix phase and forming the(Nd,Dy)2Fe14B hard shell phase.(b)the elements such as Cu and Al can improve the wettability of grain boundary phase and intensify the decoupling interaction.(c)the Ni element can diffuse into the matrix phase and enhance the Curie temperature of magnets.(d)Dy,Ni,Al and Cu elements in grain boundary phase can improve the electrochemical potential of grain boundary phase to enhance the corrosion resistance of magnets.