Investigation Of The Nd-Fe-B Sintered Magnets With High Performance Fabricated Through Grain Boundary Modification

Nd-Fe-B magnets are known as the third rare earth permanent magnets due to their excellent comprehensive magnetic properties and are widely used in various fields.Especially in the environment of advocating green energy,the rapid development of the green industry led by wind power generation and new energy vehicles has sharply increased the demand for high-performance Nd-Fe-B sintered magnets.However,since the actually produced magnets cannot reach the ideal microstructure,the coercive force of the magnets is much lower than its theoretical limit.Low coercivity will cause the magnets to undergo violent thermal demagnetization in high-temperature environments,which will lead to the deterioration of the magnets.At the same time,the traits of intergranular corrosion and intergranular fracture also limit the further application of Nd-Fe-B sintered magnets.In this paper,based on the grain boundary modification process,three low-melting eutectic alloys of Tb_62.5Co_37.5,Tb₆₈Ni₃₂,and Tb₆Fe₁₃Cu powder were designed to be mixed with Nd-Fe-B alloy powder to prepare Nd-Fe-B sintered magnets.By optimizing the addition amount of the target alloy and the heat treatment process,a magnetic hardened layer with a high magnetocrystalline anisotropy field was generated on the surface of the main phase,and a continuous thin layer of grain boundary phase with high electrode potential was formed,which improved the microstructure of the magnet.Therefore,the intrinsic coercivity,corrosion resistance,and mechanical properties of the magnet were significantly improved,while the remanence of the magnet could be kept unchanged or slightly reduced.The main results are as follows:Combining the coercivity theory of Nd-Fe-B sintered magnets and related alloy phase diagrams,three alloys of Tb_62.5Co_37.5,Tb₆₈Ni₃₂,and Tb₆Fe₁₃Cu with high potential and low melting point were designed and prepared for high-performance Nd-Fe-B sintered magnet through grain boundary modification process.Firstly,the melting points of the three terbium-containing alloys are much lower than the sintering temperature of the magnet,and will melt into liquid state during the sintering process,to assist the liquid phase sintering process of the magnet,thereby increasing the density of the magnet and forming a uniformly distributed continuous thin grain boundary phase.At the same time,Tb diffused into the surface of the main phase of the magnet to form a magnetic hardened layer with a high magnetocrystalline anisotropy field,which improved the coercivity of the magnet.Simultaneously,the high electrode potential elements Co,Ni,and Cu were enriched in the grain boundary,which was beneficial to narrow the gap in electrode potential between the main phase of the magnet and the grain boundary phase,to reduce the driving force for corrosion,and thereby improve the corrosion resistance of the magnet.Grain boundary modification process by adding Tb₆₈Ni₃₂ to the grain boundary can effectively improve the intrinsic coercivity,corrosion resistance,and mechanical properties of the magnet.By adding 3.0 wt%Tb₆₈Ni₃₂ to the main alloy（Pr,Nd）_31.2M_3.36Fe_balB_1.0（M=Dy,Ho,Al,Gd,wt%）,the intrinsic coercivity of the magnet increased from 18.44 k Oe to 27.02 k Oe.The increase rate was as high as 47%,the remanence and the maximum energy product remain at 10.91 k Gs and 29.00 MGOe,respectively;the remanence and maximum magnetic energy product of the magnet have been significantly improved when the addition amount was 1.0 wt%,from 11.88 k Gs and 34.34 MGOe to 12.88 k Gs and 40.12 MGOe,respectively.The corrosion potential of the modified magnet increased from-0.901 V to-0.831 V,and the Vickers hardness increased from 649.4 HV to 927.0 HV.At the same time,when the addition amount was 1 wt%,the bending strength of the magnet reached the highest value（262 MPa）.Using the low melting point auxiliary alloy Tb_62.5Co_37.5 for grain boundary modification,Nd-Fe-B sintered magnets with high intrinsic coercivity and high corrosion resistance can be prepared.By adding 3.0 wt%Tb_62.5Co_37.5 to the main alloy（Pr,Nd）_31.2M_3.36Fe_balB_1.0（M=Dy,Ho,Al,Gd,wt%）,the intrinsic coercivity of the magnet increased from 18.20 k Oe to 27.10 k Oe,an increase of 49%,and the remanence and maximum energy product remain at 10.84 k Gs and 28.47 MGOe,respectively.And the remanence and maximum magnetic energy product of the magnet have been significantly improved when the addition amount was 0.5 wt%,from 11.10 k Gs and 30.13 MGOe to 11.86 k Gs and 34.19 MGOe,respectively.At the same time,the improvement of the density of the modified magnet,the formation of uniform and continuous thin-layer grain boundary phases with high electrode potential,and the reduction of the difference in electrode potential between the main phase and the grain boundary phase,all in favor of reducing the driving force of grain boundary corrosion.The corrosion potential of the magnet increased from-0.902 V to-0.756 V,and the corrosion current density decreased from 6.049μA/cm² to 0.7μA/cm².The Vickers hardness of the modified magnet increased from 742.2 HV to 1055.6 HV,and the bending strength increased from 0.5 wt%to 280 MPa.Using the ternary alloy Tb₆Fe₁₃Cu to modify the grain boundary of（Pr,Nd）_29.67M_4.45Fe_balB_0.99（M=Dy,Gd,Al,Zr,Co,wt%）can prepare high-performance Nd-Fe-B sintered magnets with low heavy rare earth content.By adding 3.0 wt%Tb₆Fe₁₃Cu to the main alloy,the intrinsic coercive force of the magnet is increased from 15.01 k Oe to 21.4k Oe,an increase of up to 43%.For each addition of 1.0 wt%Tb,the intrinsic coercive force increased by about 27%compared to the initial magnet.Higher than 23.2%and 19.2%of Tb_62.5Co_37.5 and Tb₆₈Ni₃₂ grain boundary modification.At the same time,the remanence and the maximum magnetic energy product remained at 12.03 k Gs and 35.21 MGOe,which were only reduced by 3%and 6%,respectively.The corrosion potential of the modified magnet increased from-0.972 V to-0.932 V,and the corrosion resistance was significantly improved.