Investigation Of The Nd-Fe-B Sintered Magnets With High Coercivity Fabricated Through Grain Boundary Restructuring

Due to the excellent magnetic properties, Nd-Fe-B sintered magnets have been widely applied in various fields, which play an important role on the development of national economy and defense technology. However, for the low magnetocrystalline anisotropy field HA in the surface region of the 2:14:1 phase grains, the obtained coercivity is much lower than the theoretical value. This results in poor thermal stability of the magnets, limiting their applications in high-temperature environments, which is the unsolved problem since their discovery 30 years ago. Based on the grain boundary restructuring (GBR) approach, Dy71.5Fe28.5, Dy6Fe13Cu and Ho63.4Fe36.6 alloys are designed and added into the Nd-Fe-B sintered magnets in the present work. Through optimizing the adding amount of the alloys and the prepare process of the magnets, the 2:14:1 phase grains are refined, in addition, magnetic hardening shell with higher HA and continuous grain boundaries (GBs) are formed in the magnets. Consequently, nucleation of the magnetic reverse domain in the surface region of the 2:14:1 phase grains is suppressed, contributing to the enhanced coercivity. Main results of the work are as follows:According to the magnetism of the R2Fe14B compounds and the phase diagram, Dy71.5Fe28.5, Dy6Fe13Cu and Ho63.4Fe36.6 are designed and introduced into the magnets through GBR approach. For the lower melting point, all the aiding alloys can melt into liquid to increase the volume fraction of the liquid phase and improve the wettability between the GB phase and the 2:14:1 phase during the sintering, accelerating the densification and modifying the distribution of the GB phase. On the other hand, HA of Dy2Fe14B (150 kOe) and Ho2Fe14B (75 kOe) are both higher than the Nd2Fe14B (67 kOe). Diffused Ho and Dy into the surface region of the 2:14:1 phase grains to replace Nd can improve the local HA of the magnet, contributing to enlarge the nucleation field of the reverse domain. Finally, there are no nonmagnetic elements which can heavily diffuse into the 2:14:1 phase grains in these aiding alloys, remanence reduction resulting from the magnetic dilution can be avoid.Nd-Fe-B sintered magnet with high coercivity and good thermal stability can be prepared through the GBR approach to add Dy71.5Fe28.5.Adding 3 wt.% Dy71.5Fe28.5 into the near-stoichiometric magnet of (Pr, Nd)12.3FebalB6.1, the coercivity Hcj can be enhanced from 9.6 to 17.0 kOe by maintaining the remanence Br and maximum energy product (BH)max of 13.6 kGs and 45.0 MGOe.Hcj of (Pr, Nd)12.82Tb0.33Febal(Ga, Al, Zr)0.73B5.88 magnets is enhanced from 17.3 to 24.2 kOe with 3 wt.% Dy71.sFe28.5 addition. Meanwhile, thermal stability of the magnets is effectively improved by modifying the temperature coefficients of αBr and βHcj from-0.133 %/℃ to 0.113%/℃ and-0.685%/℃ to-0.587%/℃. Meanwhile, formation of (Pr, Nd, Dy)-rich phase and Dy-oxides in the GBs decreases the electrode potential difference with the 2:14:1 phase, improving the corrosion resistance of the magnets. Furthermore, co-addition of Dy71.5Fe28.5 with high-melting-point Zr can suppress the abnormal grain growth and simultaneously decrease the microstructural defects formed in the sintering, i.e. holes or discontinuous GBs. Compared to the magnet added with 0.3 wt.% Zr, the magnet co-added with 3 wt.% Dy71.5Fe28.5 and 0.3 wt.% Zr possesses much higher density, less holes and more continuous GBs. Hcj of the magnets is improved from 14.0 to 21.9 kOe with Br of 13.3 kGs and (BH)max of 43.0 MGOe.Eutectic temperature TE of the RE-rich GB phase can be reduced by adding Dy6Fe13Cu through the GBR approach. Through modifying the second-step annealing process, coercivity increment by per unit Dy addition can be effectively enhanced by modifying the distribution of the GBs and accelerating the diffusion of Dy. Adding 3 wt.% Dy6Fe13Cu into the (Pr, Nd)12.82Tb0.33Febal(Ga, Al, Zr)0.73B5.88 magnet, TE of the GB RE-rich phase is reduced from 665℃ to 445.2℃. During annealing at 465 ℃, the RE-rich liquid phase flows and envelops the 2:14:1 phase grains to form more continuous GBs to isolate them. Meanwhile, the flowing liquid phase supplies more continuous liquid diffusion channels of Dy. More Dy is diffused from the GBs towards the 2:14:1 phase grains to form (Pr, Nd, Dy)2Fe14B hardening shell, enlarging the local HA. As a result, coercivity of the magnet is enhanced from 17.3 to 22.8 kOe. The calculated coercivity increment per unit Dy addition is 3.33 kOe, much higher than that of 2.61 kOe in the Dy71.5Fe28.5 restructured magnets.Dy-free Nd-Fe-B sintered magnets with coercivity higher than 18 kOe can be prepared by adding Ho63.4Fe36.6 (price of which is approximately one fifth of to restructure the magnet. Adding 2.5 wt.% Ho63.4Fe36.6 into (Pr, magnet and optimizing the sintering process, three main microstructural changes are observed:i) the 2:14:1 phase grains are refined, decreasing the stray field surrounding them, ii) (Pr, Nd, Ho)2Fe14B hardening shell with higher HA are formed in the surface region of the 2:14:1 phase grains, iii) continuous GBs with Fe content of～9.03 wt.% are formed to isolate the neighboring ferromagnetic 2:14:1 phase grains. As a result, coercivity of the magnet is enhanced from 14.0 to 18.0 kOe with BΤ= 13.15 kGs, (BH)max=42.0 MGOe. Such magnetic properties are superior than the (Pr, Nd)13.16Ho0.8(Fe, M)balB5.78 commercial magnet with equivalent Ho content (Hcj=16.5 kOe, Br=12.3 kGs,(BH)max=35.5 MGOe). Furthermore, due to the lower cost of Ho than Dy, material cost of the Ho63.4Fe36.6-added magnets versus that of Dy71.5Fe28.5-added ones are lower by approximately 18% to realize equivalent coercivity.