Coercivity enhancement of sintered Nd–Fe–B magnets by grain boundary diffusion with Pr_(80-x)Al_(x)Cu_(20)alloys

A grain boundary diffusion(GBD)process with Pr_(80-x)Al_(x)Cu_(20)(x=0,10,15,20)low melting point alloys was applied to commercial 42M sintered Nd–Fe–B magnets.The best coercivity enhancement of a diffused magnet was for the Pr_(65)Al_(15)Cu_(20)GBD magnet,from 16.38 kOe to 22.38 kOe.Microstructural investigations indicated that increase in the Al content in the diffusion source can form a continuous grain boundary(GB)phase,optimizing the microstructure to enhance the coercivity.The coercivity enhancement is mainly due to the formation of a continuous GB phase to separate the main phase grains.Exchange decoupling between the adjacent main phase grains is enhanced after the GBD process.Meanwhile,the introduction of Al can effectively promote the infiltration of Pr into the magnet,which increases the diffusion rate of rare-earth elements within a certain range.This work provides a feasible method to enhance coercivity and reduce the use of rare-earth resources by partial replacement of rare-earth elements with non-rare-earth elements in the diffusion source.

Optimization of the grain boundary diffusion process by doping gallium and zirconium in Nd–Fe–B sintered magnets

As the channel for grain boundary diffusion(GBD)in Nd–Fe–B magnets,grain boundary(GB)phases have a very important effect on GBD.As doping elements that are commonly used to regulate the GB phases in Nd–Fe–B sintered magnets,the influences of Ga and Zr on GBD were investigated in this work.The results show that the Zr-doped magnet has the highest coercivity increment(7.97 kOe)by GBD,which is almost twice that of the Ga-doped magnet(4.32 kOe)and the magnet without Ga and Zr(3.24 kOe).Microstructure analysis shows that ZrB_(2)formed in the Zr-doped magnet plays a key role in increasing the diffusion depth.A continuous diffusion channel in the magnet can form because of the presence of ZrB_(2).ZrB_(2)can also increase the defect concentration in GB phases,which can facilitate GBD.Although Ga can also improve the diffusion depth,its effect is not very obvious.The micromagnetic simulation based on the experimental results also proves that the distribution of Tb in the Zr-doped magnet after GBD is beneficial to coercivity.This study reveals that the doping elements Ga and Zr in Nd–Fe–B play an important role in GBD,and could provide a new perspective for researchers to improve the effects of GBD.

Thermal stability improvement and microstructure optimization of high cobalt content Nd-Fe-B magnets via terbium grain boundary diffusion

The substitution of Fe by Co in the 2:14:1 phase is an effective method to increase the Curie temperature and enhance the thermal stability of the Nd-Fe-B magnets.However,the accumulation of Co ele ment at the grain boundaries(GBs) changes the GBs from nonmagnetic to ferromagnetic and causes the thinlayer GBs to become rare,In this paper,the method of diffusing Tb element was chosen to improve the microstructure and temperature stability of high-Co magnets.Three original sintered Nd_(28.5)Dy_(3)-CO_(x)e_(bal)M_(0.6)B_(i)(x=0,6 wt%,12 wt%;M = Cu,Al,Zr) magnets with different Co contents were diffused with Tb by grain boundary diffusion(GBD).After GBD,high-Co magnets exhibit more continuously distributed thin-layer GBs,and their thermal stability is significantly improved.In high-Co magnets(x=6 wt%),the absolute value of the temperature coefficient of coercivity decreases from 0.603%/K to0.508%/K in the temperature range of 293-413 K,that of remanence decreases from 0.099%/K to 0.091%/K,and the coercivity increases from 18.44 to 25.04 kOe.Transmission electron microscopy(TEM)characterization reveals that there are both the 1:2 phase and the amorphous phase in the high-Co magnet before and after GBD,EDS elemental analysis shows that Tb element is more likely to preferentially replace the rare earth elements in the 2:14:1 main phase than in the 1:2 phase and the amorphous phase.The concentration of Tb at the edge of the main phase is much higher than that in the 1:2phase and amorphous phase,which is beneficial to the improvement of the microstructure.The preferential replacement of Tb elements at the edge of the 2:14:1 phase and thin-layer GBs with a more continuous distribution are synergistically responsible for improving the thermal stability of high-Co magnets.The study indicates that GBD is an effective method to improve the microstructure and thermal stability of high-Co magnets.

Effect of annealing temperature on magnetic properties of a sintered Nd-Fe-B magnet after grain boundary diffusion with Dy_(70)Cu_(30) alloy

Grain boundary diffusion process(GBDP)has been developed as an effective approach to increase the coercivity of sintered Nd-Fe-B magnets by regulating the compositions and phase distributions near grain boundaries.This work aims to explore how to select the optimum annealing temperature after GBDP.In this work GBDP was performed on a sintered Nd-Fe-B magnet using Dy_(70)Cu_(30) alloy.After GBDP the low eutectic temperature of the grain boundary phases decreases from the initial 492 to 451℃.The magnetic property dependent on different annealing temperatures near the low eutectic temperature was studied.The magnetic properties,especially the squareness factor of demagnetization curve show a strong dependence on the annealing temperature.After GBDP the optimal magnetic properties can be obtained after annealing just above the low eutectic temperature of the grain boundary phases.The mechanism is discussed based on the microstructure analysis.

Effects of grain boundary diffusion process on magnetic properties enhancement and microstructure evolution of hot-deformed Nd-Fe-B magnets

Grain boundary diffusion(GBD)process is an important approach for producing Nd-Fe-B magnets with high coercivity and high thermal stability.The GBD for hot-deformed Nd-Fe-B magnets with nanocrystalline micro structure is more complicated compared to sintered magnets.Here,we investigated the effects of different GBD methods,i.e.,intergranular addition(in-situ GBD 1#),in-situ GBD from magnet surface during hot pressing and hot deformation(in-situ GBD 2#),and conventional GBD,on the magnetic properties and microstructure of hot deformed magnets.After the treatment by these three GBD approaches using 2 wt%Pr_(40)Tb_(30)Cu_(30)diffusion source,the coercivity of the hot-deformed magnet increases from 1281 to 1567,1412 and 2022 kA/m,respectively.The coercivity enhancement is attributed to the formation of local(Nd,Tb)2Fe14B phase with stro ng magnetic anisotropy.Reduced grain orientation is found in both in-situ GBD 1#and conventional GBD treated samples mainly due to the local stress state variation and the rotation of platelet grains.Interestingly,the in-situ GBD 2#processed sample has a high orientation at diffusion surface,which may be caused by the modified surface state of the magnet by the diffusion source.Compared with the in-situ GBD processes,the conventional GBD exhibits a higher utilization efficiency of Tb.Since the in-situ GBD is effective to treat thick hot-deformed magnets,further effort should be aimed at enhancing its diffusion efficiency.

Effect of aluminum on microstructure and magnetic properties of sintered Nd-Fe-B magnets processed by grain boundary diffusion of Tb-Al

The grain boundary diffusion process(GBDP)of Tb can improve the coercivity of sintered Nd-Fe-B magnets.In this study,the effect of AI on the diffusion of Tb in the GBDP was investigated.The content of diffused Tb-Al was precisely controlled by adjusting the magnetron sputtering process.The Tb equivalent of Al was also studied.Results show that AI promotes the diffusion of Tb deeper into the magnet,reducing the thickness of the shell in the core-shell structure.This study is helpful for further developing the process,reducing the consumption of heavy rare earth elements(Tb),and improving the coercivity of sintered Nd-Fe-B magnets.

Simultaneous enhancement of coercivity and electric resistivity of Nd-Fe-B magnets by Pr-Tb-Al-Cu synergistic grain boundary diffusion toward high-temperature motor rotors

To high-power permanent magnetic motors,it is critical for Nd-Fe-B magnets to maintain the desirable coercivity at high-temperature operating conditions.To address this,two approaches have been proven effective:(1)enhancing the room temperature coercivity;(2)reducing the eddy current loss.However,these two items are difficult to be simultaneously achieved.Here,the grain boundary diffusion(GBD)of the Pr-Tb-Al-Cu-based source is applied to enhance the coercivity and electric resistivity at room temperature from 1101 kA m-1 and 2.13×10–6Ωm to 1917 kA m-1 and 2.60×10–6Ωm,and those at 120°C from 384 kA m-1 and 4.31×10–6Ωm to 783 kA m-1 and 4.86×10–6Ωm,respectively.Such optimization is ascribed to the improved formation depth of Tb-rich 2:14:1 shells with large magnetocrystalline anisotropy and the increased intergranular Pr-based oxides with high electric resistivity,induced by the coordination effects of Tb and Pr,as proven by the atomic-scale observations and the first principles calculations.It thus results in the simultaneously improved output power and energy efficiency of the motor because of the combination of magnetic thermal stability enhancement and eddy current loss reduction,as theoretically confirmed by electromagnetic simulation.

Recent progress of grain boundary diffusion process for hot-deformed Nd-Fe-B magnets

Grain boundary diffusion process(GBDP)was first proposed for sintered Nd-Fe-B magnets to achieve the high utilization efficiency of heavy rare earth elements.Recent success of fabricating high performance nanocomposite magnets by GBDP indicates that this method also exerts huge applicable potential on hot-deformed Nd-Fe-B magnets.In this review,the development and magnetic property enhancement mechanisms of different diffusion methods proposed on hot-deformed magnets were thoroughly elucidated.Moreover,the improve room for further property enhancement and the accompanying problems of GBDP on hot-deformed magnets are also discussed in this article.

Coercivity,microstructure,and thermal stability of sintered Nd-Fe-B magnets by grain boundary diffusion with TbH3 nanoparticles

Grain boundary diffusion technique with TbH3 nanoparticles was applied to fabricate Tb-less sintered NdFe-B permanent magnets with high coercivity. The magnetic properties and microstructure of magnets were systematically studied. The coercivity and remanence of grain boundary diffusion magnet are improved by 112% and reduced by 26% compared with those of the original magnet, respectively. Meanwhile, both the remanence temperature coefficient（α） and the coercivity temperature coefficient（β） of the magnets are improved after diffusion treatment. Microstructure shows that Tb element enriches in the surface region of Nd2Fe（14）B grains and is expected to exist as（Nd,Tb）2Fe（14）B phase. Thus, the magneto-crystalline anisotropy field of the magnet improves remarkably. As a result, the sintered Nd-FeB magnets by grain boundary diffusion with TbH3 nanoparticles exhibit enhanced coercivity.

Coercivity enhancement of hot-deformed NdFeB permanent magnets with AlCuZn eutectic alloy grain boundary diffusion

It is approved that grain boundary diffusion is an effective method to increase the coercivity of hot-deformed NdFeB magnet.In this paper,a new rare earth-free grain boundary diffusion source of hot-deformed magnet was studied.AlCuZn powders blended with commercial NdFeB powders were hot-compacted to obtain fully dense magnets,hot-deformed into anisotropic magnets and finally annealed to gain better homogeneity.Initially,the influences of annealing temperature and time on the magnetic properties of the specimens were studied and the optimal parameters of 600℃ and 60 min were achieved.Then,by changing the proportions of AlCuZn grain boundary diffusion,the coercivity,remanence and maximum energy product of the hot-deformed NdFeB magnets were examined.The result showed that with 1.0 wt%AlCuZn grain boundary diffusion and annealing at 600℃ for 60 min,the coercivity rose from 828 to 987 kA·m^(-1) without deteriorating the remanence.Microstructural analysis confirmed that AlCuZn diffused into the intergranular boundaries and the magnet diffused with AlCuZn possessed finer grains than that of without AlCuZn grain boundary diffusion.

Influence of Pr-Al-Co alloy diffusion source on magnetic properties and microstructure of sintered Nd-Fe-B magnets processed by grain boundary diffusion

In this study,the influence of the content of Al and Co in the diffusion source on the magnetic performance and microstructure of the diffused magnet was studied by grain boundary diffusion treatment with Pr_(70)Al_(30-x)Co_(x)(x=0 at%,10 at%,15 at%,20 at%,30 at%)alloys.When the Co content in the diffusion source increases from 0 at%to 10 at%,the coercivity enhancement in the Pr_(70)Al_(20)Co_(10)diffused magnet is the highest,increased from 1.62 to 2.24 T,higher than 2.01 T of the Pr_(70)Al_(30)diffused magnet.With further increase of Co content in the diffused source,the coercivity of the diffused magnet decreases gradually,the coercivity of Pr_(70)Al_(15)Co_(15),Pr_(70)Al_(20)Co_(10)and Pr_(70)Co_(30)diffused magnet is 2.15,1.99 and1.81 T,respectively.Microstructural analysis shows that plenty of continuous grain boundary phases(CGBPs)can be formed in the Pr_(70)Al_(20)Co_(10)diffused magnet under the synergistic effect of Al and Co,which leads to the enhancement of magnetic isolation between more adjacent grains.However,the amount of CGBP in the diffused magnets gradually decreases with the further increase of Co content in the diffusion source.

Grain boundary diffusion and viscous flow governed mechanical relaxation in polycrystalline materials

Grain boundary(GB) diffusion and viscous flow play dominant roles in mechanical relaxation of polycrystalline materials. The pioneering work of Zener and Kê, by accounting for relaxation in GBs by viscous shearing, predicts a single peak in the internal friction spectrum. Later investigations show the existence of two to three peaks in the internal friction spectrum when taking into account both GB diffusion and viscous flow for dissipation. In this paper, we further identify the characteristic relaxation modes in polycrystalline materials. We illustrate that competitive viscous flow and diffusion for normal stress relaxation give rise to distinct dependence of relaxation time on grain size. We construct an internal friction spectrum mapping based on the competitive deformation mechanisms including viscous flow in both normal and tangential directions and GB diffusion. The essential features of internal friction spectrum of polycrystalline materials from our analysis are consistent with available experimental observations. These findings may also be applicable to study relaxation dynamics of other material systems such as metallic glasses and porous materials.

Correlation of Magnetic Properties of Co/Cr Bilayer Thin Films with Grain Boundary Diffusion

The microstructure and magnetic properties of Co/Cr bilayer films were examined before and after postdeposition annealing by using transmission electron microscopy （TEM）, X-ray diffraction （XRD） technique and vibrating sample magnetometer （VSM）. A model of grain boundary （GB） Cr-rich phase growth involving GB diffusion derived from the Cr underlayer was proposed to elucidate the kinetics of the paramagnetic Cr-rich phase growth along Co GBs within the Co layer. The correlation of the GB Cr-rich phase formation with the magnetic Co grain isolation and accordingly, improvement of magnetic properties was experimentally investigated and discussed in detail. Our analysis results are well consistent with previous micromagnetic simulations on the improvement of magnetic properties by the magnetic grain isolation. The results provide some insights into the processing-structure-property relationships of the Co/Cr bilayer films, and thus suggest that the magnetic grain isolation be feasible not only in longitudinal recording media, but also be effective in tuning the exchange coupling of magnetic grains in perpendicular recording media via the GB diffusion from underlayer and/or overlayer.

Effect of TbF_(3)diffusion on the demagnetization behavior and domain evolution of sintered Nd-Fe-B magnets by electrophoretic deposition

We studied the magnetic properties and domain evolution of annealed and TbF3-diffused sintered Nd-Fe-B magnets using the electrophoretic deposition method.After TbF_(3)diffusion,the coercivity increased significantly by 9.9 kOe and microstructural analysis suggested that Tb favored the formation of the(Nd,Tb)_(2)Fe_(14)B shell phase in the outer region of the matrix grains.The first magnetization reversal and the dynamic successive domain propagation process were detected with a magneto-optical Kerr microscope.For the TbF_(3)-diffused magnet,the magnetization reversal appeared at a larger applied field and the degree of simultaneous magnetization reversal decreased compared with an annealed magnet.During demagnetization after full magnetization,the occurrence of domain wall motion(DWM)in the reproduced multi-domain regions was observed by the step method.The maximum polarization change resulting from the reproduced DWM was inversely related to the coercivity.The increased coercivity for the diffused magnet was mainly attributed to the more difficult nucleation of the magnetic reversed region owing to the improved magneto-crystalline anisotropy field as a result of Tb diffusion.

Diffusion behavior of helium in titanium and the effect of grain boundaries revealed by molecular dynamics simulation

The microstructures of titanium（Ti）, an attractive tritium（T） storage material, will affect the evolution process of the retained helium（He）. Understanding the diffusion behavior of He at the atomic scale is crucial for the mechanism of material degradation. The novel diffusion behavior of He has been reported by molecular dynamics（MD） simulation for the bulk hcp-Ti system and the system with grain boundary（GB）. It is observed that the diffusion of He in the bulk hcp-Ti is significantly anisotropic（the diffusion coefficient of the [0001] direction is higher than that of the basal plane）,as represented by the different migration energies. Different from convention, the GB accelerates the diffusion of He in one direction but not in the other. It is observed that a twin boundary（TB） can serve as an effective trapped region for He.The TB accelerates diffusion of He in the direction perpendicular to the twinning direction（TD）, while it decelerates the diffusion in the TD. This finding is attributable to the change of diffusion path caused by the distortion of the local favorable site for He and the change of its number in the TB region.

Diffusion of Hydrogen along the Grain Boundaries in Ni_3Al Alloys

The diffusivity of hydrogen in two Ni3Al alloys (No.1 and No.2) has been measured in the temperature range of 100 degreesC to 420 degreesC using an ultrahigh vacuum gaseous permeation technique. The diffusivity data fall into two segments, in which the hydrogen diffusivity adheres to the Arrhenius form, respectively. From the hydrogen diffusivity, it is conjectured that the hydrogen diffusivity reflects the hydrogen transportation along the grain boundaries at lower temperature and the hydrogen transportation in the lattice at higher temperature. The intergranular fracture of Lit-type intermetallics induced by hydrogen at relative low temperature results from hydrogen transportation along the grain boundaries and not in the lattice.

Pr-Tb synergistic strengthening on coercivity of Nd-Ce-Fe-B magnets grain boundary diffused with Pr-Tb-Al-Ga alloys

Sintered Nd-Ce-Fe-B magnets were grain boundary diffused(GBDed) with Pr_(x)Tb_(80-x)Al_(10)Ga_(10)(at%)(x=0,20,40,60,80) alloys.The effect of Pr/Tb content in diffusion source on magnetic properties,microstructure and elements distribution of GBDed magnets was investigated.When Pr is used to substitute for 75% Tb in diffusion source,Tb consumption per unit coercivity improvement of GBDed magnet reduces by 77%,compared with the Tb_(80)Al_(10)Ga_(10) diffused magnet.Tb element diffuses into magnets and then forms Tb-rich shell with high magneto-crystalline anisotropy field surrounding main phase grains,resulting in substantial coercivity improvement.Pr with low melting point diffuses deeply along liquid grain boundary phase during GBD process.It can eliminate some sharp defects of main phase grains and make grain boundaries smooth,which provides diffusion channels for further diffusion of Tb element.Therefore,there are more diffusion channels for Tb and less Tb enriched at surface region,making Tb diffuse more deeply and improving Tb utilization efficiency.This method significantly improves the coercivity,and realizes the green,efficient and high-quality utilization of heavy rare earth(HRE)elements.

Grain boundary construction and properties enhancement for hot deformed(Ce,La,Y)-Fe-B magnet by a two-step diffusion process

The rare earth-iron-boron magnets based on high abundance rare earths(REs)show potential for costeffective permanent magnets but their hard magnetic properties have to be greatly improved.The grain boundary diffusion process(GBDP)is known as an effective way to improve the coercivity of Nd-Fe-B magnets,however,the conventional diffusion method faces a challenge for Ce-based magnets since there is no enough continuous GB layer as the diffusion channel.Here,a two-step(Nd-Cu doping followed by Nd-Cu diffusion)GBDP was introduced for hot deformed(Ce,La,Y)-Fe-B magnet,and the excellent magnetic properties ofμ0Hc=0.63 T,μ0Mr=0.68 T,and(BH)max=72.4 kJ/m^(3)were achieved.The Nd-Cu doping helps the formation of RE-rich GB layer,and then it acts as the diffusion channel for increasing the ef-ficiency of the subsequent Nd-Cu diffusion and results in the increased volume fraction of continuously distributed GB phase,whose paramagnetism was verified by 57Fe Mössbauer spectrometry.Those paramagnetic GB phases help to form the discontinuous domain walls,as observed by Lorentz transmission electron microscopy,and break the magnetic exchange coupling of RE2Fe14B grains.It thus contributes to the coercivity enhancement of the hot deformed magnet with two-step diffusion,which is further proved by micromagnetic simulation.This study proposes a potential technique to prepare anisotropic hot deformed(Ce,La,Y)-Fe-B magnet with high cost-performance.

Development of non-rare earth grain boundary modification techniques for Nd-Fe-B permanent magnets

The magnetic performance of Nd-Fe-B magnets depends on their grain boundary structure.Intergranular addition and grain boundary diffusion(GBD)process are effective approaches for enhancing coercivity with low material cost.This review summarizes the development of grain boundary modification techniques with emphasis on our recent work using cost-effective non-rare earth(non-RE)sources for GBD.Up to now,heavy rare earth(HRE)based compounds,metals and light rare earth(LRE)based alloys have been successfully employed as the diffusion sources for coercivity enhancement.Inspired from the previous investigations on the intergranular addition of non-RE compounds and alloys for Nd-Fe-B magnets,in 2015,we firstly proposed a novel GBD process based on diffusion source of Mg O.After that,various non-RE diffusion sources have been developed.The fundamentals of non-RE additives and non-RE diffusion sources for hard magnetic properties enhancement of Nd-Fe-B magnets are summarized here based on both the experimental and computational results.In particular,the properties-microstructure relationships of non-RE GBD modified magnets are discussed.The non-RE alloys or compounds modify the composition and structure of the grain boundary by diffusing into the intergranular regions,resulting in enhanced coercivity and corrosion resistance.Recently,we used Al-Cr coatings for both coercivity enhancement and surface protection,which shortens the production process and makes non-RE diffusion sources more competitive.The opportunity and future directions for non-RE GBD are also discussed in this review.

Coercivity enhancement mechanism of grain boundary diffused Nd-Fe-B sintered magnets by magnetic domain evolution observation

The grain boundary diffusion(GBD) technology was used to prepare high performance Nd-Fe-B sintered magnets by NdH3 and TbH3 nanoparticle diffusion.The factors affecting the coercivity of GBD magnets include distribution of rare earth rich grain boundary phase and substitution of the heavy rare earth.In order to distinguish the influence of various factors on the coercivity,the microstructure and magnetic domain evolution of the original,reference,Nd-diffused,and Tb-diffused magnets were analyzed.The core-shell structure formed by heavy rare earth substitution is the main factor of coercivity enhancement,and it can transform the magnetic domain reversal mode from easy-nucleation(EN) to difficultnucleation(DN).With increasing the diffusion depth,the shell of the core-shell structure gradually becomes thinner,DN grains gradually decrease while the EN grains gradually increase,indicating that the magnetic domain reversal mode is directly related to the core-shell structure.