Improved corrosion resistance and thermal stability of sintered Nd-Fe-B magnets with holmium substitution

The effects of Ho substitution for Nd on the microstructure, corrosion resistance and thermal stability of the Nd-Fe-B magnets were investigated. The(Nd,Ho)-O phase was formed with increasing Ho substitution. The results of potentiodynamic polarization and highly accelerated stress test show improved corrosion resistance with increasing Ho substitution. The optimum mass loss 0.29 mg/cm~2 is achieved.Moreover, the average temperature coefficients for remanence and coercivity in the range of 25-150℃are both closer to zero, indicating improved thermal stability. The mechanisms for the improved corrosion resistance and thermal stability are discussed in relation to the microstructure featuring the(Nd,Ho)-O phase.     

Recycling of waste Nd-Fe-B sintered magnets by doping with dysprosium hydride nanoparticles

Recycling of waste sintered Nd-Fe-B permanent magnets by doping DyH3 nanoparticles was investigated. The effect of the DyH3 nanoparticles on the microstructure and magnetic properties of the recycled m...     

Improvement of coercivity and thermal stability of Nd-Fe-B sintered magnets by intergranular addition of Tb80Fe20 alloy

To improve the coercivity and temperature stability of Nd-Fe-B sintered magnets for high-temperature applications, the eutectic Tb₈₀Fe₂₀ (wt%) alloy powders were added into the Nd-Fe-B sintered magnets by intergranular method to enhance the coercivity (H_cj) and thermal stability. The microstructure, magnetic properties and thermal stability of the Nd-Fe-B magnets with different Tb₈₀Fe₂₀ contents were studied. The experimental results demonstrate that the coercivity (H_cj) of the sintered Nd-Fe-B magnet is significantly enhanced from 14.12 to 27.78 kOe, and the remanence (B_r) decreases not obviously by introducing 4 wt% Tb₈₀Fe₂₀ alloy. Meanwhile, the reversible temperature coefficients of coercivity (β) and remanence (α) of the Nd-Fe-B magnets are increased from ?0.5634%/℃ to ?0.4506%/℃ and ?0.1276%/℃ to ?0.1199%/℃ at 20–170 ℃, respectively. The Curie temperature (T_C) of the Nd-Fe-B magnet is slightly enhanced with the increase of Tb₈₀Fe₂₀ content. Moreover, the irreversible flux magnetic loss (h_irr) is obviously reduced as Tb₈₀Fe₂₀ addition increases. Further analysis of the microstructure reveals that a modified microstructure, i.e. clear and continuous RE-rich grain boundary layer, is acquired in the sintered magnets by introducing Tb₈₀Fe₂₀ alloy. The associated mechanisms on improved coercivity and thermal stability were comprehensively researched.     

Effect of niobium on thermal stability and impact toughness of Nd-Fe-B magnets with ultra-high intrinsic coercivity

The effects of Nb on the thermal stability and impact toughness of ultra-high intrinsic coercivity of Nd-Fe-B magnets were investigated.The results showed that the addition of Nb could improve the thermal stability,and obviously increased the impact toughness of sintered Nd-Fe-B magnets.The optimum thermal stability of sintered Nd-Fe-B magnets was obtained when the content of Nb was 1.0 at.%.The maximum impact toughness of sintered Nd-Fe-B magnets was obtained when the content of Nb was 1.5 at.%,but the magnetic properties of sintered Nd-Fe-B magnets drastically deteriorated when the content of Nb increased from 1.0 at.% to 1.5 at.%.The microstructure showed that overfull Nb addition made many ultra-fine grains get together,which led to the density of sintered Nd-Fe-B magnets decline and drastically deteriorated the magnetic properties of sintered Nd-Fe-B magnets.     

Improvement of microstructure and coercivity for Nd-Fe-B sintered magnets by boundary introducing low melting point alloys

Different from the grain boundary diffusion process(GBDP),which is suitable for modifying thin magnet,a green-pressing agents permeation process(GAPP) that uses low melting point alloys was applied to the Nd-Fe-B green compact with a thickness over 15 mm to reconstruct the boundary microstructure of a sintered Nd-Fe-B magnet.The coercivity increases from 12.3 kOe for the sample free of Pr_(80)Al_(20) to16.8 kOe for the sample with 2 wt% Pr_(80)Al_(20).By further increasing the Pr_(80)Al_(20) content to 3 wt%,the coercivity increases slightly,but the remanence and H_k/H_(cj) deteriorate obviously.The optimal comprehensive properties of H_(cj)=16.8 kOe,B_r=13.4 kG and H_k/H_(cj)=0.975 are obtained at 2 wt% Pr_(80)Al_(20),since matrix phase grains are separated by relatively continuous thin grain boundary layers,which weaken the magnetic coupling between adjacent grains.The coercivities of the samples from the GAPP that use2 wt% Pr_(80)Al_(20),Pr_(70)Al_(30) and Pr_(60)Tb_(20)Al_(20) alloys,respectively,can be enhanced to a large extent.However,the coercivity of the magnet reconstructed with Pr_(80)Al_(20) is lower than that of the sample with Pr_(60)Tb_(20)Al_(20) but is higher than that of the sample reconstructed with Pr_(70)Cu_(30) alloy.Moreover,the coercivity of the sample from the GAPP using 2 wt% Pr_(80)Al_(20) is much higher than that of the sample from the GBDP,which is due to a nearly uniform boundary microstructure from the surface to the interior of the thick magnet from the GAPP,thus providing new insights into the fabrication of thick and bulky permanent magnets with high coercivity.     

Microstructural investigation of Nd-rich phase in sintered Nd-Fe-B magnets through electron microscopy

The distribution, morphologies and structures of intergranular Nd-rich phase in sintered Nd-Fe-B magnets were studied through electron microscopy. Backscattered electron (BSE) imaging revealed that Nd-rich particles with various morphologies and sizes were randomly distributed at the grain boundaries and the triple junctions of the tetragonal Nd2Fe14B matrix. Through selected area electron diffraction (SAED) analysis under a systematic tilting condition, most intergranular Nd-rich phase particles, with sizes ranging from hundreds of nanometres to several micrometres, were identified as face-centred cubic (FCC) structure. Such particles possessed several approximate orientation relationships with their adjacent Nd2Fe14B matrix grains, such as (002) Nd2Fe14B//(200) FCC_Nd-rich [120] Nd2Fe14B//[001] FCC_Nd-rich , (002) Nd2Fe14B//( 220 ) FCC_Nd-rich [110] Nd2Fe14B//[112] FCC_Nd-rich , as well as (011) Nd2Fe14B//(131) FCC_Nd-rich [111] Nd2Fe14B//[114] FCC_Nd-rich , which could be attributed to minimising interfacial energy. The combination of highresolution electron microscopy with energy-dispersive X-ray spectroscopy revealed the internal inhomogeneous nature of Nd-rich phases. The large lattice distortion and nanoscale-ordered structures within a single Nd-rich grain were observed.     

Effect of Terbium addition on the coercivity of the sintered NdFeB magnets

The effects of Tb addition on the microstructure and magnetic properties of the NdFeB magnets prepared by HD method were investigated by X-ray diffraction(XRD) and BH magnetometers.The results of the microstructure showed that both the Tb-doped and undoped permanent magnets were composed mostly of Tetragonal phase Nd2Fe14B(space group P42/mnm) and a trace amount of Nd-rich phase.Accordingly,addition of Tb led to a decrease of the pole density factor of(004),(006) and(008) crystal plane of the Nd2Fe14B phase calculated by Horta formula,but the coercivity of the magnets increased from 2038 kA/m up to 2302 kA/m as a consequence of Tb addition.The study of the Hc(T)/Ms(T) versus/Ms(T) behavior showed that the nucleation was the dominating mechanism for the magnetization reversal in both sintered magnets,and the microstructural parameters of αk and Neff were obtained also.The Kronmüller-plot showed an increase of the αk responsible for an increase of the coercivity.     

Recycled Nd-Fe-B sintered magnets prepared from sludges by calcium reduction-diffusion process

Environmental friendly recycling process for Nd-Fe-B sintered magnet sludges generated in the manufacturing process, which contain large amount of rare earth, including Nd, Pr and Dy, is badly needed so far. In present study, we have developed an effective route to obtain recycled sintered magnets from Nd-Fe-B sintered magnet sludges by calcium reduction-diffusion (RD) process. Compared to conventional recycling process, our research is focused on recovering most of the useful elements, including Nd, Pr, Dy, Co, and Fe together instead of just rare earth elements. To improve the recycling efficiency and reduce pollution, the co-precipitating parameters were simulated and calculated using MATLAB software. Most of useful elements were recovered by a co-precipitation method, and the obtained composite powders were then directly fabricated as recycled Nd-Fe-B powders by a calcium reduction-diffusion (RD) method. The recovery rates are 98%, 99%, 99%, 93%, and 99%, for Nd, Pr, Dy, Co, and Fe, respectively. The amount of useful elements contained in the recovered composite powders is greater than 99.71 wt%. The process of RD for synthesizing Nd₂Fe₁₄B and subsequently removing CaO was thoroughly investigated. Furthermore, the recycled Nd-Fe-B magnet exhibits a remanence of 1.1 T, a coercivity of 1053 kA/m, and an energy product of 235.6 kJ/m³, respectively, indicating that recycled Nd-Fe-B sintered magnet was successfully recovered from the severely contaminated sludges via an effective recycling route.     

Improvement in magnetic properties,corrosion resistance and microstructure of Nd–Fe–B sintered magnets through intergranular addition of Tb68Ni32

New energy vehicles and offshore wind power industries have a high demand for sintered Nd–Fe–B magnets with high intrinsic coercivity and high corrosion resistance. In this study, the magnetic properties, anticorrosion properties, and microstructure of Nd–Fe–B sintered magnets with the intergranular addition of low-melting-point eutectic Tb₆₈Ni₃₂ alloy powders were investigated. The aim is to determine if the addition of Tb₆₈Ni₃₂ can improve these properties. A low melting-point eutectic alloy Tb₆₈Ni₃₂ powders was prepared as a grain boundary additive and blended with the master alloy powders prior to sintering. The coercivity of the resultant magnets gradually increases from 1468 to 2151 kA/m by adding increasing amounts of Tb₆₈Ni₃₂. At the same time, the remanence first increases and then slightly decreases. After studying the microstructure and elemental composition of the Tb₆₈Ni₃₂ added magnets, it is found that the significant increase in coercivity and the negligible reduction in remanence is due to densification, improved grain orientation, a uniform and continuous boundary phase distribution, as well as the generation of a (Nd,Pr,Tb)₂Fe₁₄B “core–shell” structure surrounding the main-phase grain. Moreover, the corrosion resistance of the magnet is greatly improved owing to the enhancement of electrochemical stability, as well as the optimization of the distribution and morphology of the intergranular phase.     

Optimization of core-shell structure distribution in sintered Nd-Fe-B magnets by titanium addition

In view of the uneven distribution of the core-shell structure of sintered Nd-Fe-B magnets after grain boundary diffusion,this study proposes to use high-melting-point and reactive element titanium(Ti) as an additive to increase the diffusion channels and to enhance the diffusion of heavy rare earth elements along the grain boundary phase.By adding Ti element,the diffusion depth and hence the intrinsic coercivity of magnets are increased significantly.The addition of Ti increases the coercivity ...     

烧结Nd-Fe-B磁体腐蚀动力学行为

研究了不同成分的烧结Nd-Fe-B磁体在恒定湿热和中性盐雾腐蚀环境中的腐蚀动力学行为,讨论了不同合金成分和腐蚀条件对磁体腐蚀速率的影响,探讨了烧结Nd-Fe-B磁体在不同介质中的电化学行为.结果表明:烧结Nd-Fe-B磁体在湿热环境中的重量变化表现出明显的抛物线衰减规律,而在盐雾腐蚀环境中的重量变化表现出先增后减的规律.以重稀土Dy替代少量Nd,并添加少量Co等元素,可以提高烧结Nd-Fe-B磁体的耐湿热和耐盐雾腐蚀性能.     

Microstructure and corrosion resistance of sintered NdFeB magnet modified by intergranular additions of MgO and ZnO

Microstructure and corrosion resistance of sintered Nd15Dy1.2Fe77Al0.8B6 and Nd22Fe71B7 magnets modified by intergranular addition of MgO and ZnO were investigated. Both the remanence and sintering density of the magnets increased slightly with intergranular additions of MgO and ZnO. There was a remarkable increase in coercivity of Nd22Fe71B7 after addition. Besides, the effects on magnetic properties and an improved corrosion resistance were observed. Compared with the native magnets without addition, corrosion potential of the magnets with MgO and ZnO additives was more positive and the current density in the anodic branch of the polarization curve was reduced. Corrosion resistance resulting from autoclave testing （2×10^5 Pa of steam pressure, 120 ℃） showed that the corrosion rate of NdFeB magnets reduced with the increase of additive amount. Microstructure observation revealed that MgO and ZnO additives were incorporated into the intergranular phases in the magnets. With the introduction of MgO and ZnO, more intergranular phase with high oxygen content was formed while keeping the volume fraction of all the intergranular phases almost unchanged, which may contribute to improved corrosion resistance. Furthermore, addition of MgO and ZnO refined the grain size of Nd22Fe71B7.     

Effect of yttrium substitution on magnetic properties and service performances of Nd-Fe-B sintered magnets

We successfully fabricated partial Y substituted NdY-Fe-B magnets with nominal compositions of(Nd_1-xY_x)_13.80Fe_ba1Al_0.24Cu_0.1B_6.04(at%,x=0,0.1,0.2,0.3,0.4) by powder metallurgy process and the magnetic properties as well as service performances of the magnets were also systematically investigated.The phase constituents of the magnets have no obvious variation within the whole range of Y content,while the main phase grain...     

Effects of Pr-Cu-Ti intergranular addition on microstructure and magnetic properties of heavy-rare-earth-free Nd-Fe-B sintered magnets

By intergranular addition of Pr-Cu-Ti alloy powders in the Nd-Fe-B sintered magnets with the normal B component, we propose an approach to the optimization of grain boundary and local Nd-Fe-B composition system. The coercivity is enhanced from 1.42 to 1.86 T, while further addition leads to a reduction in remanence and coercivity. The analyses of phase composition reveal that Ti mainly exists in the form of metallic Ti alloy, and part of Ti combines with B to form the TiB₂ phase after the liquid phase sintering process. This process results in a consumption of B in the local Nd-Fe-B composition system and a change of the grain boundary component, which contributes to the formation process of the RE₆(Fe,M)₁₄ phase after the annealing process. Therefore, with the modification of grain boundary and composition system, the intergranular addition of Pr-Cu-Ti induces the generation of continuous thin grain boundary phases. It promotes the intergrain exchange decoupling, increasing the coercivity in the annealed magnet. While the excess addition results in the segregation of TiB₂, as well as the precipitation of TiB₂ into the Nd-Fe-B phase, which leads to structural defects. Thus, the further effort for the addition alloy with Ti to reduce the deterioration of the microstructure will lead to further improvement in magnetic properties.     

Magnetic properties and microstructures of terbium coated and grain boundary diffusion treated sintered Nd-Fe-B magnets by magnetron sputtering

Tb coating on the surface of commercial sintered Nd-Fe-B magnet was prepared by DC magnetron sputtering.The secondary heat treatment was used to regulate the microstructure for the enhancement of coercivity,namely diffusion treatment and annealing treatment.The coercivity increases significantly from 18.3 to 28.0 kOe,the remanence decreases slightly from 14.1 to 14.0 kGs,and the comprehensive magnetic properties are higher than 75(Hcj+(BH)_max=76.7).SEM results indicate that,on the one hand,950℃is the optimal diffusion temperature.Lower diffusion temperature results in insufficient diffusion of Tb element.Higher diffusion temperature can lead to the main phase grain growth,the decrease of Nd-rich phase,and forming holes in the magnet.On the other hand,500℃is the optimal annealing temperature.Lower annealing temperature can result in the reduction of Nd-rich phase.Higher annealing temperature can generate the non-defined Nd-rich thin layer between grains.     

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 Pr₇₀Al_30–xCo_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 Pr₇₀Al₂₀Co₁₀ diffused magnet is the highest, increased from 1.62 to 2.24 T, higher than 2.01 T of the Pr₇₀Al₃₀ diffused magnet. With further increase of Co content in the diffused source, the coercivity of the diffused magnet decreases gradually, the coercivity of Pr₇₀Al₁₅Co₁₅, Pr₇₀Al₂₀Co₁₀ and Pr₇₀Co₃₀ diffused magnet is 2.15, 1.99 and 1.81 T, respectively. Microstructural analysis shows that plenty of continuous grain boundary phases (CGBPs) can be formed in the Pr₇₀Al₂₀Co₁₀ 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.     

Relationship between controllable preparation and microstructure of NdFeB sintered magnets

The double hard magnetic phase magnets with nominal compositions of Nd_30–xDy_xFe₆₉B₁(x=2, and 4) (wt.%) were prepared. The magnetic properties of the magnets were measured with a NIM-2000H hysteresigraph. The crystalline structures of the magnets were identified by X-ray diffraction (XRD). The Rietveld refinement was carried out using the FULLPROF software. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses were carried out in order to investigate the microstructure of the magnets. It showed that the magnets consisted mainly of Nd₂Fe₁₄B phase, and some Nd-rich phase. Two types of matrix-phase grains in dark grey and light grey were found in the magnets with x=2 and 4. The Dy content was obviously different in the two types of grains, which proved that the double hard magnetic phases (Dy-rich and Dy-lean phases) coexisted in the magnet. It revealed that the Nd-rich phases in junction regions had fcc structure, with the unit cell parameter of about 0.52–0.56 nm. The weak superlattice spots were found in the SAD patterns of the junction Nd-rich phases with large scale. The double hard magnetic phase structure seemed to improve the magnetic properties of NdFeB magnets with high coercivity, while decrease the consumption of Dy element, compared with the single alloy magnet.     

Enhanced mechanical properties in die-upset Nd-Fe-B magnets via die-upsetting process

The mechanical properties of die-upset Nd-Fe-B magnets produced at different die-upset processes were investigated. The results showed that the optimum comprehensive mechanical properties of die-upset Nd-Fe-B magnets were obtained at the deformation temperature of 680 oC. The anisotropy of Vickers hardness was more obvious at the die-upset level of 55%, and the Vickers hardness measured parallel to the c-axis was significantly lower than that perpendicular to the c-axis. The fracture toughness measured parallel to the c-axis first increased, and then decreased with increase in die-upset level. The maximum fracture toughness of Nd-Fe-B magnets was obtained at the die-upset level of 60%. The microstructure showed that the width of defect layers and the average size of large grains increased, and the layered structure of die-upset Nd-Fe-B magnets was obviously different with increase in the die-upset level.     

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 NdH_3 and TbH_3 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.     

Structure and magnetic properties of bulk nanocrystalline Nd-Fe-B permanent magnets prepared by hot pressing and hot deformation

Structure and magnetic properties were studied for bulk nanocrystalline Nd-Fe-B permanent magnets that were prepared at 650 °C for 3 min under 300 MPa using the SPS-3.20-MK-V sintering machine and the hot pressed magnets were then submitted to hot deformation with height reduction of 50%,60%,70%,80%,and 85%.Effects of height reduction(HR) and deformation temperature on the structure and magnetic properties of the magnets were investigated.The crystal structure was evaluated by means of X-ray diffraction(XRD) and the microstructure was observed by transmission electron microscopy(TEM).The magnetic properties of the magnets were investigated by vibrating sample magnetometer(VSM).As the height reduction increased,the remanence(B r) of the magnets increased first,peaks at 1.3 T with HR=60%,then decreased again,and the coercivity(H ci) of the magnets decreased monotonically.On the other hand,as the deformation temperature increased,the B r of the magnets increased first,peaks at 1.36 T with HR=60%,then decreased again,and the H ci of the magnets decreased monotonically.Under optimal conditions,the hot deformed magnet possessed excellent magnetic properties as B r =1.36 T,H ci =1143 kA/m,and(BH) max =370 kJ/m 3,suggesting the good potential of the magnets in practical applications.