Temperature stability and microstructure of ultra-high intrinsic coercivity Nd-Fe-B magnets

The variations of intrinsic coercivity and remanence of sintered Nd-Fe-B magnets with ultra-high intrinsic coercivity were investigated. The results showed that the intrinsic coercivity and remanence declined simultaneously with increasing temperature, but the squareness of the magnets has hardly been changed. The temperature coefficients of remanence （α） and coercivity （β） for the magnets were calculated by two different methods, and the variations of the temperature coefficients and the microstructure of sintered Nd-Fe-B magnets were analyzed. The temperature coefficients of remanence （α） and coercivity （β） for the sintered magnets are very small, and the existence of fine microstructure is necessary to obtain sintered Nd-Fe-B magnets with ultra-high intrinsic coercivity.

Effects of Ti and(or) Cu on microstructures and magnetic properties of sintered Nd-Fe-B magnets

The alloying elements Ti and(or) Cu were added into the intergranular regions of sintered Nd Fe B magnets and their effects on microstructures and magnetic properties of the magnets were investigated. The results showed that a small amount of Ti and(or) Cu additions can enhance the coercivity and have little effect on the remanence of Nd Fe B magnets. Compared with individual addition of pure Ti or Cu elements, Ti and Cu co addition of intergranular region is more efficient to improve the coercivity of the magnets. The improvement of the coercivity can be attributed to the segregation of Cu element on the surface of the magnetic phase (Nd 2Fe 14 B) and the occurrence of fine Nd Fe Ti particles near grain boundaries. The former can prevent the magnetic coupling of Nd 2Fe 14 B grains to a certain degree and impede effectively the propagation of reversed domain walls through the magnetic phase grains. The latter can inhibit the growth of magnetic phase grains during the sintering process, resulting in a finer grain size. Both are beneficial to the coercivity enhancement. With increasing Ti content above 0.8%, a strip Ti rich phase appears in the intergranular region, resulting in the dramatic reduction of the remanence of Nd Fe B magnets.

Effect of Weak Magnetic Intergranular Phase on the Coercivity in the HDDR Nd-Fe-B Magnet

Assuming that intergranular phase (IP) existing between adjacent grains is a weak magnetic phase, we study the effect of IP on the coercivity in the HDDR Nd-Fe-B magnet. The results indicate that the coercivity increases with the increasing IP’s thickness d, but decreases with increasing its anisotropy constant K1(0). When the structure defect thickness r0 =6nm, d=1nm and K1(0)=0.15K1 (K1 is the normal anisotropy constant in the inner part of a grain), our calculated coercivity is in agreement with available experimental data.

Toward understanding the microstructure characteristics,phase selection and magnetic properties of laser additive manufactured Nd-Fe-B permanent magnets

Nd-Fe-B permanent magnets play a crucial role in energy conversion and electronic devices.The essential magnetic properties of Nd-Fe-B magnets,particularly coercivity and remanent magnetization,are significantly infuenced by the phase characteristics and microstructure.In this work,Nd-Fe-B magnets were manufactured using vacuum induction melting(VIM),laser directed energy deposition(LDED)and laser powder bed fusion(LPBF)technologies.Themicrostructure evolution and phase selection of Nd-Fe-B magnets were then clarified in detail.The results indicated that the solidification velocity(V)and cooling rate(R)are key factors in the phase selection.In terms of the VIM-casting Nd-Fe-B magnet,a large volume fraction of theα-Fe soft magnetic phase(39.7 vol.%)and Nd2Fe17Bxmetastable phase(34.7 vol.%)areformed due to the low R(2.3×10-1?C s-1),whereas only a minor fraction of the Nd2Fe14B hard magnetic phase(5.15 vol.%)is presented.For the LDED-processed Nd-Fe-B deposit,although the Nd2Fe14B hard magnetic phase also had a low value(3.4 vol.%)as the values of V(<10-2m s-1)and R(5.06×103?C s-1)increased,part of theα-Fe soft magnetic phase(31.7vol.%)is suppressed,and a higher volume of Nd2Fe17Bxmetastable phases(47.5 vol.%)areformed.As a result,both the VIM-casting and LDED-processed Nd-Fe-B deposits exhibited poor magnetic properties.In contrast,employing the high values of V(>10-2m s-1)and R(1.45×106?C s-1)in the LPBF process resulted in the substantial formation of the Nd2Fe14B hard magnetic phase(55.8 vol.%)directly from the liquid,while theα-Fe soft magnetic phase and Nd2Fe17Bxmetastable phase precipitation are suppressed in the LPBF-processed Nd-Fe-B magnet.Additionally,crystallographic texture analysis reveals that the LPBF-processedNd-Fe-B magnets exhibit isotropic magnetic characteristics.Consequently,the LPBF-processed Nd-Fe-B deposit,exhibiting a coercivity of 656 k A m-1,remanence of 0.79 T and maximum energy product of 71.5 k J m-3,achieved an acceptable magnetic performance,comparable to other additive manufacturing processed Nd-Fe-B magnets from MQP(Nd-lean)Nd-Fe-Bpowder.

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.

Influence of zirconium addition on microstructure,magnetic properties and thermal stability of nanocrystalline Nd_(12.3)Fe_(81.7)B_(6.0) alloy

Effect of Zr addition on microstructure, magnetic properties and thermal stability of Nd12.3Fe81.7-xZrxB6.0 (x=0-3.0) ribbons melt-spun and annealed was investigated. Magnetic measurement using vibrating sample magnetometer (VSM) revealed that Zr addition was significantly effective in improving the magnetic properties at room temperature. The intrinsic coercivity Hci of the optimally processed ribbons increased monotonically with increasing Zr content, from 751.7 kA/m for x=0 to 1005.3 kA/m for x=3.0. Unlike the coercivity, the remanence polarization Jr increased first with Zr addition, from 0.898 T up to 1.041 T at x=1.5, and then decreased with further Zr addition. The maximum energy product (BH)max behaved similarly, increasing from 103.1 kJ/m3 to a maximum of 175.2 kJ/m3 at x=1.5. Microstruc- ture studies using atomic force microscopy (AFM) and transmission electron microscopy (TEM) had shown a significant microstructure refinement with Zr addition. The absolute values of temperature coefficients of induction and coercivity were significantly increased with increasing Zr content, indicating that Zr was detrimental to thermal stability of the melt-spun Nd2Fe14B-type material.

Influence of dysprosium substitution on magnetic and mechanical properties of high intrinsic coercivity Nd-Fe-B magnets prepared by double-alloy powder mixed method

The double-alloy powder mixed method is very proper for developing new small-mass products by changing the composi- tion of sintered Nd-Fe-B magnets, and there is little research on this aspect. The variation on magnetic and mechanical properties of high intrinsic coercivity Nd-Fe-B magnets prepared by double-alloy powder mixed method was discussed, which is a method blend- ing two-type main phase alloy powders with different components. The results showed that the intrinsic coercivity and density of sin- tered Nd-Fe-B magnets increased gradually with the increase in Dy content, and the double-alloy powder mixed method could obtain high intrinsic coercivity Nd-Fe-B magnets with good crystallographic alignment and microstructure. The bending strength of sintered Nd-Fe-B magnets declined, and the Rockwell hardness of sintered Nd-Fe-B magnets first declined, and then increased with the in- crease in Dy content. The microstructure showed that there existed the phenomenon that the Dy element diffused into main phase dur- ing sintering process, and the distribution of Dy content in main phase had some variation in homogeneity as a result of incomplete reaction between the double-alloy powder types.

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.

THE EFFECT OF Mo ADDITION ON COERCIVITY OF NdFeB SINTERED MAGNET PREPARED BY BLENDING METHOD

Alloy modification, accompanying with proper heat treatment, is commonly used to improve the thermal stability of NdFeB magnet. Traditional alloy modification is performed through melting process with alloy elements to form the multi-alloy. In doing so, these alloy elements not only are introduced into the inter-ranular boundaries, but partly into the main phase, thus decreasing to some extent the magnetism of the main phase. In this paper, the blending method is used to prepare the Nd22Fe71B7/Mo sintered magnet, and its magnetic properties and microstractures are investigated. The results show that by adding 1.5% （mass fraction） Mo, the intrinsic coercivity 24, of the magnet reaches the maximum value of 1719.36KA/m, while continually increasing the amount of Mo has a less effect on iHc Microstructures analysis indicates that Mo-free Nd-Fe-B magnet has not uniform grains in size, while that with Mo element has uniform grains in size and smooth grain boundaries. Experiments show that after the NdFeB magnet is sintered at 1273K and annealed at 873K, the added Mo element could prevent the equilibrium transformation between the main phase and Nd-rich phase, thus resulting in the precipitation of fine second main phase （Nd2Fe14-xMoxB） from the main phase boundaries, preventing the nucleation and expansion of anti-magnetic domain, and enhancing the coercivity.

Effect of Cu doping and annealing treatment on the microstructure and mag-netic properties of nanocrystalline single-phase Nd-Fe-B alloys

The effects of Cu addition and annealing treatment on the magnetic properties and microstructure of Ndl2.3Fe81.7-xCuxB6 （x=0-1.2） ribbons melt-spun and annealed were systematically investigated by the methods of vibrating sample magnetometer （VSM）, X-ray diffraction （XRD）, and transmission electron microscopy （TEM）. Optimum magnetic properties were achieved by annealing melt-spun Nd12.3Fe81.5Cu0.2B6 ribbons at 550℃ for 15 min, which only contained Nd2Fe14B phase. The remanence, coercive force, and maximum energy product increase by 18.4%, 36.2%, and 49% respectively compared with those of Cu-free samples. The sig- nificant improvement in magnetic properties originates from the freer grains of the samples by introducing Cu, which leads to the stronger exchange-coupling between neighboring grains.

Uneven Evolution of Microstructure,Magnetic Properties and Coercivity Mechanism of Mo-Substituted Nd-Ce-Fe-B Alloys

The purpose of this paper is to study the influence of Mo addition on the phase morphologies,microstructures and magnetic properties of the designated alloys.It is found out that the coercivity H_(cj) increases unevenly from 12.2 kOe for(Nd_(0.8)Ce_(0.2))_(13)Fe_(82)B_(5) to the maximum value of 13.3 kOe for(Nd_(0.8)Ce_(0.2))_(13)Fe_(80)B_(5)Mo_(2).The transmission electron microscopy images demonstrate that the grain size decreases with the addition of Mo,which indicates that Mo has grain refinement effect.The correlative analysis gives rise to the conclusion that the coercivity mechanism of the investigated alloys is dominated by pinning type.All in all,the enhancement of the magnetic properties is mainly attributed to the synergistic impact of grain refinement,pinning effects and the micro structural homogenization.The research may shed light on the potential development and application of rare earth-based counterpart magnets.

BOUNDARY MICROSTRUCTURE AND MAGNETIC HARDENING OF SINTERED NdFeB MAGNET

The boundary microstructure of sintered alloy Nd_(15.5)Fe_(77)B_-(7.5) has been studied by TEM,AES and SAED.The boundary structure may be distinguished into 4 types.The first three types remain the same during annealing,and the fourth changes its microstructure remarkably.The 4th type is composed of two different regions,i.e.,the central Nd-rich phase and the epitaxial laver of the Nd_2Fe_(14)B grains.Owing to the atomic diffusion and other types of mass trans- port,magnetic hardening occurred in the epitaxial layer,thus the coercivity of the alloy has been improved.

Effect of CeO_(2)doping on the coercivity of 2:17 type SmCo magnets

The effects of CeO_(2)doping on the magnetic properties and microstructure of 2:17 type SmCo magnets are studied.With the increase of CeO_(2)from 0 wt.%to 3 wt.%,the coercivity of the magnets increases from 22.22 kOe to over 29.37 kOe,which is an increase of more than 30%.When the doping content is lower than 1 wt.%,the remanence and magnetic energy product of the magnets remain almost constant.Both decrease sharply as the doping concentration further increases.After CeO_(2)doping,the oxide content in the magnet increases significantly and the Ce element is uniformly distributed in the magnet.Observing the magnetic domains reveals that doping with CeO_(2)can refine the magnetic domains and make the magnetic domain wall more stable,resulting in a significant increase in the coercivity of the magnets.

Effect of heterogeneous microstructure on magnetization reversal mechanism of hot-deformed Nd-Fe-B magnets

The hot-deformed(HD) Nd-Fe-B magnets show heterogeneous microstructure composed of coarse and fine grain regions. It is significant to fully understand the influence of this complex microstructure on the magnetization reversal process which can give the guidance for the enhancement of the magnetic properties. In this paper, the heterogeneous microstructure of the(HD) Nd-Fe-B magnets were characterized from the morphology, size, macro-texture and micro-structure. In addition, the magnetization reversal process of the HD Nd-Fe-B magnets was systematically analyzed by magnetic measurement, insitu domain evolution observation and micromagnetic simulation. The results indicate that the HD NdFe-B magnets mainly consist of fine grain regions(FGRs) and coarse grain regions(CGRs). The FGRs show plate-like grains with fine grain size and strong c-axis texture, while the CGRs show equiaxial grains with large grain size and weak c-axis texture. In particular, it is worth noting that the texture in homogeneity exists not only between FGRs and CGRs, but also inside both the FGRs and CGRs. The dominant coercivity mechanism of the HD Nd-Fe-B magnets is domain wall pinning. Also, the experimental analysis shows that the reverse domain is formed and expanded in the CGRs at low reverse applied field, while the reverse domain occurs in the FGRs at higher reverse applied field. The micromagnetic simulation results also confirm the above magnetization reversal process. In addition, micromagnetic simulation results also show that the orientation of the grains also affects the pinning strength, besides the grain size.

Microstructure improvement related coercivity enhancement for sintered NdFeB magnets after optimized additional heat treatment

The micro structure, especially the Nd-rich phase and the grain boundary, in sintered NdFeB magnets plays an important role in magnetic reversal and coercivity mechanism. To better understand the effects of the microstructure on the coercivity, we investigated the microstructure and properties improvements of a commercial sintered NdFeB magnet after optimized additional heat treatment. The coercivity is enhanced from 1399 to 1560 kA/m. This enhancement has been explained in terms of the evolution of the grain boundary structure, and the formation of continuous thin layers of Nd-rich phase is important for high coercivity. The micromagnetic simulation together with the numerical analysis based on the nucleation model suggest that the reversed magnetic domains nucleate mainly at the interface of multijunctions of Nd_2 Fe_（14）B grains with high stray fields during the demagnetization process. Both improved anisotropy fields at grain boundaries and reduced stray fields at multi-junction Nd-rich phases contribute to the coercivity enhancement. This work has importance in understanding the crucial micro structure parameters and enhancing the obtainable properties for sintered NdFeB magnets.

Microstructure evolution and coercivity mechanism of hydrogenation-disproportionation-desorption-recombination(HDDR) treated Nd-Fe-B strip cast alloys

In this paper,we systematically investigated the microstructure evolution and coercivity mechanism of hydrogenation-disproportionation-desorption-recombination(HDDR)treated Nd-Fe-B strip cast alloys by transmission electron microscopy(TEM)and three-dimensional atom probe(3 DAP)analyses.The rodlike NdH_(2+x) phases with diameters of 10-20 nm are embedded intoα-Fe matrix,which hereditarily leads to textured grains in HDDR alloy.The migration of NdH_(2+x) from Nd-rich region toα-Fe matrix during hydrogen absorption process contributes to the uniform redistribution of Nd-rich phases after HDDR treatment The HDDR alloy with single domain grain sizes of 200-300 nm exhibits relatively low coercivity of 1.01 T that arises from pinning magnetic domain motion.The weak c-axis orientation of HDDR alloy results in a lower reverse magnetic field(coercivity)to reduce remanence to 0.Moreover,the direct contact of Nd_(2)Fe_(14)B grains and the high concentration of ferromagnetic elements(Fe content≈66.06 at%,Co content≈0.91 at%)in Nd-rich grain boundary layer lead to strong magnetostatic coupling effect among Nd_(2) Fe_(14)B grains.The nano-sized a-Fe inside Nd_(2) Fe_(14)B matrix makes the magnetization reversal easily and decreases the coercivity of HDDR alloy.

Microstructures and coercivity mechanism of 2:17-type Sm-Co magnets with high remanence

Sm(Co_(bal)Fe_(0.245)Cu_(0.07)Zr_(0.02))7.8(at%)sintered magnets with high remanence(B_(r)~1.15 T)were prepared using a traditional powder metallurgy method.Tunable magnetic properties,especially intrinsic coercivity(H_(cj)),were obtained through adjusting isothermal procedure parameters.H_(cj)of the magnets increases from 305 to 752 kA·m^(-1)with isothermal annealing time increasing from 3 to 20 h,while B_(r) of the magnets almost keeps constant.From the bright field transmission electron microscopy(TEM)images,it is found that:(1)there is dispersed precipitated phase with very small size in the magnet annealed for 3 h,while the magnets annealed for 20 h have distinct and intact cellular structure;(2)the number density of Z-phase in magnet annealed for 20 h is bigger than that for 3 h.Besides,the finer microstructures were studied with high-resolution transmission electron microscopy(HRTEM).

Magnetic properties and microstructure of nanocrystalline bulk Nd-Fe-B with Ce-Co addition

The influence of Ce-Co alloy addition and sintering holding time on permanent magnetic properties and micro structure of nanocrystalline Nd-Fe-B bulk alloy were investigated.The coercivity of Nd-Fe-B bulk alloy can be enhanced greatly by more than 100% after adding Ce-Co powders.However,when the concentration of Ce-Co is up to 30 wt%,the density of the magnet can reach the maximum value of 7.58 g/cm^(3),but the coercivity does not increase significantly.On the other hand,with the increase of holding time to 10 min,the density and coercivity of magnets increase gradually,reaching up to 7.55 g/cm^(3) and 1134.3 kA/m,respectively.After the addition of Ce-Co alloy,Ce-Co may easily diffuse into the Nd-Fe-B matrix during hot-pressing and under the high pressure and temperature,thus increasing the content of grain boundary phase and the pinning effect of grain boundary,which leads to the increase of coercivity.The extension of the hot-pressing holding time may be more conducive to the diffusion of CeCo into the Nd-Fe-B matrix.In addition,the effect of Ce-Co addition on the magnetic properties of Nd-FeB with different content of rare earth was also studied.The addition of Ce-Co can effectively increase the coercivity of nanocomposite Nd_(2)Fe_(14)B/α-Fe magnets.The addition of Nb to the parent alloy can further improve the coercivity.For Nd_(11)Fe_(81.5)Nb_(1)Ga_(0.5)B_(6) alloy with 10 wt% Ce-Co addition,the coercivity can increase from 740.28 to 1098.48 kA/m.

Coercivity Mechanism of Melt-spun Nd-Fe-B Bonded Magnet

The microstructures of melt-spun Nd_135- Fe_81.74B4.76 ribbons have been investigated by means of X-ray diffraction,Mossbauer spectroscopy and HREM.Experimental results show that optimal magnetic properties of the bond- ed magnet can be obtained by melt-spun alloys at wheel velocity of 24m/s,the microstructure of which was single phase Nd_2Fe_(14)B crystallites,and no other phases,such as Nd-rich,B-rich or x-Fe phase,were present.It is considered that the high intrinsic coercivity ~iH_c can be attributed to the sin- gle-domain behaviour of Nd_2Fe_(14)B crystallite phase.

Sensitivity of coercivity and squareness factor of a Nd-Fe-B sintered magnet on post-sintering annealing temperature

An N38SH-grade magnet with low oxygen content was used to study the evolution of magnetic properties upon post-sin-tering annealing. Phase transformation of as-sintered magnet was investigated by differential scanning calorimetry （DSC）. Three low temperature eutectic transition points were detected. Little change could be found when annealed below the lowest eutectic transition point. A wide annealing temperature range （460–560 oC） between the lowest and highest eutectic transition point was available for this magnet to achieve a relatively high coercivity （~1671 kA/m） at a relatively low Dy content （~3 wt.%）. However, squareness fac-tor （SF） of the demagnetizing curve and its temperature stability were found to decrease after annealing above the highest eutectic transition point. This was attributed to the change of Cu content in the Nd-rich phase under different annealing temperatures.