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Hydrogen decrepitation is a very effective method of pulverizing Nd‐Fe‐B‐type alloys for sintered magnets. Jet‐mill response of hydrided materials is much better than that of unhydrided materials, i.e., shorter milling time is required and finer particle size is obtained for hydrided materials. The sinterability of hydrided powder is also improved from that of unhydrided powder. The minimum sintering temperature for obtaining full density (≳98% of theoretical density) magnets made from hydrided powders ranges between 900 and 950 °C, while that for magnets made from unhydrided powders ranges between 1050 and 1100 °C. Magnets made from hydrided powder sintered at low temperatures usually exhibit high coercivity comparable to or greater than that of the magnets made from unhydrided powder sintered at high temperatures followed by subsequent heat treatment. The enhancement of sinterability and the increase in coercivity of Nd‐Fe‐B magnets are discussed.

The microstructure of Nd 13.5 Dy 1.5 Fe 67 Co 5 V 4 B 8 Al 1 sintered magnets produced by Aimants Ugimag is studied by transmission electron microscopy. The structure of FeV 2 B 2 precipitates inside the hard magnetic phase and the precipitate-matrix relations (relative crystallographic orientations, interface planes and growth directions) are discussed.

The magnetic properties of sintered Nd‐(Dy)‐Fe‐B magnets prepared with different annealing conditions have been investigated. Thermomagnetic analysis was carried out on the sintered and annealed Nd‐(Dy)‐Fe‐B magnets in the temperature range of room temperature to 320 °C. The coercivity of gas‐quenched Nd14.1Fe78.3B7.6 magnets after annealing for 1 h at temperatures between 600 and 900 °C is higher by 3 kOe than the furnace‐cooled one after annealing. Thermomagnetic analysis found that the sintered Nd14.1Fe78.3B7.6 magnet consists of the Nd2Fe14B phase as well as an unknown magnetic phase (A1) with Tc around 150 °C. The annealed magnets consist of two unknown magnetic phases (A1,A2) and the Nd2Fe14B phase with a Tc of 150, 240, and 305 °C, respectively. Thermomagnetic analysis also showed that the Mr value of the A2 phase in the gas‐quenched Nd‐Fe‐B magnet was higher than that of the furnace‐cooled one. These results seem to indicate that increase of coercivity is related to the formation of the A2 phase.

Measurements of temperature dependence of the coercive field Hc are reported for Nd-Fe-B sintered magnets. The relationship between log Hc and log Hf was deduced from Gaunt's localized weak-domain-wall pinning model, which can be written in the following form, log Hc=log Hf+log(25 ((31rb2/25kT)-1)), where r is the domain-wall energy and b is the range of interaction between wall and pin. The values of log Hc were calculated using the above equation, and are in good agreement with experiment.

The magnetic properties of Nd/sub 16/Fe/sub 77/B/sub 7/ and Nd/sub 16/Fe/sub (61-x)/Co/sub 16/Ga/sub x/B/sub 7/ (x=0, 1, 2, 4, 7) have been measured. It was found that the remanent magnetization (B/sub r/), the maximum energy product (BH)/sub max/, and the Curie temperature (T/sub C/) decrease with increasing Ga content. The coercive field (H/sub c/) increases with increasing Ga content when x is smaller than 2, but decreases when x>2. At x=2, the coercive field reaches its maximum value. It was also found in all the samples investigated that there is a linear relationship between H/sub c//sup 1/2/ and T/sup 2/3/, which cannot be explained by Gaunt's wall-barrier model. The temperature dependence of the calculated values of H/sub 0/ and (4bf) is examined. >

The temperature dependence of the coercive field (H/sub c/) and the thermal fluctuation field (H/sub f/) of Nd-Fe-B sintered magnets was measured from 100 K to 500 K. It was found that a linear relationship exists between l/sub g/H/sub f/ and l/sub g/H/sub c/ in this temperature range. The relationship between l/sub g/H/sub c/ and l/sub g/H/sub f/ was deduced from Gaunt's localized weak pinning model. Values of l/sub g/H/sub c/ calculated from the derived relationship are in very good agreement with experimental values. >

Applying RIP (Rubber Isostatic Pressing) to the ferrite sintered magnets, the following have been developed : (1) Ferrite sintered magnets having energy product as high as 5.12 MGOe ; (2) RIP setup for arc-segment magnets.

The effects of DyHx addition on magnetic properties and microstructure of Nd-Fe-B sintered magnets were investigated. In order to obtain the best magnetic properties, the effects of sintering temperature and annealing time on magnetic properties were also investigated. The results shown that the addition of DyHx can affect the microstructure, increase the intrinsic coercivity (Hcj), and improve the thermal stability of sintered Nd-Fe-B magnets. The remanence (Br) and Hcj showed different variation trends as sintering temperature changes. Within a reasonable range, sintering at higher temperature can get higher Br but lower Hcj. The Hcj was further increased by heat treatment and annealing time was extended.

The coercivities (Hc), saturation magnetizations (Js), and anisotropy constants K1 of (Nd1−x, Dyx)2Fe14B sintered magnets (x = 0 to 1.0) are determined using high magnetic field measurements up to about 8 MA/m. The Ha, Js, and K1 values of samples with x

Synchrotron pole figures have been recorded from the Nd2Fe14B, Nd2O3, and NdO grains in commercial Dy-free, Nd-Fe-B sintered magnets in the as-sintered and fully annealed states. In contrast to the well known fiber texture in the Nd2Fe14B grains, the Nd-oxide phases showed no preferred orientation in the as-sintered or annealed states. The random orientation of the Nd-oxide grains was confirmed from histograms of the pole density values. These showed only small deviations from the pole density of a non-textured material.

The grain boundary diffusion (GBD) process is an effective process for enhancing the coercivity of Nd–Fe–B sintered magnets. When the GBD process is applied to a thick magnet, the magnet exhibits a coercivity distribution. Partial coercivity at the centre is the same as that of the original magnet, but the partial coercivity increases on going towards the magnet surface. In this paper, the effects of conditions of the GBD process on the characteristics of coercivity distributions were investigated. Surface coercivities increase with increasing amount of coatings, while diffusion depths are almost the same. The diffusion treatment temperature varied from 800 to 900 °C, and does not affect the coercivity distributions. A long-time diffusion treatment is effective in enhancing the inner coercivities, but a very long treatment causes a significant reduction in surface coercivity.

The interface between the Nd2Fe14B particles and the grain boundary surrounding them in Nd-Fe-B sintered magnets was modified by diffusion Dy metal selectively through the grain boundary involving the reduction of Dy2O3 or DyF3 with CaH2 and the coercivity of them was effectively enhanced from 0.98 MA/m to 1.42 MA/m with keeping the high value of remanence. The resultant coercivity was higher than that of the magnets modified with Dy2O3 (HcJ = 1.10 MA/m) and DyF3 (HcJ = 1.24 MA/m) without using any reductant such as CaH2, respectively.

For the study of the relationship between the domain wall motion (DWM) and the coercivity in the samples of Dy-free Nd-Fe-B sintered magnets, the DC susceptibility (• •DC) in the initial magnetization from the thermally demagnetized state and the DWM in the partially demagnetized state after the full magnetization in 5T-pulse field were measured. In the high coercivity (μ.Hc) samples of μ.Hc=1.35T and 1.62T, the stronger pinning in initial magnetization was detected as the• •DC compared to other samples of the smaller coercivities such as μ.Hc=0.80T. The former samples also showed little DWM in the partially demagnetized state even the domain structures were clearly detected by magneto-force microscopy (MFM). In the latter samples, however, the obvious DWM was measured as the differences (ΔM) between the magnetizations under applied field (Ma) and the remanences (Mr) in demagnetization process (ΔM = Mr - Ma). The pinning in DWM in initial magnetization process and the DWM in the partially demagnetized state are both originated from the interaction between the domain walls and grain boundary phases. The enhancement in coercivity, therefore, should come from the same interactions in the sintered samples.

For the purpose of enhancement of coercivity (H c ) in sintered Nd-Fe-B magnets, we study the effects of high magnetic felds of up to 140 kOe applied during the annealing. We investigated the annealing temperature (T a ) dependence of H c systematically. Coercivity enhancement phenomena induced by the magnetic annealing were observed only when the sample contains a small amount of Al and Cu additives, and the annealing temperature is in the vicinity of T a = 500°C or 550°C. DSC measurements suggest that these temperatures can be interpreted as the eutectic points of Nd70Cu30 and Al17Cu83 phase, respectively. Temperature dependence of H c and maximum energy product (BH)max in the coercivity-enhanced samples were measured up to 250°C. We have found that a sample with H c = 14.6 kOe at 20°C gives a twice larger (BH)max value at 125°C as compared with those in the sample with H c = 9.8 kOe at 20°C.

The effects of rare-earth contents, substitutions, and heat treatment parameters on the impact resistance of the Nd–Fe–B sintered magnets were investigated. Analysis shows that the impact resistance increases linearly with increasing plastic rare-earth-rich phase in the investigated magnets. Heavy rare-earth Dy substitutions for Nd help us to improve the impact resistance. The bending strength and impact resistance during annealing process was investigated. It is found that the annealing process affects the bending strength and impact resistance remarkably. Both the bending strength and the intrinsic coercivity are poor in the as-sintered specimen. In the postsintering annealing process, both of them get improved and the change of intrinsic coercivity is similar to that of the bending strength. Study showed that the optimized heat treatment parameters help us to improve the bending strength and impact resistance of the sintered Nd–Fe–B magnets. Study shows that the sintered Nd–Fe–B magnets with improved i...

The microstructure, magnetic properties and corrosion behaviour of RE 16Fe7(1-x)M(x)Co5B8 (RE = Nd, Pr, Dy, Tb ; M = Si, Al, Cr) sintered magnets have been examined. It was established that the presence of Re-(Co, Fe, M) compound along the grain boundaries of these alloys was responsible for inhibition of the corrosion in acid solution and in atmospheric environment. The best magnetic properties obtained in these studies are Br = 1.26 T and MHc = 1 730 kA/m for the Nd14DyTbFe70SiCo5B8 alloy.

A quantitative correlation between the microstructure and squareness factor of R16Fe76B8 (R=Nd or Pr) sintered magnets is proposed. The expression was derived based on the micrographs of 19 magnets, making a total of more than 11 000 grains analyzed. This expression is independent of the determination of the sample’s magnetic properties. Magnet processing conditions were used to modify the microstructure and, hence, the squareness factor. Six microstructural parameters—grain size, grain elongation, grain roundness, and their respective standard deviations—were quantified using an image analyzer software program. The squareness factor was higher for magnets with a microstructure showing rounder grains and a sharp size distribution. A comparison of the squareness factor obtained by the proposed expression with that calculated from the demagnetization curve showed a fairly good agreement. Some disparities were also found and are discussed herein.

An effort to increase the impact toughness of Nd–Fe–B sintered magnets by adding small amounts of Al, Nd, Ga, Cu, and Nb was successful. No significant compromise to magnetic properties occurred. Based on this work, a series of sintered Nd–Fe–B magnets with improved toughness was developed, which we call ToughNEO™. Small precipitates, which may contribute to the improvement of toughness, were observed using scanning electron microscope for all samples with improved toughness. Tumbling and drilling tests further verified the improved toughness of these developed ToughNEO™ magnets.

The angular dependence of the nucleation field H n in high-coercivity grains of sintered NdFeB and SmCo 5 magnets has been studied by direct observations of the domain structure in an external magnetic field. The effect of the stray field of surrounding grains on the angular dependence of H n has been estimated quantitatively. It is found that changes in H n of individual grains as large as 0.45 T may be observed, depending on the micromagnetic state of neighbouring grains.

An analysis of Wohlfarth's remanence relationship is used to clarify the nature of magnetization processes in different types of permanent magnets based on the Nd 2 Fe 14 B phase. The deviations from ideal Wohlfarth behaviour are due to domain walls in sintered magnets and to magnetic interaction in melt-spun materials, respectively.

Using sintered specimens of Ba-hexaferrite powders as a model system, a study was made of the dependence of coercivity on the particle dimensions and on the temperature from RT up to the Curie temperature. The temperature behaviour of the coercive and anisotropy field are the same in single-domain particle specimens while they are rather different in polydomain particle specimens. However, the coercivity behaviour of both systems can be fitted well up to 350°C using a simple relation between H c , H A , and M s . Further contributions to the coercivity have to be taken into account at higher temperatures.

By means of the magneto-optical Kerr effect the domain structure of the surface grains of RE-Fe-B-based sintermagnets is investigated. The dependence of the saturation field on the angle φ between external field and preferred direction of the sample follows a 1/cos φ-law whereas the coercive field follows a Stoner-Wohlfarth-like behaviour.

Until now feedback magnetization measurements have been carried out only on soft magnetic materials. The realization of such measurements by computer-controlled feedback systems gives new results in the case of the low-temperature behaviour of Nd-Fe-B sintered magnets. Low pinning fields are detectable with high resolution.

The intrinsic energy barriers 31rb2, as well as the lgHc–lgHf plot, were calculated from a localized weak domain wall pinning model which fit the experimental data well. These results support the localized weak domain wall pinning model in NdFeB sintered magnets.

It is well known that the Pr2Co14B intermetallic compound has a high magnetocrystalline anisotropy (100 kOe at 294 K). This value is higher than that of the Pr2Fe14B compound (70 kOe at 294 K). Hard magnets, using the rapid‐quenching technique, have been prepared for both the Co‐ and Fe‐based materials. In the case of the conventional powder‐metallurgy technique, only the Fe‐based magnets have been successfully prepared. So far, there is no report of achieving any significant value of coercivity in the Pr‐Co‐B system. In this paper we report preliminary results on Pr‐Co‐B sintered magnets with room‐temperature intrinsic coersivity, Hci, equal to 5.2 kOe and an energy product, (BH)max equal to 10.3 MGOe. At liquid‐nitrogen temperature (77 K) Hci is greater than 18 kOe (which was the maximum available magnetic field) and (BH)max is equal to 12.5 MGOe. These values of (BH)max are twice those reported for the rapidly quenched magnets.1 The Curie temperature of the magnet is 988 K (corresponding to the presenc...

The magnetic aftereffect was measured for SmCo/sub 5/ sintered magnets. The effects of machining down to small size and to heat treatment were investigated. Marked magnetic aftereffect and permanent deterioration were observed. It was found that the magnetic aftereffect could be reduced by heat treatment to about 1/2 and the permanent deterioration to about 1/3. In contrast, the aftereffect constant, which is the coefficient of logarithmic time decay, remained constant irrespective of heat treatment. >

This paper reviews the results of studies in our laboratory in the 2 to 12% Co range of Cr‐Co‐Fe permanent magnet alloys. Our salient results are as follows: (1) Isotropic ternay alloys, typified by 10.5Co‐28Cr‐Fe (Chromindur II) with values of Br = 9800, Hc = 400 Oe and (BH)max = 1.8 MGOe. This alloy is now in volume production for telephone receiver applications. (2) Anisotropic ternary alloys prepared by a newly developed ’’deformation‐aging’’ process, with values of (BH)max ranging from 2.3 MGOe for a 5Co‐33Cr‐Fe alloys to 5.5 MGOe for an 11.5Co‐33Cr‐Fe alloy. Adding 2% Cu raises the value of (BH)max by 0.5–1.0 MGOe. (3) Anisotropic alloys prepared by magnetic‐field aging, with values of (BH)max ranging from 4.2 MGOe for a 2.5Co‐32Cr‐Fe alloy to 6.2 MGOe for a 9Co‐27Cr‐Fe alloy. (4) Sintered magnets typified by the 5Co‐31Cr‐Fe alloy with (BH)max∼4.4 MGOe.

The effect of 20 kinds of additional elements have been studied to increase the intrinsic coercivity for Co substituted Nd-Fe-B sintered magnets. The addition of Ga was found to increase iHc more than any other elements.The effects of Ga addition on magnetic properties and thermal stability were investigated for Nd (Fe0.72Co0.2B0.08)5.6 and Nd0.8Dy0.2 (Fe0.86Co0.06B0.08)0.5 magnets. A combination of both Ga and Nb or Ga and W additions improved thermal stability which was attributed to the increase of iHc and good squareness by finer grains.Typical Ga added 2 sintered magnets and their magnetic properties and exposure temperatures to keep the irreversible losses under 5% (Pc=2) were as follows.[1] Nd(Fe0.7Co0.2B0.08Ga0.02)5.6: Br=12.2 KG, iHc=16.5 KOe and (BH)max=35.2 MGOe/140°C[2] Nd0.8Dy0.2(Fe0.835CO0.06B0.08Nb0.015Ga0.01)5.5: Br=10.45 KG, iHc=27.1 KOe and (BH)max=26.2 MGOe/260°C

The coercive force and residual magnetization have been examined as functions of angle for textured powder specimens of Sm0.5Pr0.5Co5 of particle size 1–50Μ made by mechanical crushing; similar results are presented for sintered magnets.

The microstructure of Nb-containing NdDyFeCoB magnets has been investigated using transmission electron microscope. Very small particles whose diameters are about several to 40 A have been found in as-sintered magnets. It may be formed by self-tempering. Dislocations and stacking faults have been found inside grains. It seems that the increase of coercive field in Nb-containing magnets is closely related with defects and precipitations.

The influence of additive Dy2O3 on the magnetic properties of Nd16(Fe0.96Al0.04)77B7 has been investigated. It is shown that when the Dy2O3 concentration is 4wt% the intrinsic coercivity iHc of the sintered magnets increases to 17850 Oe.

The magnetic field and temperature dependences of the magnetic aftereffect constant S/sub v/ were measured for Nd-Fe-B sintered magnets in the temperature range between 200 and 500 K on a J-H demagnetizing curve. It was found that S/sub v/ is constant and independent of the magnetic field in the range from H/sub m/ to H/sub cJ/. S/sub v/ decreases linearly between 200 and 500 K as the temperature increases and is larger in materials whose H/sub cJ/ is large. The temperature dependence of S/sub v/ for Nd-Dy-Fe-B and Nd-Fe-Co-B sintered magnets differs from that in SmCo/sub 5/ magnets and is similar to that in Sm/sub 2/Co/sub 17/ sintered magnet. However, the value of S/sub v/ for Nd-Fe-Co-B sintered magnet is larger than that of Sm/sub 2/Co/sub 17/ magnets by roughly 2.5 to 3 times. >

The advent of MAGNEQUENCH neodymium-iron-boron materials having excellent magnetic properties and potential economic advantages has initiated a new era in permanent magnet technology. These magnets have already been used in cranking motors, stepping motors, etc. Different kinds of isotropic powders have been developed having high and low coercivities for use in varied applications. The properties of these powders are discussed. The properties of fully dense isotropic (MQ2) and anisotropic magnets (MQ3) are given. Anisotropic powders produced from hot deformed magnets are stable and it contrasts sharply with those prepared from sintered magnets. These anisotropic powders can be molded into bonded magnets with energy products of 15-17 MGOe.

Pr-Co binary ribbons with 11.1∼17.3 at% Pr were produced by rapid quenching techniques. The samples were melted and quenched in Ar gas atmosphere to avoid oxidation. A maximum coercivity of about 10 kOe was obtained for 17.3 at% Pr-Co ribbons (approximately the stoichiometric content of PrCo5), which was twice the value of sintered PrCo5 magnets. Oxygen content of these ribbons was 300∼500 ppm (by weight), less than about 1/10 the content of sintered magnets. These 17.3 at% Pr-Co ribbons were magnetically and chemically stable under atmospheric conditions for a 30 month period.

Magnetic viscosity in Nd-Fe-B sintered magnets was measured at different fields between 4.2 and 300 K. At a given temperature, the viscosity S ( H ) and the irreversible susceptibility χ irr are found to be proportional. The temperature dependence of the coefficient S v = S / χ irr is deduced and an expression for the activation energy is derived. It is found that the intrinsic activation energy E 0 is proportional to the domain wall energy, whilst the activation volume is proportional to the cube of the domain wall width.

The new process for the production of pure iron powder with an iron content of the order of 99.9% is simple and can be mechanized. The resulting powder has excellent processing characteristics and in particular, as demonstrated by tests conducted at various organizations, can be successfully utilized for the production of sintered magnets, magnetic cores, high-density constructional parts, etc. In addition, the powder is also likely to find application as a charge material in the manufacture of steel powders and in the production of alloys with special electrical, magnetic, and physical properties.

The heat treatment effects on the magnetic properties, especially on the coercivity for SmCo 5 single crystals, are reported. There is a peak at 1080°C in the annealing temperature dependence of coercivity for SmCo 5 single crystals. The variations of the coercivity with annealing temperature are similar between SmCo 5 single crystals and Sm-Co sintered magnets. It is noted that the coercivity of SmCo 5 single crystals remarkably decreases by occurrence of scratches and oxides on the surface of the specimens. It seems that the decrease of coercivity in this crystal is due to nucleation of domain walls caused by crystalline defects.

The research of the Nd-Fe-B magnet was started from an expectation that B and/or C might expand Fe-Fe interatomic distances in rare-earth iron alloys, leading to increase the Curie temperature of them. After the success in developing the powerful Nd-Fe-B sintered magnets with record high energy products, however, a crystal structure analysis revealed that Fe-Fe pairs having small interatomic distances existed also in the Nd-Fe-B alloy. It is emphasized that the success in this work is a result of many fortunate matters which were not expected in the beginning.

To improve the thermal stability of (Nd 0.8 Dy 0.2 ) (Fe 0.86 Co 0.06 B 0.08 )5.5 sintered magnets for heavy duty applications, such as starter motors, effects of additional elements(M=Al, Nb and Ga) On the thermal stability were investigated in the (Nd 0.8 Dy 0.2 ) (Fe 0.86-u -Co 0.06 B 0.08 M u )5.5 sintered magnets. The addition of Al increased i H c , but did not decreased irreversible losses at 220°C(Pc= 2). The sintered magnet with M=Nb and u=0.006 had lower i H c and higher irreversible losses than u=0, while the sintered magnet with M=Nb and u=0.015 had higher i H c and lower irreversible losses. Additions of Ga were resulted in higher i H c and lower irreversible losses. A combination of both Nb and Ga additions was also studied and a very low irreversible loss of under 5 % after exposure at 260°C(Pc= 2) was obtained for (Nd 0.8 Dy 0.2 ) (Fe 0.835 Co 0.06 B 0.08 Nb 0.015 Ga 0.01 )5.5 sintered magnets. The obtained magnetic properties for the magnet are B r =10450 G, b H c =10000 Oe, i H c =27100 Oe and (BH)_{\max}=26.2 MGOe.

In Co 5 Sm, the coercive force of aggregates of powders or sintered magnets is much greater at 77 K than at room temperature. In single particles, the field at which an individual magnetization jump occurs increases rapidly and monotonically with decreasing T. On the other hand, the K reported for Co 5 Sm is the same at 77 K and room T, with a broad maximum between. It thus appears that the reversal process is not controlled by the properties of bulk Co 5 Sm. The observed T dependence must be ascribable to the imperfections that govern the magnetization reversal process.

Commercially viable and economic techniques are described to enable the production of consistent quality anisotropic NdFeB permanent magnets by the powder metallurgy route. Throughout the development work associated with this project, particular attention has been paid to the safety aspects, whilst optimisation of process and pilot plant variables enable the fabrication of sintered magnets up to 33 MGOe (265 kJm−3). The plastic injection moulding route has also been considered and has culminated in the experimental formation of isotropic, thermoplastic magnets up to 5.0 MGOe (40 kJm−3).

Hysteresis loops of small but still multi‐grain samples taken from sintered Co5Sm magnets show a wide range of behavior, samples from the same magnet bracketing the bulk properties. Magnetization discontinuties often appear and are associated with large grains. These jumps show a quantized dependence of jump field on magnetizing field and often a good 1/cosϑ dependence, exactly as in particles prepared from cast material. A sample consisting of only one grain gave a completely rectangular loop. Measurements on sintered magnets indicate that large grains tend to reverse in small fields, detracting from the overall properties.

The composition dependence of Curie temperature, spin reorientation temperature and magnetic anisotropy field are investigated for Nd2 (Fe1-xTx)14B (T = Nb, Mo and Ga) alloys. The contribution of pinning to the magnetic hardening for NdFeNbB sintered magnets increases with the Nb addition. Precipitation has been found in NdFeNbB sintered magnet.