RU2476947C2 - METHOD FOR OBTAINING HIGH-COERCIVITY MAGNETS FROM ALLOYS ON BASIS OF Nd-Fe-B - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method is implemented by diffusion annealing of sintered magnets at the temperature of 600-1000°C, the surface of which contacts disperse powders of materials - diffusion sources, on the basis of dysprosium and/or terbium. As diffusion sources, disperse powders of metallic dysprosium (Dy) or terbium (Tb), powders of Dy and/or Tb hydrides, or alloys on the basis of RmTn intermetallic compounds with low fusion temperature of (500-1200°C) are used, where R-Dy or Tb; T - Co, Fe, Ni, Al, Cu, Ga or their combination and m>n (i.e. content of Dy or Tb is at least 50 at %). Diffusion annealing is performed during 0.5-20 hours at the temperature of 550-850°C, with further additional annealing at the temperature of 550-600°C during 0.5-1.0 h.

EFFECT: increasing coercitive force of sintered Nd-Fe-B magnets, without sufficient reduction of their residual induction and maximum energy product.

4 tbl, 5 dwg

Description

The invention relates to the field of permanent magnets and can be used in the production of high-energy permanent magnets based on rare-earth alloys and, in particular, based on alloys of the neodymium-iron-boron (Nd-Fe-B) system.

High-energy sintered Nd-Fe-B magnets are widely used in magnetic systems of various technical devices (for example, computer hard drive drives, air conditioning compressors, focusing systems, mobile phones, motors and generators for hybrid and electric cars, etc. ) In many systems, magnets are operated in large demagnetizing fields and at elevated ambient temperatures, and therefore they must possess not only a high residual induction B _r and a large value of the maximum energy product (BH) _m , but also a high coercive force H _c . Since the magnetization reversal mechanism of sintered Nd-Fe-B magnets is associated with the formation of nuclei of the inverse magnetic phase, the suppression of defects at grain boundaries with locally lower values of the magnetocrystalline anisotropy field H _a in which the nuclei are formed is an important task to achieve high values of H _c . Solving the problem of increasing H _{c of} Nd-Fe-B permanent magnets without lowering the values of B _r and (HH) _m allows one to increase the power of magnetic systems, their temperature and time stability, and therefore, expand the scope and improve the quality of products with such magnets.

It is well known that the substitution of a part of neodymium for heavy rare-earth elements (TRE), such as dysprosium (Dy) or terbium (Tb), is effective for increasing the magnetic anisotropy field in (Nd, TPЗE) ₂ Fe ₁₄ B compounds and the coercive force of magnets based on of these compounds [S. Hirosawa. Y. Matsuura, H. Yamamoto, S. Fujimura, M. Sagawa, J. Appl. Phys. 59 (1986), pp. 873-879]. Table 1 summarizes the main properties of the compounds R ₂ Fe ₁₄ B (R - Pr, Nd, and Tb): density ρ, Curie temperature T _C , saturation magnetization M _s, and anisotropy field Н _а [Buschow, Materials Science Reports, 1986]. Values of H _a compounds (HREE) ₂ Fe ₁₄ B, where HREE - Tb and Dy, in three and two times, respectively, higher than the values N _and Nd ₂ Fe ₁₄ compound B. Therefore, replacement of a part of neodymium heavy rare earth elements is effective for improving the values of H _and in quasi-triple compounds (Nd, TRZE) ₂ Fe ₁₄ B. On the principle of substitution of Nd neodymium for Dy and Tb in Nd-Fe-B alloys, a method is based that is widely used to increase the coercive force, Curie temperature and thermal stability of magnets based on these compounds [S. Hirosawa. Y. Matsuura, H. Yamamoto, S. Fujimura, M. Sagawa, J. Appl. Phys. 59 (1986), pp. 873-879].

Table 1 Compound formula ρ * 10 ³ , kg / m ³ T _C , K M _s , μ _{B /} f.e. µ ₀ M _s , T N _a , MA / m Pr ₂ Fe ₁₄ B 7.45 569 1.41 1.56 6.96 Nd ₂ Fe ₁₄ B 7.59 588 32.5 1.61 5.36 Tb ₂ Fe ₁₄ B 7.92 620 14.0 0.664 19.0 Dy ₂ Fe ₁₄ B 8.05 593 14.0 0.712 13.4

In order to increase the values of H _c from 1000 kA / m, which are inherent in sintered magnets of Nd-Fe-B ternary alloys, to 2000 kA / m or more, which are necessary for operation of the magnet at elevated temperatures, usually from 15 to 30% neodymium , which is part of the alloy, is replaced by Dy or Tb.

One of the significant drawbacks of this method is a significant decrease in the residual induction and the maximum energy product of sintered magnets, since, as follows from Table 1, the values of μ ₀ M _s for (TRSE) ₂ Fe ₁₄ B are approximately 2.5 times lower than for Nd ₂ Fe ₁₄ B. The decrease is due to the fact that the magnetic moments of TRE atoms in the (Nd, TRE) ₂ Fe ₁₄ B compound line up antiparallel to the iron moments, in contrast to Nd moments that are aligned in parallel. Another disadvantage is that Dy, and even more Tb, are scarce and expensive elements compared to neodymium, therefore, in conditions of rapidly increasing consumption of magnets of the Nd-Fe-B system with increased temperature stability, in particular for the production of electric vehicles and hybrid cars Their widespread use is becoming a big problem that will worsen over time.

There is also known a method for producing permanent magnets of the Nd-Fe-B system with increased temperature stability by preparing a mixture of alloys consisting of a base alloy based on a neodymium-iron-boron system and an additive alloy enriched in Dy and / or Tb, which includes the smelting of alloys, obtaining alloy powders, mixing and grinding powders, compacting them in a magnetic field, sintering compacts at a temperature of 1000-1120 ° C and subsequent heat treatment [US Patent 7244318]. In this method, high-energy sintered permanent magnets of optimal composition are obtained by joint grinding, pressing and sintering of powders of two alloys, which significantly differ from each other not only in chemical composition, but also in the production method. One of the alloys is the base alloy, the concentration of which in the mixture varies from 25 to 95 wt.%, Is close in composition to the stoichiometry of the magnetically hard intermetallic compound Nd ₂ Fe ₁₄ B, contains these components in the following proportions, wt.%: Neodymium 23-31%, boron 1.0-1.2%, iron - the rest. The alloy is obtained by vacuum induction melting. The second alloy, the concentration of which in alloy mixtures varies between 5-75 wt.%, Is substantially enriched with Dy or Tb up to 32-97 wt.%, Contains boron 0.1-1.0% and iron - the rest. The method has many modifications in the field of choosing the composition of the additives and the amount of their introduction [Patent EPO 0261579; RF patent 2174261] and allows you to flexibly vary the composition, which provides the ability to quite simply obtain a wide range of hysteresis properties of sintered magnets. The concentration of TRES near the grain boundaries in this method is slightly increased in comparison with magnets in which Dy and Tb were introduced directly into the alloy.

However, the main drawback is that a decrease in B _r cannot be completely ruled out, since TRE atoms diffuse intensively into matrix grains during sintering of magnets at high temperatures (1000–1120 ° С).

A qualitatively different approach to solving the problem of producing highly coercive magnets without reducing B _r was proposed by Park et al. [K.T. Park, K. Hiraga, M. Sagawa, Effect of metal coating and consecutive heat treatment on coercivity of thin Nd-Fe-B sintered magnets, Proc. of 16 ^th Int. Workshop on REPM and their Applications (Sendai, Japan, 2000), p. 257]. The idea of the approach is to localize Dy or Tb mainly in the paramagnetic intergranular phase on the surface of the grain of magnets and thus increase the magnetocrystalline anisotropy in the region of defects at the grain boundaries and, therefore, increase the coercive force, while maintaining the original magnetization or at least not allowing its strong reduction. Since the diffusion coefficient at the grain boundaries of polycrystals is hundreds of thousands of times larger than the volume diffusion coefficient, diffusion annealing was proposed to achieve the goal. Technically, the method was implemented as follows. Dy layers with a thickness of about 3 μm were vacuum-deposited on both sides onto thin plates of sintered Nd-Fe-B magnets. After diffusion annealing of such plates for 5 min at 800 ° С and additional annealing at 600 ° С for 5 min, their Н _s almost doubled, and B _r decreased by about 5%.

The described method has two disadvantages. Firstly, the result is realized on very thin wafers (50 microns thick) and, secondly, a technically sophisticated method of radio frequency sputtering was used to apply dysprosium layers.

The closest to the claimed technical essence and the achieved result is a method for producing highly energy-intensive high-coercive magnets from Nd-Fe-B alloys by diffusion annealing of sintered magnets, the surface of which is in contact with powders of materials - diffusion sources, for which dispersed powders of TRZE oxides and fluorides were used (Dy, Tb) containing a high concentration of dysprosium and / or terbium [US Patent 7488393]. To implement the method, blocks of sintered Nd-Fe-B magnets were cut into plates with a thickness of 1 to 5 mm. The surface of the plates was etched with a solution of nitric acid. TRZE fluoride powders with an average particle size of less than 5.0 microns and TRZE oxide powders with an average particle size of less than 0.1 microns were mixed in proportions of 50:50 (wt.%) With water. The magnets were immersed in a mixture activated by ultrasound, and then immediately dried with hot air. Magnets coated with TbF ₃ , DyF _3, or Dy ₂ O ₃ powders were annealed at 800–900 ° C for several hours in an argon atmosphere, followed by low-temperature treatment (UTR) at 500 ° C for 1 h. Coercive force of magnets treated in contact with DyF ₃ and TbF ₃ powders, increased by approximately 500 kA / m and 680 kA / m, respectively. The residual magnetization after the process with any TRZE compound decreased by 20 mT. This decrease in B _r values is significantly lower compared to a magnet produced using the standard method of sintering a mixture of powders.

Using scanning electron microscopy and microanalysis, it was shown that the distribution of the concentration of terbium, neodymium, oxygen, and fluorine in a magnet coated with TbF ₃ powder is not uniform. At a site located at a depth of 250 μm from the surface of the magnet, it was found that Tb diffuses into the magnet through the phase along the grain boundaries and forms terbium-enriched layers on the surface of the Nd ₂ Fe ₁₄ B grains by replacing neodymium. The thickness of the Tb-enriched layers was approximately 0.5 μm for grains near the surface of the magnet and less than 0.5 μm for internal grains. Fluorine also diffused into the magnet, forming NdOF oxofluoride. The use of only fluoride powders having the lowest melting points in the range of contact materials used led to higher values of H _{from the} magnets.

The disadvantage of this method is two circumstances associated with the fact that dispersion fluorides and dysprosium and terbium oxides are selected as diffusion sources. Firstly, since the oxides and fluorides of TRZE are stable chemical compounds with a very high melting point, for example, for compounds Dy ₂ O ₃ , Tb ₂ O ₃ it is 2400 and 2390 ° C, respectively. In this regard, the decomposition of these compounds in order to separate the TRE atoms for subsequent diffusion along grain boundaries requires a sufficiently large activation energy. In the end, this leads to the need for long-term annealing (8-10 hours), required for the observed increase in coercive force. Secondly, along with Dy and Tb, undesired fluorine and oxygen elements diffuse along the boundaries of the magnets to form NdOF oxofluorides. The essential task of producing high-energy-intensive magnets from Nd-Fe-B alloys is to ensure that they have an extremely low oxygen concentration (<1000 ppm), and the appearance of oxofluorides as a result of diffusion annealing can lead to a deterioration in the temporal stability of magnetic hysteresis properties.

The basis of the present invention is the task of increasing the coercive force of the permanent magnets of Nd-Fe-B without reducing the values of B _r and (BH) _m by localizing the Dy or Tb atoms mainly along the grain boundaries of Nd ₂ Fe ₁₄ B.

The problem is solved in that in the method for producing highly energy-intensive high-coercive magnets from Nd-Fe-B alloys, diffusion annealing of sintered magnets is used, the surface of which is in contact with dispersed powders of materials - diffusion sources based on dysprosium and / or terbium. According to the invention, powders of metallic dysprosium or terbium, or powders of hydrides of dysprosium and / or terbium, or powders of alloys based on intermetallic compounds R _m T _n with a particle size in the range of 5-500 μm and with a low melting point of 500-1200 are used as diffusion sources ° C, where

R is Dy or Tb;

T - Co, Fe, Ni, Al, Cu, Ga, or a combination thereof;

m and n are atomic fractions of R and T elements, respectively, and m≥n (i.e., the content of Dy or Tb is at least 50 at.%),

while diffusion annealing is carried out for 0.5-20 hours at a temperature of 750-850 ° C, followed by additional annealing at a temperature of 550-600 ° C for 0.5-1.0 hours

The task of increasing the coercive force of the magnets in the proposed invention, as in the known method [US Patent 7488393], is solved by diffusion annealing of the magnets, however, differs in that dispersed pure metal powders Dy and / or Tb are used as diffusion sources in the claimed technical solution, powders of their hydrides Dy (Tb) H _x (x = 2.0-2.2), as well as alloy powders based on intermetallic compounds rich in dysprosium and / or terbium: Dy (Tb) -Co, Dy (Tb) -Cu and Dy (Tb ) -Ga having a low melting point. The coercive force of the permanent Nd-Fe-B magnets without decreasing the values of B _r and (BH) _m is increased by localizing the Dy or Tb atoms mainly along the grain boundaries of Nd ₂ Fe ₁₄ B due to the fact that upon annealing, grain-boundary diffusion of atoms from the magnet surface occurs hundreds of thousands of times faster than bulk diffusion into the body of grains.

Hydrogen from hydrides was easily removed as a result of dehydrogenation during diffusion annealing in vacuum, and additional alloying of Nd-Fe-B alloys with Cu, Ga, and Co elements, which are part of the selected compounds, also contributes to an increase in the corrosion resistance of magnets, their coercive force, and temperature stability [J. Fidler, T. Schrefl, Overview of Nd-Fe-B magnets and coercivity, J. Appl. Phys., 79 (1996), pp. 5029-5034].

The temperature of diffusion annealing T _DA , as well as the particle size of powders D, are process parameters that significantly affect the change in the properties of magnets during diffusion annealing. The lower temperature limit of diffusion annealing is determined by the melting temperature (about 650 ° C) of a neodymium-enriched eutectic localized between the grains of the Nd ₂ Fe ₁₄ B magnet phase. Below this temperature, a noticeable effect of an increase in H values _from magnets annealed in contact with powders containing Dy was not detected. The upper temperature limit of diffusion annealing was determined by the effect of sintering and diffusion welding of powders with the surface of the magnets. The melting points of the materials selected for the preparation of powders - sources of diffusion are given in table 2 [State diagrams - Binary alloy phase diagrams. Second edition, 1996]. The melting points of the dysprosium-rich compounds of the Dy-Al system are not precisely established. In TRZE-T systems with T = Co, Cu, and Ni, there is a fairly large number of intermetallic compounds with low melting points. However, it should be borne in mind that in the case of the choice of Dy (Tb) -Ni compounds, the diffusion of Ni atoms into the body of Nd ₂ Fe ₁₄ B grains can lead to a significant decrease in B _r magnets. The melting points of compounds with terbium are, as a rule, somewhat lower than the corresponding compounds with dysprosium.

table 2 T Compounds and their melting points, ° C Al Dyal
~ 900 Dy ₃ al ₂
~ 900 Dy ₂ al
~ 900 TbAl
1050 Tb ₃ Al ₂
950 Tb ₂ Al
880 With Dy ₄ co ₃
746 Dy ₁₂ co ₇
765 Dy ₃ co
875 Tb ₄ Co ₃
605 Tb ₁₂ Co ₇
800 Tb ₃ Co
840 Cu Dycu
955 TbCu
900 Ga Dyga
1280 Dy ₅ ga ₃
1210 TbGa
1210 Tb ₅ Ga ₃
1130 Ni Dyni
1173 Dy ₃ ni ₂
928 Dy ₃ ni
762 Fe Dyfe ₂
1275 TbFe ₂
1187 Dy
1409 Dyf ₃
1157 Dy ₂ f ₃
2400 Tb
1356 TbF ₃
1177 Tb ₂ O ₃
2390

The use of compounds with Tb should lead to a larger increase in H _c , but due to the higher cost and deficiency of this metal compared to Dy, its use in practice is very limited. Compounds of the Dy (Tb) -Fe and Dy (Tb) -Ga systems have fairly high melting points, and compounds with Fe have stoichiometry (TRZE) of Fe ₂ with a low content of TRZE, which limits their prospects for solving this problem. For comparison, the lower row of the table shows rather high melting points of fluorides and oxides Dy and Tb used in the solution closest to the claimed one.

Figure 1 shows the dependence of μ _o M _m , B _r and H _{c of} thin Nd-Fe-B plates pressed into Dy, DyCu, DyGa, Dy ₃ Co and DyH _x powders on diffusion annealing time t _DA at 750 ° С ;

figure 2 shows the microstructure of a magnet annealed at 750 ° C for 10 hours in contact with DyH _x powder (a), and the change in the concentration of Fe, Nd and Dy from the surface of the plate to its center, defined in the selected rectangular sections of the thin section (b) ;

figure 3 shows the change in the concentration of Fe, Nd and Dy along the radius of grains located at distances of 40 (a), 80 (b) and 130 μm (c) from the surface of the magnet annealed at 750 ° C for 10 hours in contact with the powder DyH _x , and the microstructure of the magnet at a depth of 130 μm from the surface (g);

figure 4 shows the demagnetization curves of plate samples of Nd-Fe-B, annealed in a free state (a) and in contact with Dy ₃ Co powder (b): initial state (curve 1), after annealing at 750 ° C, 2 h (curve 2) and after additional low-temperature treatment — 550 ° С, 0.5 h (curve 3);

figure 5 shows the demagnetization curves of a magnet annealed in contact with Dy ₃ Co powder at 850 ° C for 20 h (a), and the dependence of H _c on the temperature of additional annealing T _A of a cylinder cut from it with a diameter of 9.5 mm (b).

The claimed method is as follows. Sintered magnets were prepared from an alloy of 31.2 wt.% Nd - 1 wt.% B - ost. Fe, using the previously described technology [A.G. Popov, N.V. Kudrevatykh, V.P. Vyatkin, D.Yu. Vasilenko, D.Yu. Bratushev, TZPuzanova, BCGaviko, A.V. Ogurtsov . The experience of obtaining high-quality magnets from Nd-Fe-B alloys prepared by the strip casting method // Promising Materials, Special Issue (6), 2008, pp. 348-353; A.G. Popov, N.V. Kudrevatykh, V.P. Vyatkin, D.Yu. Vasilyenko, D.Yu. Bratushev, T.Z. Puzanova, E.G. Gerasimov. Obtaining high energy-intensive permanent magnets from plate alloys Nd-Fe-B // FMM, 2010, Volume 109, No. 3, p.257-266]. Samples were either cut from sintered magnet blanks by the electrospark method either in the form of thin plates 4 × 4 × 1–3 mm ^{3 in} size or in the form of cylinders 9–20 mm in diameter and 3–6 mm high. The alloys used for diffusion annealing — DyCu, DyGa, Dy ₅ Ga ₃ Tb ₅ Ga _3, and Dy ₃ Co — were prepared by induction melting, and the hydrides DyH _x and TbH _x were treated by treating pure dysprosium and terbium metals in a hydrogen atmosphere at a pressure of 3 bar and a temperature of 500 ° C. Powders of these materials were obtained by grinding in a mortar or in a vibration mill to an average particle size D in the range from 3 to 500 microns. After grinding the surface of the sintered Nd-Fe-B magnets, they were placed in a matrix and filled with powders containing Dy and / or Tb on all sides, and then pressed into these powders at a pressure of about 2 kbar. As a result, the sintered magnet was coated with a layer of pressed powder about 1 mm thick. Samples prepared in this way were annealed in vacuum at 750–850 ° С for 0.5–20 hours, followed by low-temperature annealing at 550 ° С for 0.5 hours. After removing the powder shell, the magnetic properties of the plates were measured using a vibrating magnetometer, and of cylindrical magnets in a closed magnetic circuit of the Permagraph apparatus. Observations of the microstructure and determination of the distribution of the concentration of elements in the magnets were performed on TSCAN VEGA 2LMH and QUANTA-200 scanning electron microscopes with microanalyzers.

The effect of diffusion annealing temperature and particle size of the powders on the properties of sintered magnets in the form of plates with a thickness of 2 mm is shown in table 3.

Table 3 Powder D (μm) t _DA (h) The temperature of diffusion annealing, ° C 750 800 850 µ _o M _m , T B _r , T H _c , kA / m µ _about M _m , T B _r , T H _c , kA / m µ _o M _m , T B _r , T H _c , kA / m Dyh _x ≤40 0.5 1.36 1.34 1000 1.35 1.32 1080 1.34 1.32 1216 2 1.35 1.33 1176 1.32 1.28 1336 1.33 1.28 1272 TbH _x 0.5 1.35 1.34 1056 1.34 1.33 1560 1.33 1.32 1616 2 1.34 1.33 1264 1.34 1.33 1600 1.33 1.31 1664 Dy ₃ co 0.5 1.34 1.32 1080 * * * 1.31 1.25 1072 2 1.33 1.31 1160 1.32 1.29 1296 1.29 1.27 1368 Dycu 0.5 1.35 1.30 992 1.32 1.29 992 * * * 2 1.33 1.28 976 1.31 1.26 1032 * * * Dy ₅ ga ₃ 0.5 1.37 1.36 1120 1.35 1.33 1552 1.34 1.33 1616 2 1.35 1.34 1344 1.34 1.33 1608 1.33 1.32 1664 Tb ₅ Ga ₃ 0.5 1.35 1.34 1056 1.34 1.33 1592 1.34 1.33 1768 2 1.34 1.33 1384 1.34 1.33 1688 1.33 1.31 1864 Dyga ≤40 2 - - - 1.35 1.31 968 - - - 9 0.5 1.35 1.33 984 1.34 1.32 1040 1.33 1.29 1232 9 2 1.34 1.32 1008 1.35 1.33 1192 1.35 1.32 1272 4.9 2 - - - 1.35 1.32 1108 - - - * Note: Failed to remove the shell without destroying the magnet.

With increasing temperature and diffusion annealing time, the values of H _s increase and the values of _{R r} decrease slightly. Coercive force of Nd-Fe-B magnet plates pressed into DyH _x , Dy ₃ Co, DyGa, Dy ₅ Ga ₃ and TbH _x , Tb ₅ Ga ₃ powders after diffusion annealing at 750-800 ° С for 0.5-2 h and additional annealing at 550 ° C increases by 320-680 and 400-760 kA / m, respectively, without a significant decrease in B _r . The smallest increase in H _s occurs upon diffusion annealing with DyCu powder.

The degree of increase in H _c depends on the particle size of the powders. Powders from brittle materials DyH _x , TbH _x , By ₃ Co, DyGa, Dy ₅ Ga ₃ and Tb ₅ Ga _{3 are} readily obtained in the dispersed state. A decrease in the particle size of the DyGa powder from 40 to 9 μm effectively increases the values of H _c , however, the coercive force decreases if D decreases to 4.9 μm.

It was not possible to obtain powders with a small particle size from ductile Dy and mechanically durable cast alloy DyCu. As a result of annealing in contact with the powder of the latter material, the increase in coercive force did not exceed 240-320 kA / m. In addition, due to the formation of a fusible eutectic in the intergrain space of the magnets, DyCu powder was sintered with the magnet plate at annealing temperatures of more than 800 ° C, and this limits the diffusion annealing at higher temperatures. The abnormal temperature dependence of the coefficient of thermal expansion of Nd-Fe-B magnets in the region of the Curie point of the Nd ₂ Fe ₁₄ B phase contributed to the removal of the shell from the contact material after diffusion annealing [A.V. Andreev, A.V. Deryagin, S.M. Zadvorkin , S.V. Terentyev. Thermal expansion and magnetostriction of R ₂ Fe ₁₄ V compounds (R = Y, Nd, Sm) // FTT, 1985, v. 7. No. 6. S.1641-1645]. Since the shells made of paramagnetic contact materials do not have such a KTP anomaly, a crack appeared at the interface between the magnet surface and the sintered powder, which made it easy to remove the shell. At elevated temperatures, in cases of particularly strong welding with a magnet, it was necessary to use grinding or electric spark cutting of the shell.

The dependence of the properties of the plates of Nd-Fe-B magnets pressed into powders of various compositions on the diffusion annealing time t _DA at 750 ° C is shown in Fig. 1. The properties are shown after additional heat treatment at 550 ° C for 0.5 h. Qualitatively identical dependences of H _c , B _r and maximum magnetization µ _o M _m , measured in a field of 1.2 MA / m in strength, on t _{DA are} obtained for all diffusion contacts. The coercive force increases sharply at t _DA ≤1 h, and then monotonically and almost linearly increases at a speed of about 1.6 kA / m * h. The smallest increase in H _c provides contact with Dy powder. This is due to the fact that a coarse powder of metallic Dy (D≤500 μm) was used. The highest values of H _{c were} achieved in magnets pressed into Dy ₃ Co and DyH _x powders. The values of μ _o M _m also change slightly at t _DA ≤1 h, and then linearly decrease at a rate of about 9 mT / h. This is due to the fact that at the initial stages of annealing, the diffusion of dysprosium atoms occurs mainly in the intergranular enriched neodymium phase. At t _DA ≥1 h, the diffusion of Dy into Nd ₂ Fe ₁₄ B grains causes a monotonic decrease in µ _o M _m .

Confirmation in favor of such a sequence of diffusion processes is obtained by studying the microstructure and determining the distribution of the concentration of elements in magnets, performed using a scanning electron microscope. Figure 2a shows the microstructure of a magnet annealed at 750 ° C for 10 hours in contact with DyH _x powder after etching with nital. A section is prepared at a section perpendicular to the plane of the plate. The left edge of the thin section corresponds to the contact surface of the magnet with the powder. In sections of the thin section marked by rectangles, the concentration of Fe, Nd, and Dy was measured. The change in the concentration of elements from the surface of the magnet to its center is shown in Fig.2b. These results relate mainly to the content of elements in grains, since their volume significantly exceeds the volume of the intergranular phase enriched in Nd and Dy. In a layer 250 μm thick from the surface of the magnet plate, the concentration of Dy replacing Nd decreases linearly from 10 wt.% To zero. Thus, after diffusion annealing at 750 ° C for 10 h, the presence of dysprosium in the grains, which reduces the residual induction, is detected only in the surface layer of the magnet, not exceeding 250 μm.

To obtain an idea of the diffusion of Dy directly into the grain body, we measured the concentration of Fe, Nd, and Dy along the radius of grains located at distances of 40, 80, and 130 μm from the surface of this magnet. The measurement results are shown in figure 3. In a grain located at a distance of 40 μm from the surface of the magnet, the concentration of Dy is constant (about 10 wt.%) Up to the center of the grain. In a grain located at a distance of 80 μm, a diffusion zone about 2.5 μm wide is revealed, in which the concentration of Dy decreases from 10 wt.% On the grain surface to zero. In the grain at a depth of 130 μm, shown in Fig.3g, there is a shell containing not more than 8 wt.% Dy and having a thickness of about 1 μm. At a depth greater than 250 μm, Dy was not found in grains, and it can be present only in the intergranular phase enriched in Nd, leading to an increase in H _{from the} magnet without a decrease in B _r .

Case Study 1

Diffusion annealing of thin plates. Figure 4 compares the demagnetization curves of two plate samples annealed in the same mode in a free state without diffusion contact (a) and in contact with Dy ₃ Co powder (b). The demagnetization curves of the plates were measured in the initial state (curve 1), after annealing at 750 ° С, 2 h (curve 2), and after additional low-temperature treatment — 550 ° С, 0.5 h (curve 3). Annealing a free sample at 750 ° C reduces its coercive force by about a factor of two. Additional annealing at 550 ° C for 0.5 h restores H _c , but the squareness of the demagnetization curve worsens. Diffusion annealing of the Nd-Fe-B magnet in contact with Dy ₃ Co powder increases H _c , which, after additional low-temperature treatment, increases to 1160 kA / m. In this case, the residual induction B _r decreases by 7%, and the demagnetization curve maintains good rectangularity, which indicates homogeneous magnetic hardness over the entire volume of the magnet.

Case Study 2

Diffusion annealing of cylindrical magnets. Cylindrical magnets with a diameter of 12 mm and a height of h = 3, 4 and 6 mm were annealed at 750 ° C for 2 and 10 hours, and a magnet with a diameter of 20 mm and a height of 5.5 mm was annealed at 850 ° C for 20 hours. In both cases, Dy ₃ Co. powder was used as a contact material The hysteretic characteristics of the magnets in the initial state and after annealing are shown in Table 4. The residual induction of magnets with a diameter of 12 mm decreases by no more than 3.5 and 6.5%, and the values of H _c increase by more than 40 and 64%, respectively, for samples annealed for 2 and 10 hours at 750 ° C.

Table 4 The initial state After BS and annealing 550 ° С, 30 min h mm To mode B _r , T H _c , kA / m (HV) _m , kJ / m ³ B _r , T -ΔB _r , T (%) H _c , kA / m + ΔH _c , kA / m (%) (HV) _m , kJ / m ³ 3 750 ° C, 2 h 1.312 629 318.4 1.267 0.045 (3.4) 992.0 363.0 (57.8) 298.4 four 1.287 678 297.6 1.244 0.043 (3.3) 976.0 298.0 (44.0) 287.2 6 1.307 650 308.0 1.266 0.041 (3.1) 916.8 266.8 (41.0) 298.4 3 750 ° C, 10 h 1.307 610 309.6 1.224 0.083 (6.4) 1071.2 461.2 (75.5) 284.8 four 1.301 638 304.0 1.228 0.073 (5.6) 1048.0 410.0 (64.4) 283.2 6 1.318 626 313.6 1.242 0.066 (5.0) 984.0 358.0 (57.3) 291.2 5.5 850 ° C, 20 h 1.322 520 307.2 1.248 0.074 (5.6) 1124.0 604 (116.2) 292.0

The demagnetization curves of a magnet with a diameter of 20 mm and a height of 6 mm before and after diffusion annealing at 850 ° С for 20 h and additional annealing at 600 ° С, 1 h are shown in Fig. 5a. After annealing, the magnet had the following characteristics: B _r = 1.22 T, H _c = 704 kA / m and (HV) _m = 257 kJ / m ³ . Using the method of X-ray microanalysis, it was found that the thickness of the surface layer L, in which Dy is found in the volume of Nd ₂ Fe ₁₄ B grains, is about 650 μm, and the qualitative form of the dependences of the distribution of dysprosium concentration along the radius of grains located at distances of 200, 440, and 650 well consistent with the results presented in figure 3 for a plate magnet.

In order to get an idea of the effect of diffusion annealing on the properties of volumes remote from the surface, a cylinder with a diameter of 9.5 mm and an initial height was cut from a magnet with a diameter of 20 mm and a height of 5.5 mm at a distance of 3 mm from the side surface. Only the surface layers from the ends of this magnet should influence the decrease in its B _r . The dependence of H _c on the temperature of the additional annealing T _{A of} this sample is shown in Fig.5b. The coercive force of the magnet increases with a sequential decrease in the annealing temperature, reaching a maximum at T _A = 550 ° C. The demagnetization curves immediately after cutting (curve 3) and in the state with the maximum coercive force (curve 4) are compared with the demagnetization curves of the original magnet in Fig. 5a, and the parameters of the demagnetization curves are presented in the last row of table 4. The increase in H _{c of the} magnet with respect to the initial state was 116.2%, which of course cannot be explained by the influence of only surface layers at the ends, the volume fraction of which is only about 20%. Consequently, the diffusion of dysprosium along the grain boundaries of large-volume magnets also extends to a depth far exceeding the thickness of the surface layer L≈650 μm.

Claims (1)

A method for producing highly energy-intensive highly coercive magnets from Nd-Fe-B alloys by diffusion annealing of sintered magnets, the surface of which is in contact with dispersed powders of materials - diffusion sources, based on dysprosium and / or terbium, characterized in that dispersed metal dysprosium is used as diffusion sources or terbium, dysprosium or hydrides and / or terbium, or alloy powders based on intermetallic compounds R _m T _n having a particle size in the range 5-500 microns and having a melting point 500-1200 ° C,
where R is Dy or Tb;
T - Co, Fe, Ni, Al, Cu, Ga, or a combination thereof;
m and n are atomic fractions of R and T elements, respectively, and m≥n,
while diffusion annealing is carried out for 0.5-20 hours at a temperature of 750-850 ° C, followed by additional annealing at a temperature of 350-600 ° C for 0.5-1.0 hours

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