Structural and magnetic engineering of (Nd, Pr, Dy, Tb)–Fe–B sintered magnets with Tb3Co0.6Cu0.4Hx composition in the powder mixture

Journal Pre-proofs Structural and magnetic engineering of (Nd, Pr, Dy, Tb)–Fe–B sintered magnets with Tb3Co0.6Cu0.4H x composition in the powder mixture K. Skotnicova, G.S. Burkhanov, N.B. Kolchugina, M. Kursa, T. Cegan, A.A. Lukin, O. Zivotsky, P.A. Prokofev, J. Jurica, Y. Li PII: DOI: Reference:

S0304-8853(19)33088-4 https://doi.org/10.1016/j.jmmm.2019.166220 MAGMA 166220

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Journal of Magnetism and Magnetic Materials

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3 September 2019 1 November 2019 26 November 2019

Please cite this article as: K. Skotnicova, G.S. Burkhanov, N.B. Kolchugina, M. Kursa, T. Cegan, A.A. Lukin, O. Zivotsky, P.A. Prokofev, J. Jurica, Y. Li, Structural and magnetic engineering of (Nd, Pr, Dy, Tb)–Fe–B sintered magnets with Tb3Co0.6Cu0.4H x composition in the powder mixture, Journal of Magnetism and Magnetic

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Structural and magnetic engineering of (Nd, Pr, Dy, Tb)–Fe–B sintered magnets with Tb3Co0.6Cu0.4Hx composition in the powder mixture K. Skotnicova1, G.S. Burkhanov2, N.B. Kolchugina2, M. Kursa1, T. Cegan1, A.A. Lukin3, O. Zivotsky1, P.A. Prokofev2, J. Jurica1, Y. Li4 1VSB

– Technical University of Ostrava, Faculty of Materials Science and Technology, Regional Materials Science and Technology Centre, Ostrava, Czech Republic, EU, [email protected] 2Baikov

Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Moscow, Russian Federation 3JSC

SPETSMAGNIT, Moscow, Russian Federation

4School

of Materials Science and Engineering, Shanghai University, P. R. China

Abstract High-coercivity Nd–Fe–B magnets are required for clean energy applications, particularly for hybrid and electric vehicles. This study was focused on the structural design of a Nd2Fe14Bbased magnet by precise engineering of its microstructure, which included grain-boundary diffusion and grain-boundary structuring processes, through the introduction of a hydrogenated Tb3Co0.6Cu0.4Hx composition in the powder mixture. A low-rare-earth-metal strip-cast Nd-24.0, Pr-6.5, Dy-0.5, B-1.0, Al-0.2, Fe-balance (wt.%) alloy was used as the base component of the powder mixture. The distributions of the components of the blended powder mixture in the sintered magnet with the added 2 wt.% of Tb3Co0.6Cu0.4Hx and stability of the structure-sensitive magnetic parameter (coercive force) during a low-temperature heat treatment were studied. The sample exhibited a high coercive force up to 1480 kA/m at a heavy rare-earth element content in the 2–14–1 phase of ~1 at.%. Keywords: Nd–Fe–B sintered magnet; hydrogenated addition; grain-boundary diffusion; grainboundary structuring; microstructure; hysteretic properties. 1.

INTRODUCTION

High-coercivity Nd–Fe–B magnets are in demand for clean energy applications, particularly for hybrid and electric vehicles. The use of magnets has recently increased and is expected to further increase in the future considering the environmental issues. Typically, these applications require magnets having high coercive forces (jHc) and remanences (Br). It is well known that added heavy rare-earth metals (HREMs), such as Dy and Tb, effectively increase the coercivities of Nd2Fe14B-based magnets owing to the higher magnetic anisotropy fields of Dyand Tb-substituted Nd2Fe14B (2–14–1) phases. On the other hand, the remanence decreases owing to the antiferromagnetic coupling of the Fe and Dy magnetic moments and therefore the maximum energy product of the magnet simultaneously decreases. Recently, the development of high-coercivity Nd–Fe–B magnets with HREM-less or HREM-free compositions has attracted considerable attention, as the metals are scarce and exhibit price fluctuations. The techniques used to increase the coercivity of a sintered Nd–Fe–B magnet, so-called extrinsic approaches, are based on the formation of a thin magnetic hardening shell within the 2–14–1-phase grains by the intergranular addition or diffusion of Dy/Tbcontaining powders [1–3]. The grain-boundary diffusion (GBD) process has been developed as a very effective technique for reducing HREM (in particular, Dy) usage by more than 50% [3, 4]. Various methods for GBD, such as diffusion saturation (from rare-earth-fluoride [5], rare-earth [6], and rare-earth-alloy coatings [7]) and powder mixing with REMs and their compounds [8–12], have been reported. The GBD provides Nd-enriched (HREM-learned) core grains coated by HREM-

enriched (Nd, HREM)2Fe14B shells, which impede the nucleation of reversed domains and domain wall displacements and thus leads to an increase in coercivity [7]. By controlling the GBD process time and temperature, the coercivity of the magnet can be largely increased without marked decreasing the remanence. However, the traditional REM saturation method can be applied only for small and thin magnets (≤ 5 mm) as the channels for the diffusion of Dy and/or Tb into the magnet are limited in the heat treatment (HT) [6, 9]. Grain-boundary restructuring, a relatively new alloying method, is used for the fabrication of high-performance Nd–Fe–B magnets. The addition of a low-melting-point eutectic alloy to a HREM-free matrix powder can improve the wettability of the Nd2Fe14B phase grains, optimise the grain boundary structure, and form the phase surrounding Nd2Fe14B phase grains. Consequently, the coercivity of the Nd–Fe–B magnet can be significantly increased without a considerable reduction in remanence. Alloys based on (at.%) Dy–Fe (Dy71.5Fe28.5), Dy–Ni (Dy69Ni31), Dy–Mn (Dy88Mn12), (Pr37Dy30Cu33)-Hx, DyHx, etc. have been used as effective additions [12–16]. The alloying of the basic Nd–Fe–B composition with REMs added in the form of hydrides is reported in [17–20]. The hydrides decompose into REM powders during the sintering of the magnet blanks enabling HREM atom diffusion into the grains with an accompanying improvement in coercivity of the magnet. Our previous studies demonstrated that Tb and Dy hydride additions led to increases in coercive force without affecting the remanence [19] and stability of the hysteretic properties of the magnet during low-temperature annealings [20], respectively. This paper presents the precise engineering, i.e., the optimisation of the microstructure of a near-stoichiometric Nd2Fe14B-based magnet, which included the processes of GBD and grainboundary structuring through the addition of the Tb3Co0.6Cu0.4Hx composition to the powder mixture. The magnetic properties of the magnets fabricated using the blending technology for the addition of theTb3Co0.6Cu0.4Hx composition in the powder mixtures are discussed by considering their microstructures and component distributions. 2.

EXPERIMENTAL

The base alloy having a composition of (wt.%) Nd-24.0, Pr-6.5, Dy-0.5, B-1.0, Al-0.2, Fe-balance was prepared by strip-casting. It was subjected to hydrogen decrepitation during heating to 270 °C under a hydrogen flow at a pressure of 0.1 MPa and subsequent 1-h dwell at the same temperature. The Tb3Co0.6Cu0.4 alloy was prepared by arc-melting the starting components in an argon atmosphere on a water-cooled copper bottom using a nonconsumable tungsten electrode. This alloy was subjected to a homogenising annealing at 600 °C for 90 h. It was a multiphase alloy containing the Tb3(Co,Cu) compound as the base phase and Tb(Cu,Co) and Tb12(Co,Cu)7 compounds as impurity phases. The hydrogenation was carried out in two regimes, heating to 270 °C under a hydrogen flow at a pressure of 0.1 MPa and subsequent 1-h holding at the same temperature (conditions applied also for the strip-cast alloy, which enabled the simultaneous hydrogenation of the base alloy and addition) and stepped heating in a hydrogen atmosphere and holding at 500 °C in a glass Sievert-type apparatus. It yielded a powder of TbH2-3 plus (Co, Cu) fine mixture. The Tb3Co0.6Cu0.4Hx composition and hydrogendecrepitated strip-cast alloy were mixed and subjected to fine milling for 40 min using a vibratory mill and isopropyl alcohol medium, which yielded an average particle size of 3 μm. After the wet compaction of the pulp in a transverse magnetic field of 1500 kA/m, blanks of magnets were sintered at 1080 °C for 2 h and subjected to an optimal HT at 500 °C for 2 h + quenching in argon. The following subsequent stepped low-temperature HTs (LTHTs) were carried out: 20 °C  (40 min)  500 °C (20 min)  (6 h)  400 °C (10 h)  quenching in nitrogen. High-resolution field-emission scanning electron microscopy (SEM; FEI QUANTA 450 FEG equipped with an energy-dispersive X-ray spectroscopy (EDS) microprobe) and

transmission electron microscopy (TEM; JEOL JEM-2100F equipped with an EDS microprobe) were used to investigate the structures, chemical compositions, and distributions of the magnet components (X-ray mapping). The distributions of Nd and Pr and alloying elements in a 2–14–1phase grain were also studied using the local electrode atom probe (LEAP; LEAP 4000X HR) technique with a 100-nm-long tip cut from the 2–14–1-phase grain. The thin foil and tip used in the TEM and LEAP analyses, respectively, were obtained from the magnet sample using focused ion beam SEM (FEI Helios 600i). The hydrogen and oxygen contents were determined using an ONH – 2000 ELTRA analyser. The particle size distribution was determined using a laser diffraction particle size analyser (MasterSizer 3000, Malvern). The magnetic properties of the finished cylindrical permanent magnet with a diameter of 30 mm and height of 10 mm were measured at room temperature (RT) using an Automatic Hysteresis Graph system. To analyse the magnetic properties at higher temperatures up to 150 °C, we used a Microsense EZ9 vibrating-sample magnetometer (VSM) with an oven operating in the temperature range of RT to 1000 °C. For these measurements, the dimensions of the magnet sample were reduced to (1 × 1 × 1) mm3 owing to the small space in the oven and in order to not saturate measuring coils in the VSM. The maximum applied magnetic field of the VSM was 2400 kA/m (3 T). 3.

RESULTS AND DISCUSSION

3.1 Microstructures and compositions of the phases of the prepared magnets The microstructure of the magnet fabricated from the powder mixture with 2 wt.% of Tb3Co0.6Cu0.4Hx and subjected to the optimum HT is presented in Fig. 1. The results of the local electron microprobe analysis of the phases are presented in Table 1. For phases 1, 2 and 4, the average values of three measurements are presented. The stoichiometric composition of the grains was close to that of the Nd(R)2Fe14B phase (Fig. 1a). Phases 1 and 4 differed in the REM content. Grains having an increased Nd content and reduced Tb content were present in the magnet structure (phase 4 in Fig. 1a). This was confirmed by a line chemical analysis over two grains (Fig. 2). Moreover, grains having core–shell structures are observed in the image presented with an increased contrast (Fig. 1b – red circle). Such a structure is typical for magnetic materials fabricated using the GBD techniques or REM-alloy-containing powders.

Fig. 1 SEM images of the microstructure of the Nd–Fe–B magnet fabricated from the powder mixture with 2 wt.% of Tb3Co0.6Cu0.4Hx; the digits represent the analysed phases in Table 1; red circle shows core–shell structure of a 2-14-1 grain.

Table 1 Chemical analysis of the Nd–Fe–B magnet fabricated from the powder mixture with 2 wt.% of Tb3Co0.6Cu0.4Hx. Area/phase Area Phase 1 Phase 2 Phase 3.1 Phase 3.2 Phase 3.3 Phase 4 Area Phase 1 Phase 2 Phase 3.1 Phase 3.2 Phase 3.3 Phase 4

O

11.6 13.1 20.1

51.0 56.1 68.1

Tb

Dy

Al

2.3 2.1 0.0 4.5 5.2 4.5 1.1

1.5 1.2 0.0 2.4 2.4 2.2 1.0

0.3 0.3 0.4 0.0 0.0 0.0 0.3

1.0 0.9 0.0 2.0 2.2 1.5 0.5

0.6 0.5 0.0 1.0 1.0 0.7 0.4

0.7 0.7 1.6 0.1 0.0 0.0 0.8

Pr (wt.%) 6.9 5.8 24.6 17.5 17.8 17.0 6.1 (at.%) 3.5 2.8 18.4 8.7 8.6 6.5 3.0

Nd

Fe

Co

Cu

23.8 21.5 50.7 56.0 57.6 52.6 22.2

63.8 67.9 9.5 6.9 2.7 2.5 67.6

1.0 0.9 4.1 0.8 0.9 0.8 1.1

0.5 0.4 10.7 0.3 0.4 0.3 0.7

11.7 10.2 37.1 27.3 27.3 19.7 10.6

80.8 83.4 17.9 8.6 3.3 2.4 82.8

1.2 1.0 7.3 0.9 1.1 0.7 1.3

0.6 0.4 17.7 0.3 0.4 0.3 0.7

Fig. 2 (a) Line chemical analysis over two grains (Nd(R)2Fe14B) and (b) corresponding SEM image, where the decreased content of Tb is evident. The main magnetic phase of the sample exhibited Tb-enriched, Tb-depleted, and core– shell grains. This Tb distribution was realised owing to the GBD of Tb formed by the decomposition of TbH2-3 hydrides originating from the hydrogenation of the Tb3Co0.6Cu0.4 multiphase alloy. The reactive Tb powder (originating from TbH2-3 thermally destroyed during the sintering) determined the diffusion of Tb atoms to the 2–14–1-phase lattice, whose radii are smaller than that of Nd atom. It was accompanied by ousting Nd atoms to peripheral areas. As the diffusion coefficient of Nd atoms is lower than that of Tb atoms [21], the HREM diffusion was more substantial. The inequality of diffusion flows of atoms induced lattice stresses and led to inhomogeneous Tb and Nd(Pr) distributions over the 2–14–1-phase grains. Notably, a higher Tb content corresponded to a lower Nd (Pr) content. As the contents of the rare-earth components of the 2–14–1 phase were variable, we carried out a precise analysis of their distributions over the magnetic phase grains.

The intergranular Nd-rich phases at triple junctions differing in Co, Cu, and REM contents were analysed (phases 2 in Fig. 1a). According to the chemical analysis data, the Pr and Nd contents varied in the ranges of 9.2 to 18.4 and 23.4 to 37.1 at.%, respectively. The REMbased oxide phases (phase 3 in Fig. 1a) might correspond to NdO (oxygen content of 50 at.%), Nd2O3 (oxygen content of 60 at.%), or NdO2 (oxygen content of 67 at.%) [22–23]. The total oxygen content in the sample was approximately 5000 parts per million (ppm). The low hydrogen content in the magnet of 3 ppm (mean value) determined using the inert-gas fusion method indicates the complete decomposition of Tb3Co0.6Cu0.4Hxand removal of hydrogen during the sintering of the magnet. The distributions of the REMs, Co, and Cu in the matrix grains and in the intergranular Nd-rich phases were studied using X-ray mapping and LEAP analysis. A nonuniform Tb distribution was observed in the 2–14–1 grains (Fig. 3). Notably, Co depletion and Cu enrichment were observed at the triple junctions.

Fig. 3 SEM image and X-ray maps of the REMs, cobalt, and copper in the matrix grains and Nd(Pr)-rich phases of the Nd–Fe–B magnet fabricated from the powder mixture with 2 wt.% of Tb3Co0.6Cu0.4Hx. According to the LEAP data (Fig. 4), the Tb content in the main magnetic 2–14–1 phase was approximately 0.6 at.%. It varied compared to the other diffused elements, Cu and Co, but was almost constant over a distance of 100 nm.

Fig. 4 LEAP data on the distributions of the REMs, Cu, and Co. a) Concentration profiles of the REMs, Co, and Cu, b) detailed concentration profile of Tb, and one-dimensional distributions of c) Nd and d) Tb. The sample was in the form of a 100-nm-long tip cut from the 2–14–1-phase grain. Along with the formation of core-shell structure due to Tb entering from Tb3Co0.6Cu0.4Hx composition, also other components, i.e., Cu and Co, are known to be useful additions for Nd– Fe–B-based magnets. The distributions of components in a selected triple-junction phase (TJP) were investigated using TEM. Figures 5 and 6 show the bright-field TEM image of the TJP and X-ray map of the main components and elements added in the Nd–Fe–B-based magnet fabricated from the powder mixture with 2 wt.% of Tb3Co0.6Cu0.4 hydrided composition. It is obvious from the images that the TJP is enriched in Nd and Pr and is depleted of Tb. The Co content demonstrating the variable distribution within the triple junction was higher near the grain boundaries and decreased towards the centre of the triple junction. The distributions of Cu and Co suggest that beside Nd-Pr-Cu rich triple junction phase, Nd-Pr-Co-Cu rich phase can exist in the microstructure. Thus, according to the TEM data, Cu concentrated at triple junctions (Cu tends to avoid entering 2–14–1 phase since Cu destabilizes the 2–14–1 structure), whereas Co tended to enter the 2–14– 1-phase grains. However, according to LEAP data (Fig. 4), both Co and Cu were detected within the 2-14-1 grain and the Co content is higher than that of Cu. Boron concentration is slightly higher within TJP.

Fig. 5 (a) Bright-field TEM image of the TJP and (b) X-ray map of the elements in the Nd–Fe–B magnet fabricated from the powder mixture with 2 wt.% of Tb3Co0.6Cu0.4Hx.

Fig. 6 X-ray maps of the TJP in the Nd–Fe–B magnet fabricated from the powder mixture with 2 wt.% of Tb3Co0.6Cu0.4Hx. The possibility of the presence of Cu in 2-14-1 phase grains was considered in [24–25]. It was shown by calculations that due to the small energy barrier, certain amount of Cu (about 1.5 at.%) will dissolve in 2-14-1 main phase during induction-melting process (above 1330 °C). The substitution of Cu for Fe in 16k1 site in 2–14–1 could occur at high temperatures. Thus, the found presence of Cu in the 2–14–1 agrees with the data. However, during the optimum annealing process, Cu diffuses from the 2–14–1 grains to the grain boundary region (to a certain degree or completely) so as to decrease the total energy [25]. Such a behavior of Cu results in the redistribution of Cu along the grain boundary of the 2–14–1 phases during the optimum HT and LTHT and the enhancement of the coercivity owing to a decrease in the activation energy for

wetting and thus decrease in the surface imperfection [26–30]; the compositions of Cucontaining Nd-rich phase were studied with advanced methods [24, 31–33]. According to [34], Cu could be present in the intergranular (and triple junction) phase in the form of a eutectic phase of the (Pr,Nd)–Cu–Me (Me could be Al) system. The possibility of Cu-driving REM diffusion to form depth of core-shell type grains is indicated in [12]. The formation of Cu-rich grain boundary phase might have enhanced the diffusivity of Tb-atoms. In magnets fabricated using the Tb3Co0.6Cu0.4Hx composition, the additional effect of Cu cannot be excluded since grains differing in the Tb distribution, namely, Tb-depleted, Tb-enriched, and Tb shelled are observed. According to the data in [12], the improved thermal stability of the coercive force was attributed to the core/shell microstructure as the Nd-rich grain boundaries provided a high ambient coercivity, while the Co-rich shell provided an improved coercivity stability. The proposed Nd2Fe14B-core/Nd2(Fe,Co)14B-shell microstructure was realised by diffusion processing. According to our data (Fig. 5), the observed higher Co content near the grain boundaries as compared to that in the centre of the triple junction agrees with data of [12]. 3.2 Magnetic properties The magnetic properties of the sintered magnets with 2 wt.% of Tb3Co0.6Cu0.4Hx are presented in Table 2 and Fig. 7. Notably, the coercive force of the magnet subjected to the stepped LTHT is close to that of the magnet sintered in our previous study [19] using 2 wt.% of TbH2 in the powder mixture, but Br and (BH)max exceed those of the previously reported magnet. We expect less oxidised states of the additions in the case of Tb3Co0.6Cu0.4Hx. Another possible factor for the increase in coercive force is the improved wettability of the Nd2Fe14Bphase grains with grain-boundary phases alloyed with Co and Cu. Table 2 Magnetic properties of the sintered magnets with 2 wt.% of Tb3Co0.6Cu0.4Hx subjected to the optimal HT at 500 °C for 2 h and subsequent LTHT (20 °C → (40 min) → 500 °C (20 min) → (6 h) → 400 °C (10 h)); Br – remanence of magnetic flux density, jHc – coercivity of magnetic polarization, Hk – parameter adopted as a criterion of coercivity (it is the magnetic field determined at 0.9× Br, (BH)max – maximum energy product. Addition/annealing conditions Tb3Co0.6Cu0.4Hx/optimal HT Tb3Co0.6Cu0.4Hx/optimal HT + LTHT 2 wt.% of TbH2/optimal HT [19] 0 wt.% of Tb, optimal HT [19]

Br (Т) 1.35 1.35 1.30 1.36

jHc

(kА/m) 1336 1480 1520 1000

Hk (kА/m) 1200 1330 1440 850

(BH)max (kJ/m3) 360 ≥360 332 358

Fig. 7 RT J–H and B–H curves of the Nd–Fe–B magnets fabricated from the powder mixture with 2 wt.% of Tb3Co0.6Cu0.4Hxand subjected to the optimal HT and stepped LTHT (Automatic Hysteresis Graph system, the sample size was 30 mm in diameter and 10 mm in high). Studies on the stability of the structure-sensitive parameter (coercive force jHc) of a sintered magnet subjected to the LTHT (annealing at a temperature below the optimal HT temperature of 500 C) showed its increase (Table 2) up to 1480 kA/m upon hydrogenated Tb3Co0.6Cu0.4 addition. These studies were carried out by estimating the time and temperature stabilities of the magnets during their operation. This result is not typical for sintered Nd–Fe–B magnets, which usually exhibited decreased (or constant) coercive forces after LTHTs at 350– 450 C [35–36]. A constant coercive force during a low-temperature annealing was observed in the case of the DyH2 addition to the powder mixture [20]. This was explained by the Invar effect, attributed to changes in crystal-lattice rigidity owing to the structuring of the 2–14–1 phase alloyed with dysprosium by GBD. Thus, in the case of the alloying of the 2–14–1 phase with the HREM by GBD and use of Tb3Co0.6Cu0.4Hx, jHc and Hk were not decreased. The increases in these parameters can be related to the effects of Cu in the intergranular REM-rich phase and Co [12]. Figure 8 shows a series of J–H curves measured using the VSM at higher temperatures. They exhibit obvious tilting shoulder compared to the hysteresis loop in Fig. 7. We expect that such behaviour is connected with different sample dimensions used for the hysteresis loops measurements. The automatic hysteresis graph system determines averaged bulk magnetic response of finished cylindrical magnets (30 mm in diameter, 10 mm height), while for the VSM much smaller samples with dimensions (1 ×1 ×1) mm3 are used. Therefore, the VSM magnetic characteristics can be considered as local and strongly depend on what part of the magnet the sample was obtained from. Moreover, it seems that the shape of the VSM curve evinces two magnetisation reversals. It could be explained by the effect of the surface layer of a small sample that exhibits a higher surface fraction and thus a larger effect. This separation of phases was also observed for room- and high-temperature curves. Notably, the Tb content in the magnet was not sufficient to ensure its thermal stability.

Fig. 8 J–H curves of the Nd–Fe–B magnet fabricated from the powder mixture with 2 wt.% of Tb3Co0.6Cu0.4Hx measured at different temperatures using the VSM. The sample dimensions were (1 ×1 ×1) mm3. 4.

CONCLUSIONS

A detailed investigation was carried out on the microstructures, distributions of components, and magnetic properties of the Nd–Fe–B magnets fabricated from the powder mixture with 2 wt.% of Tb3Co0.6Cu0.4Hxand subjected to the optimal HT and stepped LTHT. The strip-castNd-24.0, Pr-6.5, Dy-0.5, B-1.0, Al-0.2, Fe-balance alloy (with the total REM content of 31 wt.%) was used as the main component of the powder mixture to enable the GBD of Tb atoms. The Tb3Co0.6Cu0.4Hx composition efficiently increased the coercivity of the Nd–Fe–B magnet, with a small reduction in remanence. The achieved increase in coercive force was associated with the microstructure formed through the combined GBD of Tb and grain-boundary restructuring of Cu and Co during the sintering and HT mode. The increase in coercive force of the sintered Nd–Fe–B magnet upon the LTHT is not typical for magnets, which usually exhibit decreased (or constant) coercive forces after LTHTs at 350–450°C. ACKNOWLEDGEMENTS This study was carried out within the project LTARF18031 “Development of physicochemical and engineering foundations for the initiation of innovative resources–economy technology of high-power and high-coercivity (Nd,R)–Fe–B (R = Pr, Tb, Dy, Ho) low-REM permanent magnets” and within the Project No. 14.616.21.0093 (unique identification number: RFMEFI61618X0093). The analyses were performed using the research infrastructure of the Regional Materials Science and Technology Centre, VSB – Technical University of Ostrava (Czech Republic) and that of the Centre of Collaborative Access for Functional Nanomaterials and High-Purity Substances, Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences. REFERENCES [1] G. Yan, P.J. McGuiness, J.P.G. Farr, I.R Harris. Optimisation of the processing of Nd-Fe-B with dysprosium addition. J. Alloy. Compd. 2010 (491) L20-L24. [2] I.V. Mitchell, J.M. Coey, D. Givord, I. R. Harris, and R. Hanitsch, ed. Concerted European Action on Magnets (CEAM). Essex: Elsevier Science Publishers LTD, 2012, 928 p.

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HIGHLIGHTS The detailed investigation of the microstructure, distribution of magnet components, and magnetic properties of Nd-Fe-B sintered magnet prepared from the powder mixture with 2 wt.% Tb3Co0.6Cu0.4Hx and subjected to optimal heat treatment and low-temperature stepped heat treatment was carried out. The low-REM strip-casting Nd-24.0, Pr-6.5, Dy-0.5, B-1.0, Al-0.2, Fe-balance alloy is used as the main component of the powder mixture in order to allow the grain-boundary diffusion of Tb to be realized. It was demonstrated that the application of hydrogenated Tb3Co0.6Cu0.4Hx composition is expected to efficiently enhance the coercivity of Nd-Fe-B magnets with slight sacrifice of their remanence.