EFFECTS OF ADDITIONS AND HEAT TREAMENT ON THE MICROSTRUCTURE AND MAGNETIC PROPERTIES OF SINTERED ND-FE-B MAGNETS V.P. MENUSHENKOV 1, A.G. SAVCHENKO 1, K. SKOTNICOVA 2, M. KURSA 2 1 National Research Technological University MISiS, 119049, Leninskij prospect 4, Moscow, Russia. menushenkov@gmail.com 2 VŠB Technical University of Ostrava, Faculty of Metallurgy and Materials Engineering, Department 606 - Regional Materials Science and Technology Centre, Av. 17. listopadu 15, 70833 Ostrava-Poruba, Czech Republic, Katerina.Skotnicova@vsb.cz Abstract Sintered Nd-Fe-B permanent magnets with an enhanced coercive force were fabricated (i) using the main Nd 14,5 Dy 1,5 Fe 75 Co 2 B 7 alloy and its mixtures with [Dy/Al], Tb 2 O 3, Nd 3 Co additions and (ii) applying a postsintering heat treatment of magnets. The average grain size of the Nd 2 Fe 14 B phase decreases with increasing contents of [Dy/Al], Tb 2 O 3 and Nd 3 Co additions and their melting temperatures. The mixing method is effective for all type of additions used. The maximum coercive force magnitudes were reached after aging at 550 o C, which was performed after cooling from sintering temperature to room temperature. The same aging performed after cooling from sintering temperature to 550 o C, i.e., without the intermediate cooling does not increase H ci. The correlation between structural changes and hysteretic properties of the heat-treated magnets has been discussed. Keywords: Nd2Fe14B alloy, Sintered magnets, Heat treatment, Coercive force 1. INTRODUCTION Nd-Fe-B permanent magnets are widely used for different applications and the market of them is evergrowing as their magnetic properties improve. Sintered Nd-Fe-B magnets are the most promising materials for driving motors of the hybrid electric vehicles. Since the operating temperature of magnets in these motors reaches 200 o C, the very high coercive force H ci at room temperature is necessary. The powder metallurgy method is the common technique for manufacturing high-coercivity Dy-containing Nd-Fe-B magnets [1,2]. The effective way allowing one to increase H ci is the replacing of conventional Nd-rich constituents by small additions of low-melting Dy,Tb/Co-based alloys [3,4]. Such an addition hardens the Nd 2 Fe 14 B-phase boundaries without considerable losses in the saturation polarization of a magnet. The H ci value of sintered magnets can be further enhanced by a heat treatment at ~ 500 o С [5,6], but the origin of such a coercivity enhancement during annealing is not well understood yet. The goal of this work was to compare the influence of the different type of additions on the microstructure and coercivity of sintered Nd,Dy-Fe-B magnets and to study the effect of three type of heat treatment on the magnetic properties of these magnets. 2. EXPERIMENTAL PROCEDURE The (Nd,Dy) 2 Fe 14 B magnets were prepared by two-powder blended method using the main alloy Nd 14,5 Dy 1,5 Fe 75 Co 2 B 7 and [Dy/Al], Tb 2 O 3, Nd 3 Co additions. Sintered magnets were subjected to heat treatment (HT) in accordance with three regimes which are shown in Fig. 1. Regime HT1 includes the heating to T 1 = 950 o C, cooling to the aging temperature (T 2 ), aging at T 2 for 30 min and cooling to RT. Temperature T 2 is varied from 400 to 700 o C. Regime HT2 includes the heating to T 1 = 950 o C, cooling from T 1 to T 2 at the rate V = 1,4 K/min, aging at T 2 for 30 min and cooling to RT. Temperature T 2 is varied from
400 to 700 o C. Regime HT3 includes the cyclic aging at Т 1 = 950 о С, 10 min and Т 2 = 550-560 о С, 30 min with cooling to RT. Fig. 1 Regimes of post-sintering heat treatment The compositions of the alloys were verified by plasma atomic emission spectrometry. The phases present in alloys and magnets were identified by X-ray diffraction (XRD) using Cu-K radiation and thermo-magnetic analysis (TMA). Simplified Rietveld method and program Phan% were used for quantitative phase analysis [7]. Magnetic measurements were carried out using a hysteresisgraph. 3. RESULTS AND DISCUSSION The compositions of the sintered alloys and temperatures of post-sintering aging are shown in Table. 1. Tab. 1 Composition principal alloy (PA) PA + 1 % [Dy/Al] PA + 2 % [Dy/Al] PA + 3 % [Dy/Al] PA + 4 % [Dy/Al] PA + 1 % Tb 2O 3 PA + 2 % Tb 2O 3 PA + 0.5 % Nd 3Co PA + 1 % Nd 3Co PA + 1 % Nd 3Co + 2 % [Dy/Al] Heat treatment The effects of [Dy/Al] weight fraction (x) on H ci of (Nd 14,5 Dy 1,5 Fe 75 Co 2 B 7 ) 100-x (Dy 30 Al 70 ) x magnets in sintered and heat treated states is shown in Fig. 1; the H ci(x) dependence for the magnets obeys polynomial law: H ci (sint) = -0,04 x 2 + 2,62 x +6,17; H ci (ТО1) = -0,32 x 2 + 3,79 x + 5,23; H ci (ТО2) = -0,29 x 2 + 4,71 x + 10,53. Magnetic hardening of sintered magnet is a result of growth of effective anisotropy field of (Nd 1-x Dy x ) 2 Fe 14 B phase and simultaneous reduction of M s due to the replacement of Nd atoms by Dy in Nd 2 Fe 14 B grains.
Fig. 2 H ci of sintered and heat treated magnets as a function of weight fraction of [Dy /Al] addition The aging at T 2 = 570 o C, 30 min (HT1) increases H ci by 2-2.5 times. But the same aging without intermediate cooling to 20 o C (HT2) does not change H ci (Fig. 1). The H ci increase was observed only if the magnets have been cooled to RT before aging at T 2 = 570 o C. The slopes of H ci (x) curves in sintered and aged states are identical (Fig. 1), hence the magnetic hardening after HT1 is similar to that for the sintered state. Therefore, only the structural hardening caused by changes in the microstructure of magnets is the cause for the H ci doubling after HT1. Variations of H ci at different stages of cуclic heat teratment HT3 are shown in Table 2. The values H ci (950 o C)/H ci (900 o C), H ci (550 o C)/H ci (950 o C) and H ci (560 o C)/H ci (950 o C) are shown in Fig. 2. Tab. 2 Coercive force after cyclic heat treatment HT3 No. Composition B r, T Н ci, ka/m Н ci (after heat treatment), ka/m 1 900 С,15' 950 С, 10' 550 С, 30' 950 С,10' 560 С, 30' Principal alloy (PA) 1,09 1370 925 950 1180 1005 1260 2 PA+1 % [Dy/Al] 1,04 1180 1240 1300 1590 1330 1570 3 4 PA+2 % [Dy/Al] 1,02 1820 18,25 1295 1465 1520 1560 PA+3 % [Dy/Al] 1,02 1860 1440 1430 1600 1515 1660 5 PA+4 %[ Dy/Al] 0,97 1980 1900 1900 1850 2015 1970 6 7 PA+1 % Tb 2O 3 1,09 1320 1090 1440 1510 1180 1280 PA+2 % Tb 2O 3 1,06 1430 1265 1280 1740 1295 1500 8 PA+0,5 % Nd 3Co 1,12 880 960 950 1265 1005 1290 9 PA+1 % Nd 3Co 1,12 840 830 870 1220 870 1160 10 PA+1 % Nd 3Co + +2 % [Dy/Al] 1,03 1600 1270 1260 1530 1310 1580 The data given in Table 2 and Fig. 2 show the reversible changes of H ci during high temperature-low temperature aging of sintered magnets. The magnets after HT1 (H ci = 1200-1900 ka/m) were aged at T 1 = 950 o C (H ci = 950-1400 ka/m). The following aging at 550 o C for 30 min completely restores H ci to 1300-1950 ka/m.
Fig. 3 The valuses of H ci (950 o C)/H ci (900 o C), H ci (550 o C)/H ci (950 o C) and H ci (560 o C)/H ci (950 o C) The coercivity of permanent magnets is strongly related to their microstructure and is controlled by the nucleation of reverse domains at Nd 2 Fe 14 B-phase grain boundaries. Fig. 4 The microstructure of magnets, prepared without (a) and with [Dy/Al] (b), Tb 2 O 3 (c), and Nd 3 Co (d) additions Analysis of the microstructure of sintered magnets in Fig. 3 shows that the nature of the alloy-additions has a significant effect on the characteristics of the microstructure of magnets. In particular, there is a regular decrease in the average grain size in the following cases: i) with increasing concentration of alloy-addition and ii) with increasing melting point of alloy-addition, in comparison with the microstructures of the magnets with 3% [Dy/Al], Tb 2 O 3 and Nd 3 Co. The phase transformation-induced coercivity mechanism in Nd-Fe-B sintered magnets was proposed based on the phase transformations in accordance with the specified Nd-Fe phase diagrams [8]. It was supposed
that the phase transformations result in the modification of the grain surface of the principal phase. The model of reversible modification of interfaces by HT is based on the phase s precipitation at the surface of Nd 2 Fe 14 B grains that is caused by the series of consecutive eutectoid decompositions of the Fe-rich component within the intergranular Nd-rich phase [9]. It was assumed that the formation of thin coherent layers at the surface of the Nd 2 Fe 14 B grains improves the smooth of the Nd 2 Fe 14 B grains surface and increases the coercivity. 4. CONCLUSIONS The mixing method is effective for preparing the high coercivity sintered Nd-Fe-B magnets with all type of additions: [Dy/Al], Tb 2 O 3, Nd 3 Co. The average grain size of the Nd 2 Fe 14 B phase decreases with increasing contents of [Dy/Al], Tb 2 O 3 and Nd 3 Co additions and their melting temperatures. The magnetic hardening of sintered magnets is a result of growth of effective anisotropy field of (Nd,Dy) 2 Fe 14 B phase and simultaneous reduction of M s due to the replacement of Nd atoms by Dy in Nd 2 Fe 14 B grains. The main cause for the H ci doubling after HT1 is the structural hardening resulting from changes in the microstructure of magnets. The changes of H ci are reversible during cycle of high temperature-low temperature aging of sintered magnets. ACKNOWLEDGEMENTS This work was created in the frame of the project CZ.1.05/2.1.00/01.0040 Regional Materials Science and Technology Centre with financial support of Structural Funds and from the State Budget of the Czech Republic and also was supported by Russian Federal Program "AVCP" (project 3.3292.2011). LITERATURE [1] SAGAWA, M., HIROSAWA, S., YAMAMOTO, H., FUJIMURA S., MATSUURA Y. Nd-Fe-B permanent magnet materials. Jp. J. Appl. Phys., 26, 1987, p. 785. [2] SAGAWA, M., FUJIMURA, S., TOGAWA, N., YAMAMOTO, H., MATSUURA, Y. New material for permanent magnets on a base of Nd and Fe, J. Appl. Phys., 55, 1984, p. 2083. [3] SAGAWA, M., FUJIMURA, S., YAMAMOTO, H., HIRAGA, K. IEEE Trans. Magn. 20, 1984, p. 1584. [4] SAVCHENKO, A. G., MENUSHENKOV. V. P. High-energy-product rare-earth permanent magnets: Fundamental principles of development and manufacturing. The Physics of Metals and Metalography. 91, 2001, 1, p. S242. [5] VIAL, F., JOLY, F., NEVALAINEN, E., SAGAWA, M., HIRAGA, K. T. Park, Improvement of coercivity of sintered NdFeB permanent magnets by heat treatment, J. Magn. Magn. Mater., vol. 242-245, 2002, p. 1329. [6] SEPEHRI-AMIN, H., OHKUBO, T., HONO K. Grain boundary structure and chemistry of Dy-diffusion processed Nd Fe B sintered magnets. J. Appl. Phys., 107, 2010, p. 09A745. [7] SHELEKHOV, E. V., SVIRIDOVA, T. A. Programs for X-ray analysis of polycrystals. Metal Science and Heat Treatment, 42, 2000, 8, p. 309. [8] MENUSHENKOV, V. P., ORESHKIN, M. A, ZHURAVLEV, S. A., LILEEV, A. S. Metastable Nanocrystalline A1 Phase and Coercivity in Fe-Nd Alloys. J. Magn. Magn. Matter. 203, 1999, p. 149. [9] MENUSHENKOV, V. P.. Phase Transformation-induced coercivity mechanism in Rare Earth sintered magnets, Journal of Applied Physics, 99, 2006, p. 08B523-1.