Av. dos Trabalhadores 420, Vila Santa Cecília, Volta Redonda RJ BRAZIL a

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1 Materials Science Forum Vol. 802 (2014) pp Submitted: Online available since 2014/Dec/31 at Accepted: (2014) Trans Tech Publications, Switzerland doi: / Grain Growth Kinetics of (NdPr) 2 Fe 14 B Magnets Kaio Sérgio Torres de Souza 1, Célio de Jesus Marcelo 1, Daniel Rodrigues 1 José Adilson de Castro 1, Marcos Flavio de Campos 1,a, 1 EEIMVR- Universidade Federal Fluminense Av. dos Trabalhadores 420, Vila Santa Cecília, Volta Redonda RJ BRAZIL a mcampos@metal.eeimvr.uff.br Keywords: magnets, sintering, grain size, NdFeB Abstract. Two different effects need to be considered in the sintering: (i) The densification should be maximum, to optimize the energy product BHmax and the remanence, however, (ii) the.grain size should be small, in order to maximize the coercivity. Grain growth takes place during the sintering step of the magnets, usually performed at the range o C. In this study, the grain growth kinetics is investigated. Samples of NdPrFeB magnets (proportion 3 Nd : 1 Pr) were heat treated at the temperature of 1050 o C, for times between 1 to 12 hours. The knowledge of the grain growth and coarsening kinetics allows extrapolation to other temperatures, and this information is helpful to maximize coercivity and remanence at the same time. Introduction The most important index of merit in magnets is the energy product, BHmax [1-3]. Another very relevant property is the coercivity. Magnets need to have high BHmax and high coercivity, and this can be achieved by obtaining high density but keeping the grain size as small as possible [4]. The coercivity of magnets is directly related to the grain size [5]. Thus, the sintering step is fundamental, and grain growth should be avoided. The determination of grain growth kinetics is important in order to estimate the grain growth, opening the possibility of extrapolation for other sintering temperatures. In the present study, it was investigated the grain growth for NdPr (75/25) iron boron alloy at the temperature of 1050 o C during times from one up to 12 hours. Theory For grains with spherical shape, defining D as the grain size and the curvature as C=1/D, the velocity of grain growth can be described as (dd/dt), where t is time, and K is a constant: Integration of Eq. (1) results in Eq. (2). D=D 0 when t=0. dd/dt = K C (1) D² = D 0 ² + K t (2) As the processes in thermically activated, the Constant K can be expressed as K= K 0 exp (-Q/RT), where K 0 is a constant named frequency factor, Q is an activation energy, T is temperature, and R is the gases constant. Then Eq. (2) becomes: D² = D0² + K 0 t exp[-q/rt] (3) A more general version of Eqs. (2) and (3) is Eq. (4), where K is a constant and n is growing exponent. When measurements are performed at several different temperatures, the activation energy Q can be determined, by applying Ln, see Eq (5). D = D 0 + K (t^n) (4) Ln (D - D 0 ) = Ln K 0 + n Ln t -(Q/RT) (5) All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: , Universidade Federal Fluminense, Volta Redonda, Brazil-31/12/14,15:20:42)

2 Materials Science Forum Vol In the Eq. (4) and (5), values of n near 0.5 are expected, according the assumptions of Eqs. (1) and (2) [6,7]. Deviations from n=0.5 can represent other conditions, a subject discussed in more details by Humphreys and Hatherly [6], Porter and Easterling [7] and Hulda [8]. Experimental Several samples of magnets following the commercial specification N48, and grain size around 10 m were submitted to heat treatments at the temperature of 1050 o C, in a BRASIMET, model K250 furnace according the following procedure: First, the furnace reached the temperature of 1050 o C. Then, after each hour, a new sample was inserted in the furnace. Thus, after twelve hours, twelve samples were obtained, with varying annealing times between 0-12 hours, at the step of 1 hour. For avoiding oxidation problems, each sample was in a sealed steel box. The magnets were submitted to a common metallographic preparation. The polishing followed the sequence: Sandpapers 240, 320, 600, , 1200, and 1500 (mesh). The final polishing was done using colloidal silica 0.25 m. To revealing grain size, an etching Nital 3% was used. Results and Discussion Fig. 1. Grain size evolution as function of the annealing time.

3 542 Advanced Powder Technology IX Fig. 2 Microstructure of the as-received sample, without etching Fig. 3 Microstructure of the as-received sample, with Nital 3% etching

4 D-D 0 ( m) Materials Science Forum Vol Experimental Adjust time (hours) Fig. 4 Adjust of the data for a law of the type D - D 0 = K (t^n) Table I. Values of n and K assuming equation D = D 0 + K (t^n) n 0.65 K 2.83 The Fig. 1 presents the results of grain size as function of the annealing time. Examples of microstructures are presented in Fig. 2 (no etching) and Fig. 3 (etching with Nital 3%). The data of Fig. 1 was evaluated with Eq. (4), and the obtained values are presented in Table 1 and Fig. 4. The determined values presented in Table I represent the grain growth kinetics for the evaluated material, a typical N48 class NdPr-Fe-B magnet, with approximate proportion 3Nd : 1Pr, and high copper content, around 3% wt, besides other alloying elements, as aluminum. In general, experimental values of n are below 0.5 [6-9] and, when there is grain boundary pinning by precipitates, n can be very low [8]. The n=0.65 found in the present study suggests another mechanism, probably coarsening [10,11], and not the common grain growth [9]. The data of Fig. 1 and Fig. 4 shows a significant grain growth in the first two hours, with the grain size increasing from 10 m up to 20 m after 10 hours of heat treatment. Very similar results were observed by Xianglian and coworkers [11,12]. In the case of ternary alloy Nd-Fe-B, abnormal grain growth with grain up to 400 m (temperature=1000 o C time=1.5 h) was reported [13], for magnets produced by hydrogen decrepitation. In the present study, the existence of oxides and also alloying elements as copper and aluminum at grain boundary phases possibly helps to prevent abnormal grain growth. The results of Fig. 1 and Table I may be valuable also for improvement of densification models, which evaluate coupled effects of densification and grain growth [4, 14, 15]. Some Remarks - Grain size, squareness, and conditions of operation As already mentioned, BHmax usually is the merit index that receives more attention in magnets [1-3]. Grain size has big influence on squareness [16] and, thus, affects BHmax. However, a very

5 544 Advanced Powder Technology IX important issue in magnets is the conditions of operation [17-21]. Depending on the specific application, the squareness and as consequence the grain size and grain size distribution becomes more relevant. Finally, the environment where the magnet is operating cannot be neglected, because the metals as iron and especially neodymium are very susceptible to oxidation and corrosion [21]. Conclusions For the evaluated NdPrFeB (3Nd:1Pr) high copper magnet, the kinetics of grain growth was determined, according a law of the type D - D 0 = K (t^n) where D is grain size, D 0 is initial grain size, t is time, and K and n are experimental constants. It was found n= 0.65, and K = The high value of n suggests that coarsening is a relevant mechanism for the grain growth process in the present case. The obtained data can be extrapolated for other temperatures, allowing grain growth prediction in the sintering of these materials. Acknowledgements MF de Campos and JA de Castro acknowledge CNPq. D Rodrigues thanks CAPES. Kaio S. T. Souza thanks CNPq grant. MF de Campos thanks CNPq / References [1] K.J. Strnat: Proceedings of the IEEE Vol. 78 (1990), p [2] J.F. Herbst: Rev. Mod. Phys. Vol. 63 (1991), p [3] D. Brown, B.M. Ma, Z. Chen: Journal of Magnetism and Magnetic Materials Vol. 48 (2002), p [4] J.A. de Castro, M.F. de Campos: Materials Science Forum Vols (2008), p. 80. [5].M.F. de Campos: Materials Science Forum Vol (2010), p [6] F.J. Humphreys, M. Hatherly: Recrystallization and related annealing phenomena. (2nd Edition Pergamon, Oxford UK 2004). [7] D.A. Porter, K.E. Easterling: Phase transformation in metals and alloys. (2nd Edition Chapman & Hall London, New York). [8] Z. Hulda: Materials Science Forum Vol (2004), p [9] Byung-Nam Kim, Keijiro Hiraga, Koji Morita: Materials Transactions Vol. 44 (2003), p [10] O.B.G. Assis, M. Ferrante: Journal of Materials Synthesis and Processing. Vol. 3 (1995), p. 93. [11] Liu Xianglian, Zhou Shouzeng: Journal of Rare Earths Vol. 25 (2007), p [12] L. Xianglian, Z. Shouzeng and D. Qingfei. A Study of Grain Growth Kinetics in Sintered NdFeB Magnets. In: Proceedings of the 16th International Workshop on Rare earth Magnets and Their Applications. (Sendai, Japan 2000), p [13] N.C. Liu, A.S. Kim: J. Appl. Phys.Vol. 67 (1990), p [14] J.A. de Castro, N. Karsokas Filho, D. Rodrigues, M.F. de Campos: Materials Science Forum Vols (2012), p [15] J.A. de Castro, G.V. Concilio, D. Rodrigues, L.S. Santomauro, M.F. de Campos: Materials Science Forum Vols (2012), p [16] E.A. Périgo, H. Takiishi, C.C. Motta, R.N. Faria: IEEE Transactions on Magnetics Vol. 45 (2009), p [17] M.T. Thompson: Proceedings of the IEEE Vol. 97 (2009), p [18] B. Grieb, D. Brown, D. Miller, N. Shet. Cost effective motor design based on isotropic and anisotropic bonded NdFeB magnets. In: Magnetic Materials in Electrical Machine Applications Yyteri Spa Hotell, Pori, Finland. Available at:

6 Materials Science Forum Vol [19] B. Grieb, J.P. Yang, D. Brown, D. Miller, G. Kroll. RE- Bonded Magnets Material Grades and Progress in Market Penetration. In: Advanced Magnet Materials and their Applications, October Available at: [20] B. Grieb, N. Sheth, D. Brown, H. Feng, Y. Keat, J. Herchenroeder, M. Chen, D. Miller. Cost Effective Materials Based on NdFeB Compounds. In: Magnet Materials in Electrical Machine Applications June 16 th, 2012 Pori, Finland. Available at: [21] S. R. Trout. Optimum Corrosion Protection of Nd-Fe-B Magnets. Presented in Magnetics. Denver, CO, June Available at:

7 Advanced Powder Technology IX / Grain Growth Kinetics of (NdPr) 2 Fe 14 B Magnets /