http://www.paper.edu.cn J OURNAL O F RAR E EARTHS

Vol . 22 , No. 4 , Aug. 2004 , p. 497

Preparation of Rapidly Quenching ( Nd , Pr) 12 ( FeCoZr) 82 B6 Alloy and Ξ Magnetic Properties of Bonded Magnets Zha Wusheng (查五生) 1 ,2 , Liu Ying (刘 颖) 1 3 , Gao Shengji (高升吉) 1 , Tu Mingjing (涂铭旌) 1 ( 1. College of Material Science & Engineering , University , 610065 , China ; 2. College of Material Science & Engineering , Xihua University , Chengdu 610039 , China)

Abstract : The optimum quenching rates and annealing conditions to prepare near stoichiometrical (Nd ,Pr) 12 (FeCoZr) 82B6 bonded magnets were investigated by using sub2overquenching and post annealing method. The quenching rates , annealing temperatures , and annealing time directly influence the microstructure and magnetic properties of alloy ribbons. The opti2 mum magnetic properties of bonded magnets are achieved by melt spinning at 24 m·s - 1 wheel surface speed , annealing at

655 ℃for 10 min , and bonding with 3. 25 % (mass fraction) epoxy. The best magnetic properties of remanence B r , in2 - 1 - 3 trinsic coercivity Hci and maximum energy product ( B H) max are 0. 669 T , 811 kA·m , and 75 kJ ·m , respectively.

Key words : metal materials ; permanent magnet ; Pr2based bonded magnets ; melt spinning ; magnetic properties ; rare earths CLC number : TM273 Document code : A Article ID : 1002 - 0721( 2004) 04 - 0497 - 04

The anisotropy field HA of Pr2Fe14B is about 30 % with excellent properties were prepared. higher than that of Nd2Fe14B which results in high in2 trinsic coercivity for nanocrystalline exchange coupled 1 Experimental composite permanent magnets. So the Pr2based The ingot of (Nd , Pr) 12 ( FeCoZr) 82B6 alloy was isotropic bonded magnets have currently attracted prepared by induction melting the commercial grade much attentions[1～4 ]. However this type of permanent constituents. The quenched ribbons were produced by magnet has not been used in practice. melt spinning onto a molybdenum reel rotating at cir2 There exist magnetic interactions between adja2 cumferential speeds of 20 , 24 , 28 , 32 , and 34 m·s - 1 cent grains either in nanocrystalline composite magnets respectively. These ribbons were devitrified to a crys2 or in an assembly of single hard magnetic phase of rare talline structure at 640～730 ℃for 5～20 min. Melt2 earth2transition metal intermetallic compounds[5～8 ] . It ing , quenching , and annealing were all completed in is found by numerical calculations that both short a high purity Ar atmosphere. The alloy powders were range exchange interactions and long range dipolar in2 bonded with 3. 25 % (mass fraction) epoxy and then teractions in ensembles of grains enhance the rema2 pressed into cylindrical bonded magnets with bulk nence. For an alloy with fine microstructure , the re2 density of about 6. 1 g·cm - 3. The magnetic properties manence is considerably enhanced , since the volume were measured using AMT23 automatic measure instru2 fractions of the boundary regions is much beneficial to ment. The phase components were verified by X2ray the exchange interactions. A homogeneous grain struc2 diffraction using Cu Kα radiation and the microstruc2 ture with a narrow range of grain size preserves a high ture of the ribbons were investigated using a SPA 400 coercivity. Obtaining a fine and uniform grain struc2 atom force microscope (AFM) . ture is an effective approach to improve the properties of the near stoichiometrical magnets. Therefore , by 2 Results and Discussion comparative experiments of different quenching rates and different annealing conditions the optimum pro2 2. 1 Magnetic properties of as2spun magnets cessing parameters were obtained and the near stoi2 The dependence of properties of as2spun magnets chiometrical (Nd ,Pr) 12 (FeCoZr) 82B6 bonded magnets on the quenching rates and the XRD patterns of as2

Ξ Received date : 2003 - 05 - 21 ; revised date : 2003 - 07 - 15 Foundation item : Project supported by 863 Project (2001AA324030) Biography : Zha Wusheng (1963 - ) , Male , Doctor , Associate professor 3 Corresponding author (E2mail : Liuying 5536 @163. com) 转载 中国科技论文在线 http://www.paper.edu.cn 4 98 J OURNAL O F RARE EARTHS , Vol . 22 , No. 4 , Aug. 2004

spun ribbons are shown in Figs. 1 and 2. Quenched at It is shown in Fig. 3 that there is great difference - 1 20 , 24 , 28 , and 34 m·s , ( B H) max of magnets are of magnetic properties of (Nd , Pr ) 12 ( FeCoZr ) 82 B6 measured as 33 , 13 , 7 , and 3 kJ ·m - 3 , respectively , bonded magnets quenched at different speeds. The - 1 which means that the as2spun ribbons consist of not magnet of 24 m · s has a higher Hci , B r , and only amorphous phases but also crystallites. The X2ray ( B H) max. Crystallized at 655 ℃for 10 min , B r = - 1 - 1 diffraction patterns have also verified this. The 01669 T , Hci = 544 kA·m , Hcb = 434 kA·m , - 3 - 3 ( B H) max of only 3 kJ ·m for the magnet quenched at ( B H) max = 75 kJ ·m , Hk/ Hci = 33. 2 %. The prop2 34 m·s - 1 shows very low concentration of crystallites erties of 20 m·s - 1 are inferior to that of 24 m·s - 1 : - 1 in this sample. These crystallites are mainly distribut2 Hci = 810 kA·m , B r = 0. 693T , ( B H) max = 74 kJ · [4 ,9 ] - 3 - 3 - 1 ed on the free surface of as2spun ribbons . The m , and the ( B H) max of 66 kJ ·m for 34 m·s is lower the wheel speed , the higher the volume fraction the worst in all magnets. of crystallites , the coarser the grains of ribbons , and This difference results from the microstructure more continuous crystallites distribution are expected. difference of ribbons. The free surface AFM images of With the increase of quenching rate , the thickness of ribbons are illustrated in Fig. 4. Ribbons quenched at ribbons , the volume fraction of crystallites , the mag2 24 m·s - 1 , though consist mainly of amorphous phas2 netic properties , and the diffraction peak strength es , the as2spun ribbons contain few crystallites. These monotonously decrease , whereas the grain size be2 crystallites have a very fine , even in nano2scale , grain comes nano2scale and the crystallites dispersedly dis2 size and dispersed in amorphous phases. Crystallized tribute . Quenched at an enough higher speed , the rib2 at annealing temperature , these crystallites grow. bons almost totally consist of amorphous phase. However , it is more overriding that the boundaries of the crystallites are of benefit to the nucleation of new 2. 2 Dependence of magnetic properties on crystals. The number of nucleus increases greatly and quenching rates the grains of new crystal are refined. The average grain size estimated from the free surface AFM image , Fig. 4 (b) , is about 15～20 nm and the microstructure is very homogeneous. So , the magnetic exchange in2 teraction between grains is enhanced and an excellent combination of properties is obtained : i . e. , a maxi2 mum intrinsic coercivity , a higher remanence , an im2 proved demagnetization curve , and the maximum ener2 gy product of 75 kJ ·m - 3. Similarly , the as2spun rib2 bons quenched at 20 m · s - 1 consist of amorphous phases and crystallites , but the crystallites are coars2 Fig. 1 Magnetic properties of as2spun magnets er . During crystallization the effect of grain growth overrides providing conditions for nucleation of new crystals. The average grain size is about 30 nm. But the microstructure is not homogeneous and several large grains up to 50～100 nm can be clearly seen in the free surface AFM micrograph , Fig. 4 ( a ) . The inhomogeneous and less fine microstructure favors the

Fig. 3 Dependence of crystallized magnets properties on Fig. 2 XRD patterns of as2spun ribbons quenching rates 中国科技论文在线 http://www.paper.edu.cn

Liu Y et al . Preparation of ( Nd , Pr) 12 ( FeCoZr) 82B 6 a nd Magnetic Properties 499

Fig. 5 Magnetic properties of (Nd , Pr) 12 (FeCoZr) 82B6 bonded magnets as a function of annealing temperature for 10 min (24 m·s - 1)

Crystallized under this condition , the ribbons are compounded by Nd2Fe14B/ Pr2Fe14B and have a fine and homogeneous microstructure , which yields a high2 er remanence , and a better intrinsic coercivity and en2 ergy product . If annealing temperature < 700 ℃, or annealing time < 10 min , the Nd2Fe14 B/ Pr2Fe14 B precipitate incompletely. There exist amorphous phas2 es and other non2ferromagnetic metastable phases , Fig. 4 Free surface AFM images of ribbons quenched which not only dilute the magnetic density but also iso2 - 1 - 1 - 1 (a) 20 m·s ; (b) 24 m·s ; (c) 34 m·s late adjacent grains. The exchange couplings are weakened and the magnetic properties worsened. nucleation of reversed domains leading to a reduction When annealing temperature > 700 ℃, or annealing of the coercivity , furthermore , of the energy product . time > 10 min , though the Nd2Fe14B/ Pr2Fe14B pre2 Quenched at 34 m · s - 1 the as2spun ribbons almost cipitate completely , but the grains grow furthermore. completely consist of amorphous phases. The grains The volume fraction of the boundary regions decreas2 self nucleate and self grow. From the AFM image , es . Then the exchange couplings between adjacent Fig. 4 ( c ) , a homogeneous microstructure can be grains are weakened and the magnetic properties de2 seen , but the average grain size is about 40～50 nm , grade , too . Therefore , the ribbons must be crystal2 significantly bigger than that of 24 m · s - 1. Conse2 lized at a moderate temperature for a moderate time in quently , the exchange coupling interactions between which magnetic phases precipitate completely and the adjacent grains are weakened and all of B , H , and r ci microstructure is fine and homogeneous. ( B H) decrease. max It is found in our experiments that the properties 2. 3 Dependence of magnetic properties on of as2spun ribbons quenched at 32 m · s - 1 , which annealing conditions mainly consist of amorphous phases , are insensitive to annealing temperatures. Crystallized at 670 , 685 , Both the annealing temperature and annealing 700 , 715 , and 730 ℃for 10 min , the ( B H) max of time directly determine the microstructure of ribbons magnets changes in a narrow range from 65 to 68 kJ · and , furthermore , influence the magnetic properties of m - 3 ( shown in Table 1) . This behavior can be as2 bonded magnets[10 ]. Tables 1 and 2 show the magnetic cribed to the addition of Zr dopant . The Zr addition properties of magnet prepared through different crystal2 induces precipitation of dispersive fine particles , lizing temperature and different crystallizing time. which effectively inhibit grain growth during anneal2 Temperature dependence of magnetic properties of [11 ] ing . magnet quenched at 24 m · s - 1 is shown in Fig. 5. Crystallized for 10 min. , with increasing of tempera2 3 Conclusions tures the properties increase and ( B H) reaches to max 1. There is great difference among the magnets , · - 3 ℃ 75 kJ m at 655 . After that the properties de2 which results from the constituents and microstructure grade adversely. Similarly , the optimum annealing of as2spun ribbons , from the grain size and uniformity ℃ time at 655 is 10 min. of crystallized ribbons. The best wheel speed for ob2 中国科技论文在线 http://www.paper.edu.cn 5 00 J OURNAL O F RARE EARTHS , Vol . 22 , No. 4 , Aug. 2004

taining the best magnetic properties for the bonded with high magnetic properties have been prepared us2 magnets is 24 m·s - 1. ing optimum processing parameters. Their magnetic - 1 2. The annealing temperature and time directly properties are : B r = 01669 T , Hci = 811 kA ·m , - 1 - 3 influence the properties of magnets. Crystallized under Hcb = 434 kA·m , ( B H) max = 75 kJ ·m , which is optimum conditions , the magnetic phases completely practicable and has great significance in utilizing Pr precipitate and their grains are fine and homogeneous. into permanent magnetic materials and reducing the The most favorable annealing condition to get the best manufacturing cost of permanent magnets. properties is at 700 ℃for 10 min. 3. The (Nd ,Pr) 12 (FeCoZr) 82B6 bonded magnets References : [1 ] Davies H A , Betancourt R J I , Harland C L . Table 1 Magnetic properties of ( Nd , Pr) 12 ( FeCoZr) 82 B6 Nanophase (Nd/ Pr) Fe B/α2Fe alloys : arttractive mate2 bonded magnets crystallized at different tempera2 2 14 rials for isotropic magnets with enhanced properties [J ]. tures for 10 min Scripta Mater. , 2001 , 44 : 1337.

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670 0. 649 799 422 71 [4 ] Zhou Shouzeng , Wang Zuocheng , Zhang Maocai , et al. 685 0. 628 789 400 65 The influence of quenching rates on microstructure and magnetic properties of Pr8Fe86 B6 alloy ribbons [J ]. J . 28 640 0. 645 791 406 68 Functional Materials (in Chin. ) , 2001 , 10 : 350. 655 0. 656 788 415 71 [5 ] Schrefl T , Fidler J , Kronmuller H. Remanence and 670 0. 643 777 405 68 coercivity in isotropic nanocrystalline permanent magnets 685 0. 64 743 390 64 [J ]. Physical Review B , 1994 , 49 : 6100.

32 670 0. 643 772 399 67 [6 ] Kronmuller H , Schrefl T. Interactive and cooperative 685 0. 654 725 384 65 magnetization processes in hard magnetic materials [J ]. 700 0. 638 764 393 65 J . Magn. Magn. Mater. , 1994 , 129 : 66. [7 ] Melsheimer A , Seeger M , Kronmuller H. Influence of 715 0. 649 762 403 68 Co substitution in exchange coupled NdFeB nanocrys2 730 0. 634 753 392 65 talline permanent magnets [ J ]. J . Magn. Magn. 34 670 0. 637 756 396 64 Mater. , 1999 , 202 : 458. 685 0. 642 729 381 63 [8 ] Neu V , Hubert A , Schultz L . Modelling of enhanced 700 0. 625 729 372 61 remanence of nanocrystalline , exchange2coupled hard 715 0. 636 737 385 64 magnetic grains [J ]. J . Magn. Magn. Mater. , 1998 , 730 0. 635 760 384 63 189 : 391. [9 ] Shi Yongjin , Zhou Ji , Pang Wei , et al. The distribu2 tion of the magnetic properties and microstructures for

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655 ℃ [10 ] Liu Ying , Chen Yue , Tu Mingjing , et al. Influence of heat treatment on magnetic properties of Nd9 ( Fe2 Speed/ Temperature Time/ B r/ Hci/ ( B H) max/ CoZrAl) 85B6 amorphous alloy [J ]. Journal of the Chinese · - 1 ℃ - 1 - 3 (m s ) / min T (kA·m ) (kJ ·m ) Rare Earths Society (in Chin. ) , 2000 , 18 (2) : 112.

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