Mechanically alloyed isotropic (Nd,Dy)FeC magnets

~

ELSEVIER

Journal of Magnetism and Magnetic Materials 170 (1997) L17-L21

Jewnalof magnetism and magnetic materials

Letter to the Editor

Mechanically alloyed isotropic (Nd,Dy)-Fe-C magnets Y.C. S u i * , Z . D . Z h a n g , W . L i u , Q . F . X i a o , X . G . Z h a o , T. Z h a o , Y.C. C h u a n g Institute of Metal Research, Academia Sinica, Shenyang 110015, People's Republic of China

Received9 October 1996; receivedin revised form 27 January 1997

Abstract

Structure, phase transformation and magnetic properties of mechanically alloyed samples Ndm_zDy=Fe92_mC8 (0 ~
1. Introduction

The discovery of high energy product magnets based on Nd2Fe14B has stimulated the study of the corresponding series of carbides [ 1 4 ] . The intrinsic magnetic properties of Nd2Fet4C compound are in most respects very similar to those of the corresponding boride. The carbides can be made by annealing the ingot in a temperature window between 830°C and 890°C for 21 days or by annealing the melt-spun precursors around 700°C [5,6]. RzFea~X (R = Nd~Dyl_x, X = B, C or B~C1 ~)

* Corresponding author. E-mail: [email protected].

alloys made by Liu et al. showed that coercivity higher than 12 kOe can be reached for optimum composition range without resorting to complicated powder metallurgical techniques [7]. Coercivity as large as 27.5 kOe can be obtained in cast Dy13FesoCv after proper heat-treatment; these samples consist of single-domain Dy2Fe14C particles [8]. This implies that the carbide possesses some advantages over the borides. The difficulty in obtaining Nd2Fe14C alloy limits the investigation to some extent. Mechanical alloying (MA) is a successful way to synthesis magnetic compounds such as SmFeTNx [9] NdzFex4B [10] and SmCos [11]. Recently, we have synthesised NdzFe~4C-based alloys successfully by MA and subsequent annealing [12]. It has been

0304-885i/97/$17.00 ~ 1997 ElsevierScienceB.V. All rights reserved PII S0304-88 53(97)00020-6

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Y.C Sui et al. /Journal of Magnetism and Magnetic Materials 170 (1997) L17 L21

well-known that Dy substitution can increase the coercivity of alloys. In this paper we will show that (Nd,Dy)2Fe14C bonded magnets with higher coercivity than NdzFe14C can be prepared by MA. The relationship between magnetic properties, phase transformation and composition of the (Nd,Dy)2Fe14C alloys will be studied systematically.

DY2Fel4C * Dy-rich phase -

~"

~

~

Z=16

2. Experimental details Samples of N d m _ z D y ~ F e 9 2 _ m C 8 (0 ~ z ~ m, m = 15,16) alloys were prepared from 99.5% pure Nd and Fe powders, 99.9% pure Dy powders, 99.7% pure carbon powders. The mechanical alloying was performed under pure argon atmosphere for 5 h in a high energy ball mill designed in our laboratory. The MA powders were annealed in a vacuum furnace connected to a closed glove box. X-ray diffraction analysis was conducted using Cu Kct radiation with Rigaku D/max-ra diffractometer equipped with a graphite crystal monochromator. (Nd,Dy)-Fe-C powders were embedded in epoxy resin to form magnetically isotropic magnets. Magnetic properties were measured using a pulsed field magnetometer in fields up to 10 T. The magnetization was related to the amount of magnetic powders, neglecting the dilution effect of resin and assuming that the density of the (Nd,Dy)-Fe-C alloys was 7.6g/cm 3. A.c. initial susceptibility measurements were performed to determine Curie temperatures of the phases in the (Nd,Dy)-Fe-C alloys. It was also employed to verify whether there exists (Nd,Dy)zFe~ 7Cx or (Nd,Dy)2Fe~4C and their relative contents qualitatively in the samples.

Z

25

30

35

40

45

50

55

60

2 THETA (Deg.) Fig. 1. X-ray diffraction patterns of mechanically alloyed Ndx6-zDy~Fe76C8alloysannealedat 900°Cfor 35 rain.

(Nd,DY)2Fel7C x

•

•

(Nd,DY)2Fel4C

z= 16

z=3

q

~g z=2 Q

3. Results Among the MA Nd-Fe-C powders annealed at 900°C for 35 min, it was found in our previous work [12] that Nd~sFeTvCs alloy has no NdzFelTCx and that the main phases of Nd16Fe76C8 alloy are NdzFetvC~ and Nd2Fe14C. Nd16Fe76C8 has relatively larger intrinsic coercivity than Nd15FevvC8, but the latter has higher maximum energy product than the former. Both compositions were chosen for Dy substitution.

• I 50

,

~

~

•

z =- uO L I 1 O0

,

I 150

,

I 200

~

I 250

,

i 300

i

350

Temperature (°C) Fig. 2. Z.... vs. temperature of mechanically alloyed Nd16 zDy=Fe76C8alloys annealed at 900°C for 35 rain.

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Y.C. Sui et al. / Journal of Magnetism and Magnetic Materials 170 (1997) L17 L21

Figs. 1 and 2 demonstrate X-ray diffraction patterns and Z.... vs. temperature curves of MA Nd16-zDyzFe76C8 (0 ~< z ~< 16) annealed at 900°C for 35 min. With the increment of Dy content, both the Curie temperature and the amount of (Dy,Nd)2Fel 7Cx decrease while those of (Dy,Nd)2Fel4C increase. This means that more Dy is introduced into the tetragonal phase and more (Dy,Nd)2FexTCx transforms into (Dy,Nd)2Fex4C. When z/> 5, (Dy,Nd)zFelvCx disappears. It is clear that Dy facilitates the formation of the hard magnetic phase (Dy,Nd)2Fe14C. Dy2Fe14C can be more easily formed than NdzFe14C [8] and Nd15FevTCs alloy annealed at 900:C does not contain NdzFelTCx, so no (Dy,NdhFe17Cx is found in the Ndl 5-zDy~Fe77C8 series. The thermal-stability of (Dy,Nd)zFet4C is enhanced by the substitution of Dy for Nd which is stable even at T, = 1000:'C if z ~> 2. In our previous study [12], a FCC structure Nd-rich phase with lattice parameter a = 5.15 A is found in MA Nd-Fe-C alloy. Upon Dy substitution, the lattice parameter of this phase decreases and finally it is converted to a FCC Dy-rich phase with a = 4.98 ~, for MA Dy-Fe-C alloy. Compared with the a-Fe free Nd~sFevvC8 alloy, DyxsFevvCs has both ~-Fe and more rare-earth-rich phase. The formation of more Dy-rich phase depletes the remaining material of Dy compared to the stoichiometric composition, leading to the formation of 0~-Fe. Figs. 3 and 4 show the composition dependences of the intrinsic coercivity and the maximum energy product of Nd~5 ~Dy~FevvCs and Ndl6 .~Dy~Fe76C8alloys respectively. For the two series of alloys, the coercivities increase steadily at the expense of the spontaneous magnetization regardless of the presence of the (Dy,Nd)2Fe17C., phase. This is caused mainly by the great enhancement of the anisotropy field by Dy substitution and by the antiparallel coupling of Dy with Fe. For the second series of alloys, Nd~DysFe76C8 has the highest (BH)max at the composition where (Dy, Nd)2Fel 7Cx diappears. Fig. 5 illustrates the typical demagnetization curves of four alloys annealed at 900°C for 35 rain. Either the ~x-Fe (for Dy~sFe77Cs alloy) or the (Nd,Dy)2Fel 7C~ phase (for Ndl,~Dy2Fe76C8 alloy) leaves the demagnetization curves with big step.

70~ 60 so ,~, ~

--m-- Nd15.zDyzFe77C8

/

- - ° - - Nd1 6 - z D y z F e 7 6 C 8 7

40

'- 30 20 10 0

I 0

~

I 4

i

I 8

b

112

~

6/ 1

Dy content (at%)

Fig. 3. Dependenceof the intrinsiccoercivityon Dy contentin mechanically alloyed Nd15 ,Dy=Fe77C8 and Ndl~, zDy: Fe76C8 alloysannealedat 900'C for 35 min. 7

~

- - . - - Ndl 5_zDyzFe77C8

6 ~, ~

s

~

4

0_zDyzFe76C8

°-"--°'q a 2

1 0

4

8

2

1

6

1

Dy content (at%)

Fig. 4. Dependence of the maximum energy product on Dy content in mechanically alloyed Nd15 zDy:Fe77C~ and Ndl6 -Dy:Fe76C8alloys.

E C. Sui et al. / Journal o f Magnetism and Magnetic Materials 170 (1997) L I 7-L21

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4 m

a

Nd8DY8Fe76C8

b

Nd14DY2Fe76C8

c

DY16Fe76C8

d

DYlsFe77C8

0

2

f -60

-40

-20

Appfied Field (kOe) Fig. 5. Demagnetization curves of MA (a) NdsDysFe76Cs, (b) Nda4Dy2Fe76C8, (c) Dy16Fe76C8 and (d) DylsFe7vC8 alloys annealed at 900°C for 35 rain.

Other alloys free of these soft phases do not show such step.

4. Discussion The intrinsic coercivity at room temperature for sintered (Ndl-xDyx)lsFe77B8 magnets increases linearly with the Dy concentration and reaches 50 kOe at x = 0.47. Dy concentration higher than x = 0.47 [13] results in the formation of DyFe4B along grain boundaries and decreases the intrinsic coercivity. Meanwhile, (Ndl-~DyxhsFevTB8 made by MA does not form the ferromagnetic DyFeeB phase for the entire concentration range. The intrinsic coercivity of the MA (Ndl-xDy,)lsFe77B8 alloys increases with increasing Dy content and reaches 56kOe for DylsFe77B8 alloy [14]. Dy13.sFesl.3Bs.4 alloy, which was rapidly solidified under proper quenching rate without annealing, has a maximum coercivity value of 64 kOe [15]. All the higher coercivities of (Dy,Nd)-Fe-B alloys made by different methods are attributed to

the higher anisotropy field and fine grains introduced by Dy substitution. The carbides can develop high coercivity in the annealed bulk alloys [7, 8, 16]. But the coercivity mechanism for the carbides is rather controversial. The coercivity of Nd9Dy6FeTvCv.2Bo.8 and DylsFe77C8 alloys annealed at 900°C for several hours can achieve 12.5 kOe, which was claimed to be caused by cellular structure with cell size about 1 pm [7]. DylsFe77C8 alloy contains a thin grain boundary layer with composition close to that of the principal 2:14:1 phase which impedes the movement of domain walls [16]. Van Mens et al. [8] claimed that the higher coercivity obtained in Dy13FesoC7 alloy is caused by the separated single-domain Dy2Fe~4C particles. In the present work, an FCC structure Dy-rich phase which might be non-magnetic or paramagnetic emerges in the MA Dy-Fe-C alloys during the annealing process. The homogeneous distribution of this phase separate the hard magnetic grains and reduce the coupling field [17] and enhance the coercivity. Dy16Fe76C 8 has more Dy-rich phase than Dy~sFeTsC8 and no a-Fe is found in the former. Thus no step is observed in the demagnetization curve for Dy16Fe76C8 alloy and higher coercivity is achieved. From the above discussion, it is seen that the mechanism for increasing coercivity upon Dy substitution for Nd in boride and carbide made by MA differs considerably. A close relationship between phase formation and the preparation methods is observed for both carbide and boride. More attention should be paid to the modifications of microstructure (such as distribution of phases, chemical composition and crystal structure of phases, grain size distribution, etc.) in discussing the variation of coercivity.

5. Conclusions Substitution of Dy for Nd can facilitate the formation of the hard magnetic phase in (Nd,Dy)2Fe14C-based magnets and increase both the thermo-stability of the tetragonal phase and the coercivity of the alloys. The FCC Nd-rich phase with lattice parameter of a = 5.15 ~. converts to the

Y.C. Sui et al. / Journal of Magnetism and Magnetic Materials 170 (1997) L17 L21

FCC Dy-rich phase with a = 4.98 ~, during the substitution process. The higher coercivity obtained in Dy16FevoCs is caused by both the higher anisotropy field of DyzFe14C and the formation of the Dy-rich phase which separates the hard magnetic grains. Acknowledgements This work has been supported by the National Natural Science Foundation of China and the Science and Technology Commission of Shenyang and Liaoning.

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