Origins of axial inhomogeneity of magnetic performance in hot deformed Nd-Fe-B ringmagnetsWen-Zong Yin, Ren-Jie Chen, Xu Tang, Min Lin, Don Lee, and Aru Yan Citation: Journal of Applied Physics 111, 07A727 (2012); doi: 10.1063/1.3677933 View online: http://dx.doi.org/10.1063/1.3677933 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/111/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Texture formation of hot-deformed nanocomposite Nd2Fe14B/α-Fe magnets by Nb and Zn additions J. Appl. Phys. 115, 17A704 (2014); 10.1063/1.4860942 Diffusion of Nd-rich phase in the spark plasma sintered and hot deformed nanocrystalline NdFeB magnets J. Appl. Phys. 111, 033913 (2012); 10.1063/1.3682471 Investigation on microstructure, texture, and magnetic properties of hot deformed Nd–Fe–B ring magnets J. Appl. Phys. 107, 09A725 (2010); 10.1063/1.3339817 Nanocrystalline NdFeB magnets fabricated by a modified hot-working process J. Appl. Phys. 93, 8137 (2003); 10.1063/1.1544509 Effects of some additives on the magnetic properties of single stage hot deformed NdFeB magnets J. Appl. Phys. 91, 7887 (2002); 10.1063/1.1451491

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Origins of axial inhomogeneity of magnetic performance in hotdeformed Nd-Fe-B ring magnets

Wen-Zong Yin,1,2,a) Ren-Jie Chen,1,2 Xu Tang,1,2 Min Lin,1,2 Don Lee,1,2 and Aru Yan1,2,a)

1Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology andEngineering, Chinese Academy of Sciences, Ningbo 315201, People’s Republic of China2Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology,Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences,Ningbo 315201, People’s Republic of China

(Presented 1 November 2011; received 23 September 2011; accepted 24 November 2011; published

online 9 March 2012)

Hot-deformed Nd-Fe-B ring magnets have wide potential applications. These ring magnets,

however, exhibit axial inhomogeneity of magnetic performance. In this work, the effects of

density, pressure, deformation temperature, deformation rate, and texture on axial magnetic

performance were investigated over ring magnets prepared by backward extrusion method. It was

demonstrated that the texture accounted for the variation of magnetic performance along axial

direction. Microstructures of the ring magnets were examined with SEM, which further revealed

two different origins of axial inhomogeneity of magnetic performance. The deformation degree of

Nd-Fe-B grains plays a critical role in the performance difference between the top and middle part

of ring magnet. But that between the middle and bottom part mainly results from different

alignment orientations of platelet Nd-Fe-B grains. It was both deformation degree and alignment

orientation that determined the axial texture and consequent magnetic performance of

hot-deformed ring magnets. VC 2012 American Institute of Physics. [doi:10.1063/1.3677933]

I. INTRODUCTION

Nd-Fe-B ring magnet has wide applications especially in

brushless DC motors for its high power-to-volume ratio.1 To

date, there have been three types of Nd-Fe-B ring magnets

(RMs): bonded RM, sintered RM and hot-deformed RM.

Bonded RM is isotropic. It possesses lower energy density

than the other two RMs. Sintered RM experiences a complex

preparation that costs very long time. In contrast, hot-

deformed RM can be prepared via a facile backward extrusion

method, which shows a significantly higher efficiency. There-

fore, much attention has been focused on hot-deformed RM.

The general aims of hot deformation process were sum-

marized2 (1) to obtain a near net shape geometry of the sam-

ples, (2) to produce crack-free samples with a sufficient

mechanical stability, and (3) to control the microstructure in

order to tailor the desired properties. The first two aims have

already been achieved.3,4 But the approach to the third one

was hindered by several problems. And when it comes to

backward extruded RM, the most significant problem was the

inhomogeneity of magnetic performance. Specifically, it

involves both radial and axial inhomogeneity. The radial inho-

mogeneity of magnetic performance shows an approximately

linear decrease of remanence (Br) from inner to outer surface.5

This decrease results from the decline of crystal alignment,

which was confirmed with XRD6 and SEM.7 Axial inhomoge-

neity of magnetic performance exhibits a significant increase

of Br and decrease of coercivity (Hci) from top to bottom

regions.8,9 The variations of Br and Hci along axial direction

were reported to be related to the texture and ascribed partly

to the difference of material flow during hot deformation.10

To the best of our knowledge, however, systematic investiga-

tions on the reasons for axial inhomogeneity of magnetic per-

formance have not been carried out up to date.

In our work, Nd-Fe-B RM was prepared by backward

extrusion method. The influence of several possible factors

on its axial inhomogeneity of magnetic performance was

investigated systematically. Morphology and microstructures

along axial direction were examined to further reveal the

origins of axial inhomogeneity of magnetic performance.

II. EXPERIMENT

Radially oriented RMs were prepared from commercial

melt-spun Nd-Fe-B powder (MQU-F, Magequench Inc.).

The powder was hot-pressed into fully dense precursor with

the height of 7 mm and diameter of 19.5 mm at 670 �C in

vacuum under the pressure of 90 MPa. The precursor was

put into a die with the inner diameter of 19.9 mm and then

hot-deformed into RM by backward extrusion method at

800�820 �C under argon atmosphere. The backward extru-

sion setup was illustrated in Ref. 11. At the end of hot defor-

mation, a ring-shaped die on the top surface of RM

contacted with and received the pressure from the upper

punch so as to diminish the cracks at the top of RM. The

as-prepared RM possesses a height of 13 mm, an inner diam-

eter of 14.2 mm and an outer diameter of 19.9 mm. The mor-

phology and microstructure of RM were examined with field

emission scanning electron microscope (FE-SEM, Hitachi

S-4800). Magnetic performance of RM was measured via cut

samples (1� 1� 3 mm3) on vibrating sample magnetometer

a)Authors to whom correspondence should be addressed. Electonic

addresses: yinwz@nimte.ac.cn and aruyan@nimte.ac.cn.

0021-8979/2012/111(7)/07A727/3/$30.00 VC 2012 American Institute of Physics111, 07A727-1

JOURNAL OF APPLIED PHYSICS 111, 07A727 (2012)

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(VSM, Lake Shore Model-7410) in applied fields up to

1830 kA/m. The positions of cut samples were illustrated in

Fig. 1. The distances (denoted as x) from the top surface of

RM to the cross-section center of each sample were listed in

Table S1.17 Remanent magnetization in radial direction

(easy axis) was directly measured and denoted as M//. Rema-

nent magnetization along hard direction was denoted as M?and calculated by the mean value of that in tangential and

axial directions as reported in Ref. 10.

III. RESULTS AND DISCUSSION

Density is an important factor that might influence the

performance of magnet.12 Table S1 presents the density vari-

ation of RM along axial direction. The densities at the four

positions exhibit little difference, which indicates that den-

sity has no effect on axial inhomogeneity of magnetic

performance.17

Figure 1 shows the Br and Hci of cut samples from

RMs prepared under different pressures. RM exhibits an

inhomogeneous magnetic performance along axial direc-

tion. Hci shows a monotonic decrease from position 1 to 4.

Br increases from position 1 to 3 and then decreases at

position 4. The sample at position 3 possesses the highest

Br. But Br of the sample at position 1 (0.843�0.893 T) is

near to that of hot-pressed magnet (0.857 T). When the

pressure increases from 180 to 300 MPa, Br of the sample

at position 3 increases from 1.31 to 1.38 T while Hci

declined from 1199 to 1163 kA/m. This result suggests

that pressure has slight influence on the magnetic perform-

ance of RM along axial direction, but it is not the main

factor that produces axial inhomogeneity of magnetic

performance.

Nd-Fe-B magnet exhibits variations in magnetic per-

formance over different deformation temperatures.13

Hereby, the effect of deformation temperature on the mag-

netic performance of RM was investigated. The result is

shown in Fig. 2. It is obvious that the difference in Br on

the samples prepared at 800 �C (S-800) is smaller than that

at 820 �C (S-820). But Br at position 3 of S-800 is 0.073 T

lower than that of S-820. This result indicates that the

improvement of axial difference in Br could be achieved

by lowering deformation temperature at the expense of

maximum Br. It also confirms that maximum Br on RM

could increase with the raise of deformation temperature to

some extent. On the other hand, Hci of S-800 decreases

more drastically than that of S-820. For the cut samples at

position 1, the difference in Hci between S-800 and S-820

is about 20.7 kA/m. But for the samples at position 4, Hci

of S-820 is 135.3 kA/m higher than that of S-800. The

higher Hci on S-820 might be due to improved distribution

of Nd-rich phase resulting from better material flow at

820 �C.

During hot deformation, deformation rate would become

slower gradually under certain pressure. And it was reported

that different deformation rate resulted in difference in tex-

ture and consequent magnetic performance of Nd-Fe-B mag-

net.14 To reveal the influence of strain rate on the magnetic

performance of RM, RM was prepared at a constant defor-

mation rate of 0.05 mm/s [Fig. S3(a)]. Despite of the constant

deformation rate, both Br and Hci of RM exhibit a significant

axial inhomogeneity.17 This result implies that deformation

rate is not the main reason for axial inhomogeneity of mag-

netic performance in RM.

To further clarify the influence of deformation rate on

the magnetic performance of RM, hot deformation was per-

formed at different rates. Figure S3(b) displays the influence

of deformation rate on the Br and Hci of cut samples at posi-

tion 3. The Br decreases monotonically from 1.346 T at the

deformation rate of 0.05 mm/s to 1.238 T at 0.3 mm/s while

Hci shows slight change.17 The variation of Br over deforma-

tion rate is consistent with that of cylinder magnet and attrib-

uted to higher texture at slow deformation rate.15

Besides the above factors, texture is another factor that

plays an important role in determination of Br. Hereby,

texture was evaluated with the parameter of M===M? accord-

ing to Ref. 16. The variations of Br and texture over the axial

positions of RM are displayed in Fig. 3. From position 1 to

3, Br changes in step with texture, suggesting the leading

role of texture in determination of Br. At position 4, how-

ever, the decrease of Br is out of step with that of texture.

The former is significantly slower than the latter. This result

indicates different mechanisms for the influence of texture

on Br at position 4 from that at the other positions.

To further investigate the origins of axial inhomogeneity

of magnetic performance, morphology and microstructures

FIG. 2. (Color online) Remanence Br (solid) and coercivity Hci (open) of

cut samples from the ring magnets prepared at the pressure of 300 MPa and

the deformation temperature of 800 �C (circle) and 820 �C (square).

FIG. 1. (Color online) Remanence Br (solid) and coercivity Hci (open) of

cut samples from the ring magnets prepared under the pressure of 180 MPa

(square), 240 MPa (circle), and 300 MPa (triangle). Inset: schematic illustra-

tion on the positions of cut samples in ring magnet.

07A727-2 Yin et al. J. Appl. Phys. 111, 07A727 (2012)

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were examined with SEM on the cross-section of RM along

axial direction. The result is shown in Fig. 4. The particles in

the sample at position 1 have little deformation [Fig. 4(a)].

This is in agreement with the fact that the magnetic perform-

ance of this sample is almost the same as that of hot-pressed

magnet. For the sample at position 2, the deformation of

Nd-Fe-B particles is clear [Fig. 4(b)]. But the texture value

is as low as 7.62 since the thickness and aspect ratio of

Nd-Fe-B particles vary significantly. The texture of the sam-

ple at position 3 is further enhanced to 12.2 [Fig. 4(c)]. Both

the thickness and aspect ratio of platelet particles are

improved. The easy axes of these platelets are parallel to the

radial direction of RM. The above analysis illustrates that

deformation degree is the main reason for axial inhomogene-

ity of magnetic performance at position 1, 2, and 3. Further-

more, predeformation treatment before backward extrusion

should facilitate the improvement of axial homogeneity of

magnetic performance. At position 4, the thickness and

aspect ratio of Nd-Fe-B platelets are further improved

[Fig. 4(d)]. Interestingly, Crystallographic alignment at this

position is not consistent. Besides the alignment perpendicu-

lar to radial direction as at position 2 and 3, the sample at

position 4 possesses alignment not perpendicular to radial

direction. The different orientations of crystallographic

alignment account for the decline of texture at position 4,

despite of the improved uniformity in thickness and aspect

ratio. This result reveals that the texture and magnetic

performance of RM along axial direction was determined not

only by the deformation degree but also by the orientation of

crystallographic alignment of Nd-Fe-B platelets.

IV. CONCLUSION

RM prepared by backward extrusion method exhibits

axial inhomogeneity in both Br and Hci. Density has no effect

on the axial inhomogeneity of magnetic performance of RM.

Low deformation rate together with high pressure and defor-

mation temperature improves the Br at position 3 to some

extent, but enlarges the difference in Br along axial direction.

Texture takes a leading role in determination of axial inho-

mogeneity of magnetic performance. Specifically, the varia-

tion of Br at position 1, 2, and 3 is dominantly ascribed to the

deformation degree of Nd-Fe-B particles. But the decline of

Br at position 4 is attributed to both the deformation degree

and orientation of crystallographic alignment. It is both

deformation degree and alignment orientation of Nd-Fe-B

platelets that cooperate to determine the texture and mag-

netic performance of RM.

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Fig. S3(b), Table S1, and acknowledgements.

FIG. 4. SEM images of the samples at position 1 (a), 2 (b), 3 (c), and 4 (d)

of the ring magnet. White arrows represent the radial direction of ring

magnet.

FIG. 3. (Color online) Variations of Br and texture (M===M?) over axial

position of the ring magnet prepared with the deformation rate of 0.05 mm/s

at 820 �C under the pressure of 300 MPa.

07A727-3 Yin et al. J. Appl. Phys. 111, 07A727 (2012)

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