Enhanced texture in die-upset nanocomposite magnets by Nd-Cu grain boundarydiffusionXin Tang, Renjie Chen, Wenzong Yin, Jinzhi Wang, Xu Tang, Don Lee, and Aru Yan

Citation: Applied Physics Letters 102, 072409 (2013); doi: 10.1063/1.4793429 View online: http://dx.doi.org/10.1063/1.4793429 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/102/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Texture formation of hot-deformed nanocomposite Nd2Fe14B/-Fe magnets by Nb and Zn additions J. Appl. Phys. 115, 17A704 (2014); 10.1063/1.4860942 Coercivity enhancement of anisotropic die-upset Nd-Fe-B powders by Pr-Cu alloy diffusion J. Appl. Phys. 113, 193902 (2013); 10.1063/1.4805048 High performance anisotropic NdFeB magnets prepared by dual-alloy die-upsetting J. Appl. Phys. 111, 07B540 (2012); 10.1063/1.3679866 Textured Nd2Fe14B flakes with enhanced coercivity J. Appl. Phys. 111, 07A735 (2012); 10.1063/1.3679425 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

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Enhanced texture in die-upset nanocomposite magnets by Nd-Cu grainboundary diffusion

Xin Tang (唐鑫),1,2 Renjie Chen (陈仁杰),1,2,a) Wenzong Yin (尹文宗),1,2

Jinzhi Wang (汪金芝),3 Xu Tang (唐旭),1,2 Don Lee (李 东),1,2 and Aru Yan (闫阿儒)1,2,b)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 China3Ningbo University of Technology, Ningbo 315211, People’s Republic of China

(Received 12 January 2013; accepted 11 February 2013; published online 21 February 2013)

Bulk anisotropic nanocomposite Nd2Fe14B/a-Fe magnets were prepared by hot pressing and die

upsetting coupled with Nd-Cu grain boundary diffusion. The hot workability of nanocomposite

magnets is enhanced dramatically by grain boundary diffusion of low melt point Nd-Cu alloy,

resulting in a strong anisotropy by die upsetting. The microstructure of die-upset nanocomposite

magnets is identical with that of the traditional die-upset rare earth-rich magnets. The

coercivity, remanence, and squareness degree of demagnetization curves are optimized. The

observation for microstructures and the analysis of magnetic properties suggest that the grain

boundary diffusion mainly occurs in the hot deformation process. VC 2013 American Institute ofPhysics. [http://dx.doi.org/10.1063/1.4793429]

In recent years, nanocomposite magnets consisting of

hard and soft magnetic phases have attracted much interest

for the development of high performance permanent

magnets.1–4 Texture formation in the anisotropic nanocompo-

site magnets is critical for high maximum energy product.

The hot deformation has been proved to be an effective tech-

nique to produce bulk anisotropic RE-Fe-B magnets (where

RE is rare earth) for an over-stoichiometric precursor. Some

investigators have synthesized bulk anisotropic composite

magnets by blending RE-rich melt-spun powder with RE-lean

one or a-Fe powder followed by hot pressing and hot defor-

mation. In these studies, no or weak crystallographic align-

ment found in the RE-lean or a-Fe areas demonstrates that

the blending technique may not be very promising in getting

the high performance magnets.5,6 Although a large number of

researches reveal that RE-rich phase is critical to the texture

formation, some attempts to obtain the texture in the magnets

without RE-rich phase are noteworthy. Relative studies dem-

onstrate that texture can be developed in the RE-lean precur-

sor by applying a large uniaxial stress under hot deformation

and a texture development in the RE-lean precursor with Cu

and Ga additions by die upsetting.7–9 However, the reported

texture in the die-upset RE-lean alloy is still rather poorer

compared to that obtained in the RE-rich counterparts. If the

RE-rich phase is brought into a nanocomposite system with

hard and soft phases, the hot workability of the nanocompo-

site system should be enhanced and an anisotropic nanocom-

posite magnet may be prepared by hot deformation. The

studies about grain boundary diffusion of sintered Nd-Fe-B

magnets indicate that the rare earth elements can be easily

imported into magnets along grain boundary, resulting in the

grain boundary layer thicker. Recently, the reports about

coercivity enhancement of hydrogenation disproportionation

desorption recombination (HDDR) magnets demonstrate that

the grain boundary diffusion method is effective on nano-

structure system.10,11 In this paper, we try to diffuse low melt-

ing point eutectic Nd-Cu alloy into the grain boundary

of nanocomposite magnets to improve the hot workability

of nanocomposite magnets and fabricate anisotropic nano-

composite magnets with full density by hot deformation.

The starting material was melt-spun nanocomposite

powder MQP-15-7 with size of 40–250lm purchased from

Magnequench International Inc. The low melting point eutec-

tic alloy ingot with nominal composition Nd90Cu10(wt. %)

alloy was prepared by arc melting with 99.9% pure elements,

and then were melt-spun at a speed of 40m/s in an argon

atmosphere. The as-melt-spun ribbons were ground to fine

powder with size of 80–150 lm in a controlled atmosphere

and mixed with the nanocomposite powder at a mass fraction

of x (x¼ 0–10wt. %). The as-mixed powders were hot

pressed at 700 �C under 270MPa in vacuum, and then

deformed at 850 �C under 105MPa in high purified argon

atmosphere until their height reduced by 70%. Bulk samples

with the full density of 7.6 g/m3 were obtained. The crystal

structure and morphology of grain were identified by x-ray

diffraction (XRD) and scanning electron microscopy (SEM),

respectively. Detailed microstructure was investigated by

(high-resolution) transmission electron microscopy (TEM/

HRTEM) on a Tecnai-F20 system. Thermal analysis was

conducted by differential scanning calorimetry (DSC) at a

heating rate of 40K/min. The magnetic phases and their cor-

responding Curie temperatures (Tc) were determined by

vibrating sample magnetometer (VSM, LakeShore 7410) in

a temperature range 25–900 �C in conjunction with magnetic

field 500Oe. The samples were pre-magnetized in a pulsed

magnetic field of �70 kOe and then measured in closed cir-

cuit with the BH apparatus.

a)Electronic mail: chenrj@nimte.ac.cn.b)Electronic mail: aruyan@nimte.ac.cn.

0003-6951/2013/102(7)/072409/5/$30.00 VC 2013 American Institute of Physics102, 072409-1

APPLIED PHYSICS LETTERS 102, 072409 (2013)

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Fig. 1(a) shows the XRD patterns of the die-upset mag-

nets with different mass fraction of Nd-Cu addition. The

peaks were obtained from the surface perpendicular to the

applied stress direction for all samples. With the increasing

mass fraction of Nd-Cu from x¼ 0 to 8wt. %, the relative in-

tensity of peaks for (004), (105), and (006) ascends first, and

then remains nearly unchanged. It is because that the

increased Nd-Cu content results in an improvement of hot

workability and better texture, but a higher amount of the liq-

uid phase causes smaller deformation stress and leads to a

reduction of energy available for the solution-precipitation

process.12 Therefore, when the Nd-Cu addition x is over

6wt. %, the alignment of samples is not further improved

with the increasing Nd-Cu addition. Meanwhile, considering

that the (110) reflection of a-Fe and the (006) reflection of

Nd2Fe14B nearly overlap, the intensity of a-Fe diffraction

peak becomes weakened with an increase of Nd-Cu addition,

and when the addition x is more than 4wt. %, the (110)

reflection of a-Fe becomes indiscernible. Fig. 1(b) shows the

typical XRD patterns for the raw nanocomposite powder,

hot-pressed and die-upset nanocomposite permanent mag-

nets with x ¼ 8wt. %. As is seen from Fig. 1(b), the XRD

pattern of nanocomposite powder indicates that it consists of

Nd2Fe14B and a-Fe and the grain orientation is random dis-

tribution. The diffraction peaks of hot-pressed magnets are

more broadened than those of die-upset magnets. This

indicates a fine grain size of hot-pressed magnets which

increases in the die upsetting process. Moreover, the inten-

sity of diffraction peaks of (004), (105), and (006) increases

dramatically for the die-upset magnets with 70% height

reduction, suggesting that a crystallographic alignment has

already been improved in die-upset magnets.

The field emission SEM micrographs of fractured surfa-

ces for die-upset magnets with different Nd-Cu mass fraction

indicate a consistent tendency of texture with the XRD anal-

ysis. The micrograph of Figs. 2(a), 2(b), 2(c), 2(d), 2(e), and

2(f) represents the microstructures of the samples with a Nd-

Cu mass fraction of x¼ 0, 2, 4, 6, 8, and 10wt. %, respec-

tively. As seen from Fig. 2(a), the grains are difficult to be

distinguished for the sample without Nd-Cu alloy addition,

which is because that the grains can’t grow large enough to

be observed by SEM. Fig. 2(b) shows that in the some areas

lamellar structure along the press direction can be observed.

It is considered that the hot workability of magnet should be

enhanced due to the diffusion of low melt point Nd-Cu liquid

phase, resulting in lamellar structure under hot press condi-

tion. However, in the other areas the microstructure is just as

the same as that shown in Fig. 2(a). Moreover, the areas with

the lamellar structure accounts for only a small percentage

when Nd-Cu addition x is less than 2wt. %, which indicates

FIG. 1. XRD patterns of the die-upset

magnets with different Nd-Cu alloy

addition (a) and the raw nanocomposite

powder, hot-pressed magnets and die-

upset magnets with x¼ 8wt. % (b).

FIG. 2. Field emission SEM micro-

graphs of die-upset magnets with differ-

ent Nd-Cu alloy addition. (a) x¼ 0, (b)

x¼ 2, (c) x¼ 4, (d) x¼ 6, (e) x¼ 8, and

(f) x¼ 10wt. %.

072409-2 Tang et al. Appl. Phys. Lett. 102, 072409 (2013)

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that too little Nd-Cu liquid phase addition is not enough to

give rise to sufficient hot deformation of grains. With an

increase of x, the plate-like grains emerge as shown in Fig. 2(c),

and the clear boundary between two areas with different

microstructures suggests an inhomogeneity of the Nd-Cu dif-

fusion for particles of precursor when a small amount Nd-Cu

is added. Moreover, a further observation shows that lamel-

lar areas and equiaxed grains areas with thickness of one to

several micrometers, which consists of plate-like grains and

equiaxed grains respectively, arrange alternatively along the

press direction for many diffused “initial particles.” This

phenomenon indicates the diffusion of the Nd-Cu liquid

phase is inhomogeneous yet even in the so-called diffusion

areas, but the reason and mechanism of the Nd-Cu liquid

phase diffusion in Nd-Fe-B hot press and die upset magnet is

unclear now.

Fig. 2(d) describes that the c-axis crystallographic align-

ment has been improved and only a small amount of mis-

aligned grains can be observed. The area where the Nd-Cu

has not diffused drops sharply. When the Nd-Cu amount

increases to 6wt. %, the texture is further improved and

equiaxed grains diminish as shown in Fig. 2(e). In compari-

son with Fig. 2(d), Fig. 2(e) shows the increased thickness of

platelet-shaped grains As presented in Figs. 2(e) and 2(f),

there is negligible difference in morphology between the

samples with x¼ 8 and 10wt. %. The platelet-shaped grains

with dimension 80–100 nm parallel to the compressive stress

and those with dimension 400–500 nm perpendicular to the

compressive stress are observed in die-upset nanocomposite

magnets. The microstructural morphology of die-upset nano-

composite magnets doped by >8wt. % Nd-Cu bears strong

similarities with that of traditional die-upset MQ3 magnets,

which indicates the Nd-Cu alloy has already diffused into

the magnet thoroughly along grain boundary. It is well

known that the anisotropy of the elastic properties and strain

energy of individual Nd2Fe14B grains under compressive

stress underlies the texture formation of hot deformation

magnets. In the die upset process, the grains with their c-axis

deviating from the pressing direction tend to dissolve into

the liquid Nd-Cu phase due to its high strain energy.

Meanwhile, the grains with their c-axis parallel to the press

direction have low strain energy leading to a preferred

growth by the “precipitation-growth process.”13 Besides,

grain boundary liquid phase facilitates grain boundary to

migrate and misoriented grains to rotate towards the prefer-

ential direction, i.e., the press direction.14,15

The absence of RE-rich grain boundary phase and the

existence of soft phase give rise to a relative low coercivity

for nanocomposite magnet. It can be expected that the coer-

civity of the die upset magnets will be enhanced after Nd-Cu

diffusion. Fig. 3 illustrates the demagnetization and magnetic

properties with different mass fraction of Nd-Cu. As more

Nd-Cu diffuses into the grain boundary, it increases the

effect of decoupling between hard magnetic grains and

enhances the hot workability. Therefore, as shown in Fig. 3(a),

the coercivity slightly increases when the Nd-Cu addition

x< 4wt. % and sharply increases when x> 4wt. %.

Nevertheless, the remanence increases dramatically before

x< 6wt. % and then keeps nearly unchanged, which is

ascribed to the texture change. From Fig. 3(c), as x increasesfrom 0 to 10wt. %, the coercivity increases from 2.07 kOe to

13.99 kOe, and the remanence and maximum energy product

increase from 8.26 kG and 6.29 MGOe to 12.89 kG and 37.3

MGOe, respectively.

The demagnetization curves of hot-pressed magnets and

die-upset magnets for x¼ 8wt. % are given in Fig. 3(b). For

comparison, the demagnetization curve of the hot-pressed

magnets without Nd-Cu alloy addition is illustrated in Fig. 3(b)

as well. By the addition of 8wt. % Nd-Cu, the coercivity of

hot-pressed magnets increases from the 6.67 kOe to 8.65

kOe. This is consistent with the studies on the grain bound-

ary diffusion in HDDR and sintered magnets,10,11 and dem-

onstrates that the Nd-Cu has partly diffused into grain

boundary even though in the hot press process. The demag-

netization curves measured parallel and perpendicular to the

stress direction of the die-upset magnets with 8wt. % Nd-Cu

addition exhibit a remarkable magnetic anisotropy. The

demagnetization curve measured parallel to stress direction

shows the coercivity, Hcj¼ 10.58 kOe, the remanence,

Br¼ 12.86 kG, and maximum energy product, (BH)max¼ 37

MGOe. It possesses the magnetization characteristics of a

single hard magnetic phase, which is one of the features of

effective exchange coupling between hard and soft magnetic

phases. Compared with the isotropic nanocomposite melt-

spun Nd2Fe14B/a-Fe and Pr2Fe14B/a-Fe magnets with

(BH)max� 23 MGOe,16–18 higher values of (BH)max are

FIG. 3. (a) Demagnetization curves of the die-upset magnets with different Nd-Cu alloy addition; (b) demagnetization curves of the die-upset magnets with

x¼ 8wt. % measured parallel (||) and perpendicular (\) to the stress direction and hot-pressed magnets with x¼ 0 and 8wt. %; (c) magnetic properties curves

of the die-upset samples with different mass ratio of x from 0 to 10wt. %.

072409-3 Tang et al. Appl. Phys. Lett. 102, 072409 (2013)

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obtained for die-upset magnets with Nd-Cu diffusion.

Generally, the intergrain exchange coupling (IGEC) is con-

sidered as the origin for remanence enhancement, resulting

in a high (BH)max in nanocomposite magnets. Here, the high

value of (BH)max is attributed to not only the remanence

enhancement, but even more importantly to the hard grain

texture which brings high remanence19 and excellent square-

ness of demagnetization curves. In contrast to the isotropic

hot-pressed magnets with 8wt. % Nd-Cu addition, during

die upsetting process, the Nd-Cu further diffusion into the

grain boundary increases the effect of decoupling between

the hard magnetic phases, which would be beneficial to the

increase of Hcj.

Fig. 4(a) shows TEM image of die-upset magnets with-

out Nd-Cu. It is observed that the size of equiaxed grains

with polygonal shape ranges from 100 to 300 nm and this

sample maintains the microstructure similar as hot-pressed

magnets of MQ2. In Fig. 4(c), by doping 8wt. % Nd-Cu, the

platelet-shaped grains with size of 200-1000 nm exist in die-

upset nanocomposite magnets. It indicates that Nd-Cu has

diffused into grain boundary and plays a crucial role in the

grain growth and texture formation during die upsetting

process, consistent with the results identified by XRD and

SEM images. The HRTEM images of die-upset magnets

without Nd-Cu and with 8wt. % Nd-Cu addition are shown

in Figs. 4(b) and 4(d), respectively. For traditional

RE2Fe14B-based nanocomposite magnets, the magnetization

reversal mechanism is considered as the nucleation of

reversed domains and IGEC plays a negative role for coer-

civity. As Nd-Cu diffuses into the interface of main phase,

the grain boundary phase appears and isolates the hard mag-

netic phase. This results in decoupling between the hard

magnetic phases and improves nucleation field and coerciv-

ity. Of course, the die-upset magnets have an absolutely

different microstructure with that of traditional RE2Fe14B-based magnets. Their magnetization reversal process

is very complicated and deeper research is necessary.

Thermomagnetic curves and DSC curves are employed to

take a probe into the variation in phase component. In Fig. 5(a),

the magnets consist of Nd2Fe14B and a-Fe in the die-upset

samples without Nd-Cu and die-upset samples with 8wt. %

Nd-Cu addition. The Curie temperature of hard and soft mag-

netic phase are 320 and 740 �C, respectively, revealing that

the doped Nd-Cu liquid phase has no influence on the Curie

temperature of the magnetic phases. Simultaneously, it’s evi-

dent that the amount of a-Fe in the samples without Nd-Cu

exceeds that in samples with 8wt. % Nd-Cu addition.

Therefore, a preliminary conclusion may be drawn that the

added Nd-Cu alloy reacts with part a-Fe and generating the

Fe-containing Nd-rich phase contributes to a decrease of a-Fe phase content. This is also confirmed by the DSC curves

as shown in Fig. 5(b). The DSC curves show well-

pronounced endothermic peaks at the Curie temperature of

Nd2Fe14B and a-Fe phase in die-upset magnet without Nd-

Cu. However, for the 8wt. % Nd-Cu doped samples,

the endothermic peak of a-Fe phase vanishes and weak

endothermic peak appears at temperature about 600 �C.Considering the addition of Nd-Cu alloy, the latter should be

the endothermic peak of melting point of Nd-Cu-Fe gener-

ated in hot pressing and die upsetting process.

In summary, we demonstrate a strategy to develop an

anisotropic nanocomposite Nd2Fe14B/a-Fe magnets by hot

pressing and die upsetting coupled with Nd-Cu grain bound-

ary diffusion. The Nd-Cu grain boundary diffusion has been

shown to be an effective process to improve hot workability

of nanocomposite magnets, resulting in the formation of

FIG. 4. TEM/HRTEM images of die-upset magnets with Nd-Cu alloy addi-

tion. (a) and (b) x¼ 0, (c) and (d) x¼ 8wt. %.

FIG. 5. Thermomagnetic curves (a) and

DSC curves (b) of the die-upset magnets

with x¼ 0 and 8wt. %.

072409-4 Tang et al. Appl. Phys. Lett. 102, 072409 (2013)

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lamellar structure. The magnetic properties Hcj¼ 13.99 kOe,

Br¼ 12.89 kG and (BH)max¼ 37.3 MGOe have been

obtained in the die-upset nanocomposite magnet with 10wt. %

Nd-Cu addition.

This work is supported by the National Natural Science

Foundation of China (No. 51101167, No. 60901047), the

National High Technology R&D Program of China (No.

2010AA03A402), National Science and Technology Major

Project (No. 2012ZX02702006-005), the Program of

International Science and Technology Cooperation of China

(No. 2010DFB53770), Local Cooperation Project of CAS

(DBSH-2011-013), the State Key Program of National

Natural Science of China (No. 50931001).

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