Effect of Pre-Aging Treatment on the Microstructure and Magnetic Propertiesof Sm(Co,Fe,Cu,Zr)7.8 Sintered Magnets

Yosuke Horiuchi+, Masaya Hagiwara, Keiko Okamoto, Tsuyoshi Kobayashi, Masaki Endo,Tadahiko Kobayashi, Naoyuki Sanada and Shinya Sakurada

Functional Materials Laboratory, Corporate R&D Center, Toshiba Corporation, Kawasaki 212-8582, Japan

The effects of pre-aging treatment on the microstructure and magnetic properties of Sm(Cobal.Fe0.35Cu0.06Zr0.02)7.8 were investigated. Themain phase of both solution-treated magnet and pre-aged magnet was the 1 : 7 phase, and there were no distinct differences between the X-raydiffraction profiles of these magnets. Fine Cu-rich precipitates a few tens of nanometers in size were observed in pre-aged magnet by scanningtransmission electron microscopy-energy-dispersive X-ray spectroscopy mapping, but such precipitates were not observed in solution-treatedmagnet. As for fully aged magnets, the cell size was smaller in pre-aged magnet than in non-pre-aged magnet. Thus, the pre-aging treatment gavea fine cellular structure. Mr and HcJ of pre-aged magnet were almost same as those of non-pre-aged magnet. Squareness of the demagnetizationcurve for fully aged magnet was increased by pre-aging treatment. As a result, (BH)max of magnet subjected to pre-aging treatment was greaterthan that of magnet not subjected to pre-aging treatment. The fine cellular structure seemed to result in higher squareness. The followingmagnetic properties were obtained for Sm(Cobal.Fe0.35Cu0.06Zr0.02)7.8 by pre-aging treatment: Mr = 1.24T, HcJ = 1490 kA/m, and (BH)max =266 kJ/m3. [doi:10.2320/matertrans.MBW201325]

(Received October 18, 2013; Accepted December 26, 2013; Published February 25, 2014)

Keywords: Sm2Co17 sintered magnet, cellular structure, 1 : 5 phase, 2 : 17 phase, aging treatment, pre-aging treatment, squareness

1. Introduction

Highly heat-resistant permanent magnets are needed forhigh-efficiency motors, such as traction motors in hybrid andelectric vehicles, railroad cars, and industrial equipment thatoperates at relatively high temperatures. Samarium­cobalt(Sm2Co17) sintered magnet is a candidate material for theseapplications because it has high heat resistance due to its highCurie point and its low temperature-dependent remanence.A high maximum energy product ((BH)max) is needed forminiaturization and high efficiency in motors, and ironenrichment is effective for satisfying this requirement. Inthe 1970s and 1980s, many studies investigated Sm2Co17magnets.1­7) Magnetic properties have been reported forSm(Cobal.FevCuxZy)Z sintered magnet containing relativelyhigh Fe content, V > 0.28 (= 24.5mol% Fe), and (BH)max ashigh as 264 kJ/m3 has been obtained.8,9)

We focused on Sm2Co17 sintered magnet containing alarger amount of Fe than in the magnets investigatedpreviously. We have reported that high coercivity can beobtained for Sm(Cobal.Fe0.35Cu0.06Zy0.02)7.8 sintered magnetsby selecting an appropriate solution-treatment temperaturethat does not form precipitates of Sm2Co7 phase.10) However,those magnets lack sufficient squareness, and the high(BH)max expected from its Mr could not be obtained.Therefore, improved squareness is necessary to achieve high(BH)max for these magnets. The microstructure and coercivitymechanism of Sm(Co,Fe,Cu,Zr)z magnets have been thesubject of many papers,11­16) and it is well known that agingtreatment conditions are important factors in controlling thecellular structure of these magnets. However, the relationshipbetween microstructure and squareness is not yet sufficientlyclear. Therefore, the effect of pre-aging treatment on themicrostructure and magnetic properties of Sm(Co,Fe,Cu,Zr)7.8 sintered magnets was investigated in this study.

2. Experimental Procedure

Sm(Cobal.Fe0.35Cu0.06Zr0.02)7.8 alloy powder was preparedby ball milling. The powder was then pressed at a pressure of1.5 t/cm2 in a magnetic field of 1 T. The green bodies weresintered at 1463K for 10.8 ks and then solution-treated at1403­1433K for 14.4 ks under argon atmosphere. Subse-quent isothermal aging treatments were carried out at 1103Kfor 14.4 or 144 ks, followed by quenching to room temper-ature (RT) or slow cooling to 673K with subsequent furnacecooling to RT. Some samples were pre-aged at 1023K for3.6­36 ks before aging treatment. A schematic representationof the aging treatments is shown in Fig. 1. Magnets atvarious stages of aging, namely, a, b, c, and d in Fig. 1,were used for X-ray diffraction (XRD) analysis and scanningtransmission electron microscopy (STEM) observation. Amagnet aged to stage d in Fig. 1 is called a “fully agedmagnet” in this study.

Non-pre-aged magnet

Pre-aged magnet

time

Tem

pera

ture

time

Tem

pera

ture

a b c d

a b c d

Fig. 1 Schematic representation of aging treatment.

+Corresponding author, E-mail: yosuke.horiuchi@toshiba.co.jp

Materials Transactions, Vol. 55, No. 3 (2014) pp. 482 to 488©2014 The Japan Institute of Metals and Materials

The crystal structures of specimens were characterized byXRD using Cu Ka radiation at RT. The microstructures of thespecimens were examined by STEM and TEM. STEM wasused to obtain the bright-field images and energy-dispersiveX-ray spectroscopy (EDX) mappings of Cu, and TEM wasused to obtain selected electron diffraction patterns. Magneticmeasurements were done using an automatic recordingflux meter (B-H tracer) with a maximum magnetic field of2000 kA/m at RT. Mr, HcJ, squareness, and (BH)max wereobtained from demagnetization curves. In this study, thesquareness of the demagnetization curve was defined as theratio of the measured value of (BH)max to the ideal value of(BH)max:

ðBHÞmax ðmeasuredÞ=ðBHÞmax ðidealÞ � 100% ð1ÞHere, (BH)max (ideal) was determined by using Mr in thefollowing formula:

ðBHÞmax ðidealÞ ¼ Mr2=4®0 ð2Þ

3. Results

3.1 Phase constitutionPartial XRD profiles of magnets at each stage of aging

for non-pre-aged magnets are shown in Fig. 2. At the initialstage (solution-treated magnet; Fig. 2(a)), there was no peakat around 38°. This indicates that the TbCu7-type phase (1 : 7phase) was formed by solution treatment. After aging for14.4 ks, a small diffraction peak from ð024ÞTh2Zn17 wasobserved (Fig. 2(b)). The intensity of this peak was greaterafter aging for 144 ks than after for aging for 14.4 ks. Thissuggests that phase separation occurred, giving the Th2Zn17-type phase (2 : 17 phase; cell phase) and the CaCu5-typephase (1 : 5 phase; cell boundary phase). The intensity of thepeak for a fully aged magnet was almost the same as that fora magnet after aging for 144 ks.

Figure 3 shows the partial XRD profiles at each stage ofaging for magnets that were pre-aged. As shown in Fig. 3(a),a peak from ð024ÞTh2Zn17 was not observed at the initial stage(pre-aged magnet). This means that the main phase of pre-aged magnet was still the 1 : 7 phase, as it is in solution-treated magnet. Thus, a distinct phase transition during pre-aging treatment was not observed by XRD. The change in theXRD profile upon subsequent aging treatment was almostthe same as in the case of non-pre-aged magnet, and it can beassumed that pre-aging treatment does not influence thephase change process during the aging treatment.

3.2 MicrostructureFigures 4 and 5 show the bright-field STEM images of

magnets at each stage of the aging treatment. As shown inFig. 4(a), fine, uneven contrast was observed as bands. Inprevious reports, twinning within the rhombohedral 2 : 17cell is visible in micrographs as bands of alternating intensityrunning perpendicular to the c-axis.17,18) A similar twinningstructure in the 1 : 7 phase might form in this type of magnetand the details are currently under investigation. Distinctprecipitates were not observed in solution-treated magnet. Itwas found that fine contrast become more pronounced forpre-aged magnet, as shown in Fig. 5(a). This figure also

shows that, small precipitates a few tens of nanometers insize were observed in pre-aged magnet (indicated by arrows).As for the 14.4 ks aged magnets, typical cellular structureswere formed in both non-pre-aged and pre-aged magnets andcell boundary phase was observed (arrows in Figs. 4(b) and5(b)). Figure 4(e) shows the TEM selected area electrondiffraction pattern taken at the same area as the micrographin Fig. 4(b). As shown in Figs. 4(f ) and 4(g), the indexingresults of this selected area electron diffraction patternindicate that the orientation relationship between the rhom-bohedral 2 : 17 and hexagonal 1 : 5 phases is (001)2:17R //(001)1:5H and [010]2:17R // ½�110�1:5H. The orientation relation-ship and indexing results for these phases agree well withprevious reports.17,18) The cellular structure became coarserupon further aging (Figs. 4(c) and 5(c)) and the cell size offully aged magnet was almost the same as that of magnetaged for 144 ks. This trend was the same regardless of pre-

34 35 36 37 38 39 40 41 42 43 44 45 46

(c)

(b)

(a)

Inte

nsity

(a.

u.)

2θ / degree

TbCu7

Th2Zn17

(d)

Fig. 2 Partial X-ray diffraction (XRD) profiles of non-pre-aged magnet: (a)initial state (solution-treated magnet), (b) aged 14.4 ks¼ quenching toroom temperature (RT), (c) aged 144 ks¼ quenching to RT, and (d) aged144 ks ¼ slowly cooling to RT.

34 35 36 37 38 39 40 41 42 43 44 45 46

(c)

(b)

(a)

Inte

nsity

(a.

u.)

2θ / degree

TbCu7

Th2Zn17

(d)

Fig. 3 Partial XRD profiles of pre-aged magnet: (a) initial state (pre-agedmagnet), (b) aged 14.4 ks¼ quenching to RT, (c) aged 144 ks¼ quench-ing to RT, and (d) aged 144 ks ¼ slowly cooling to RT.

Effect of Pre-Aging Treatment on the Microstructure and Magnetic Properties of Sm(Co,Fe,Cu,Zr)7.8 Sintered Magnets 483

aging treatment. However, there were differences in cell sizebetween magnet subjected to pre-aging treatment and magnetnot subjected to pre-aging treatment. For 144 ks aged magnetand fully aged magnet, cell sizes of pre-aged magnets weresmaller than those of non-pre-aged magnet. The approximatecell sizes of fully aged magnets subjected to pre-aging andnot subjected to pre-aging were 160 and 220 nm, respec-tively. The thickness of the 1 : 5 phase was almost the sameregardless of pre-aging treatment.

Figures 6 and 7 show STEM-EDX mappings of Cuobtained in the same observation areas as shown in Figs. 4and 5, respectively. In solution-treated magnet, on the onehand, it was observed that Cu dissolved homogeneously(Fig. 6(a)). In pre-aged magnet, on the other hand, Cucontrast a few tens of nanometers in size was observed(Fig. 7(a)). This Cu contrast appears to correspond to theprecipitates observed in the STEM image of Fig. 5(a).Measured by EDX, the Cu concentration of the high contrastarea in Fig. 5(a) was about 15mol% and that of low contrastarea was about 3mol%. Distinct Cu-rich precipitates wereobserved in the 14.4 ks aged magnets (Figs. 6(b) and 7(b)).

These Cu-rich precipitates correspond to the 1 : 5 phase(cell boundary phase) in the STEM bright-field images.Comparing Fig. 6(b) to Fig. 7(b) shows that the spacingbetween Cu-rich precipitates was smaller for pre-agedmagnets. Upon aging, this trend became more pronounced,in agreement with the STEM bright-field images. In the fullyaged magnets, the spacing between Cu-rich precipitates inpre-aged magnet was also smaller than that in non-pre-agedmagnet. Thus, it was found that the fine cellular structure wasformed by introducing pre-aging treatment.

The Cu concentrations of the 2 : 17 phase (cell phase) andthe 1 : 5 phase (cell boundary phase) for each magnet areshown in Table 1 and Fig. 8. The items (a), (b), (c), and (d) inthe table and figure correspond to the aging conditions a, b, c,and d in Fig. 1, respectively. For condition (a), no data areshown because a distinct cellular structure was not observedin these magnets, although Cu-rich regions were detected inpre-aged magnet as shown in Fig. 5(a). The Cu concentrationof the 2 : 17 phase decreased upon aging treatment(Fig. 8(A)). Furthermore, the Cu concentration in pre-agedmagnets was less than that in non-pre-aged magnets for all

(a)

(d)

100 nm

(c)

100 nm

(b)

100 nm

100 nm

Cell boundary phase

(300)(303)

(003)

(000)[010]2:17R

(110)(111)

(001)

(000)[110]1:5H

(e) (f) (g)

Fig. 4 Scanning transmission electron microscopy (STEM) images of non-pre-aged magnet: (a) initial state (solution-treated magnet), (b)aged 14.4 ks¼ quenching to RT, (c) aged 144 ks¼ quenching to RT, (d) aged 144 ks¼ slowly cooling to RT. (e) TEM selected areaelectron diffraction pattern taken at the same area as (b); indexing results of (f ) 2 : 17 phase and (g) 1 : 5 phase.

Y. Horiuchi et al.484

aging conditions. The Cu concentration of the 1 : 5 phaseincreased as aging treatment progressed (Fig. 8(B)). Inparticular, the increase from 14.4 ks aged magnet to fullyaged magnet was substantial, and the amount of Cu exceeded30mol% in fully aged magnet. A comparison between non-pre-aged and pre-aged magnet shows that the Cu concen-tration of non-pre-aged magnet was less than that of pre-aged magnet. However, the Cu concentration of non-pre-agedmagnet became greater than that of pre-aged magnet duringaging.

3.3 Magnetic propertiesThe magnetic properties of fully aged magnet are summa-

rized in Table 2. Mr and HcJ of pre-aged magnet were almostthe same as those of non-pre-aged magnet. However, thesquareness was increased by pre-aging treatment. As a result,(BH)max of pre-aged magnet was greater than that of non-pre-aged magnet.

4. Discussion

4.1 Effect of pre-aging treatment on microstructureIt is considered that the cellular structure of 1 : 5 phase and

2 : 17 phase was not formed by pre-aging treatment, becausethe diffraction peak from Th2Zn17 was not detected by XRDfor pre-aged magnet (Fig. 3(a)). However, the fine contrast ofCu was observed for this magnet. The Cu contrast might bedue to the modulated structure of the 1 : 7 phase, though thedetails of the crystal structure of the pre-aged magnet are notyet clear. Fine Cu-rich regions formed by pre-aging treatmentmight act as nuclei of Cu-rich 1 : 5 phase in subsequent agingtreatment. These fine Cu-rich precipitates are considered tocause the finer cellular structure in fully aged magnet. Thenumber of nuclei of Cu-rich phase is thus thought to be moregreatly increased by pre-aging treatment at lower temperaturethan by subsequent aging treatment.

It was shown that the cellular structure become finer uponpre-aging treatment by contracting the morphology of thecellular structure. The formation of the finer cellular structureby pre-aging treatment was also observed from the Cuconcentration in the each phase of the cellular structure. TheCu concentration of the 1 : 5 phase was lower in fully agedmagnet subjected to pre-aging treatment than in fully agedmagnet not subjected to pre-aging treatment. In contrast, theCu concentration in the 2 : 17 phase of fully aged magnetsubjected to pre-aging treatment was almost the same as thatof fully aged magnet not subjected to pre-aging treatment.Therefore, the volume fraction of Cu-rich 1 : 5 phase must belarger for fully aged magnet subjected to pre-aging treatmentbecause the total amount of Cu was the same regardless ofpre-aging treatment, and furthermore, the thickness of the

(a)

(d)

100 nm

(c)

100 nm

(b)

100 nm

100 nm

Cell boundary phase

Fig. 5 STEM images of pre-aged magnet: (a) initial state (pre-aged magnet), (b) aged 14.4 ks¼ quenching to RT, (c) aged144 ks¼ quenching to RT, and (d) aged 144 ks¼ slowly cooling to RT.

Table 1 Cu concentrations of 2 : 17 phase and 1 : 5 phase at each stage ofaging.

(mol%)

Sample Phase in cellular structureAging condition

(a) (b) (c) (d)

non-pre-aged2 : 17 (cell phase) ® 4.3 3.3 2.1

1 : 5 (cell boundary phase) ® 10.8 14.9 34.0

pre-aged2 : 17 (cell phase) ® 3.8 3.1 1.9

1 : 5 (cell boundary phase) ® 16.0 17.6 30.3

Effect of Pre-Aging Treatment on the Microstructure and Magnetic Properties of Sm(Co,Fe,Cu,Zr)7.8 Sintered Magnets 485

1 : 5 phase in fully aged magnet subjected to pre-agingtreatment was also the same as that in magnet not subjectedto pre-aging treatment. The increase in the volume fraction ofCu-rich 1 : 5 phase might cause the lower Cu concentration

in the 1 : 5 phase of fully aged magnet subjected to pre-agingtreatment. The fine cellular structure in fully aged magnetsubjected to pre-aging treatment can be explained by theincreased volume fraction of Cu-rich 1 : 5 phase.

(a)

(d)

100 nm

(c)

100 nm

(b)

100 nm

100 nm

Fig. 7 STEM-EDX mapping of Cu in pre-aged magnet: (a) initial state (pre-aged magnet), (b) aged 14.4 ks¼ quenching to RT, (c) aged144 ks¼ quenching to RT, and (d) aged 144 ks¼ slowly cooling to RT.

(a)

(d)

100 nm

(c)

100 nm

(b)

100 nm

100 nm

Fig. 6 STEM-energy-dispersive X-ray spectroscopy (EDX) mapping of Cu in non-pre-aged magnet: (a) initial state (solution-treatedmagnet), (b) aged 14.4 ks¼ quenching to RT, (c) aged 144 ks ¼ quenching to RT, and (d) aged 144 ks¼ slowly cooling to RT.

Y. Horiuchi et al.486

4.2 Effect of cell structure on squareness of fully agedmagnet

An increase in the squareness of fully aged magnet by pre-aging was observed (Table 2). Many microstructural studiesof SmCo-based alloys have shown that 1 : 5 phase actsas pinning sites of domain walls, though the coercivitymechanism is explained by several different models.11,19­21)

Similarly to HcJ, squareness might also be related to the Cu-rich 1 : 5 phase. As mentioned above, pre-aging treatmentappeared effective for obtaining a fine cellular structure. Thismeans that the number of pinning sites for domain walls wasgreater in pre-aged magnet than in non-pre-aged magnet.Thus, the increase in squareness can be attributed to fineprecipitates of the 1 : 5 phase.

5. Conclusion

Through observation of the phase constitution and micro-structure of non-pre-aged and pre-aged Sm(Cobal.Fe0.35-Cu0.06Zr0.02)7.8 sintered magnets and measurement of theirmagnetic properties, the following has been shown.(1) The main phase in pre-aged magnet is the 1 : 7 phase,

as it is in solution-treated magnet.(2) STEM-EDX mappings, Cu contrast a few tens of

nanometers in size was observed in pre-aged magnet.In contrast, Cu dissolved homogeneously in solution-treated magnet.

(3) The Cu-rich 1 : 5 phase precipitated in 14.4 ks agedmagnets, regardless of pre-aging treatment. The cellsize was smaller in pre-aged magnet than in non-pre-aged magnet.

(4) The cell size becomes larger with aging. In fully agedmagnet subjected to pre-aging treatment, the cell sizewas smaller than in fully aged magnet not subjected topre-aging treatment and was also smaller than in 14.4and 144 ks aged magnets.

(5) Mr and HcJ of fully aged magnet subjected to pre-agingtreatment were almost the same as those of magnet notsubjected to pre-aging treatment. However, the square-ness was increased by pre-aging treatment. As a result,(BH)max of fully aged magnet was higher for magnetsubjected to pre-aging treatment.

Thus, it is concluded that pre-aging treatment was effectivefor obtaining a fine cellular structure, and for improving thesquareness of Fe-rich Sm(Co,Fe,Cu,Zr)z sintered magnets.The following typical magnetic properties were obtained forSm(Cobal.Fe0.35Cu0.06Zr0.02)7.8 by pre-aging treatment: Mr =1.24 T, HcJ = 1490 kA/m, and (BH)max = 266 kJ/m3.

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Table 2 Magnetic properties of Sm(Cobal.Fe0.35Cu0.06Zr0.02)7.8 sinteredmagnets.

Mr/T HcJ/kA/mSquareness,

(%)(BH)max/

kJ/m3

non-pre-aged 1.24 1480 83.9 251

pre-aged 1.24 1490 86.5 266

Cu

conc

entr

atio

n, C

cu/ m

ol%

1

2

3

4

5(A)

0(b) (c) (d)(a)

Cu

conc

entr

atio

n, C

cu/ m

ol%

10

5

(B)

0(b) (c) (d)(a)

15

20

25

30

35

40Non-pre-aged magnet

Pre-aged magnet

Non-pre-aged magnet

Pre-aged magnet

Fig. 8 Cu concentration in (A) 2 : 17 phase and (B) 1 : 5 phase at each stage of aging treatment: (a) initial state (non-pre-aged magnet:solution-treated state; pre-aged magnet: pre-aged state), (b) aged 14.4 ks¼ quenching to RT, (c) aged 144 ks¼ quenching to RT, and(d) fully aged.

Effect of Pre-Aging Treatment on the Microstructure and Magnetic Properties of Sm(Co,Fe,Cu,Zr)7.8 Sintered Magnets 487

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