Secondary recrystallization of Goss texture in magnetostrictiveFe–Ga-based sheets

Chao Yuan, Xue-Xu Gao*, Ji-Heng Li,

Xiao-Qian Bao

Received: 27 May 2013 / Revised: 17 September 2013 / Accepted: 16 April 2014

� The Nonferrous Metals Society of China and Springer-Verlag Berlin Heidelberg 2014

Abstract To avoid the unfavorable effect of precipitates

on magnetic properties, low addition of 0.16 wt% NbC was

employed in Fe82.5Ga17.5 alloy, and 0.27 mm sheets were

successfully prepared by a rolling process. Strong Goss

texture was obtained in Fe–Ga sheets by two-stage

annealing process. The sulfur annealing in quartz ampoules

was made to produce the preferred growth of Goss texture,

with a 51 % increase in the observed saturation magneto-

striction, from 87 9 10-6 in the Ar-annealed sheets to

132 9 10-6 in the S-annealed sheets. However, large

amounts of S-rich and Nb-rich precipitates were formed in

the S-annealed sheets. High-temperature annealing of the

S-annealed sheets at 1,200 �C under a flow hydrogen–

argon atmosphere could effectively eliminate the precipi-

tates and promote the selective growth of (110) grains due

to the low surface energy. Large grains (*2 cm) of sec-

ondary recrystallization were achieved in the final annealed

sheets, with the maximum average saturation magneto-

striction of 180 9 10-6.

Keywords Fe–Ga alloy; Goss texture; Magnetostriction;

Secondary recrystallization

1 Introduction

In 2000, it was first reported that the addition of Ga could

greatly increase the magnetostriction of a-Fe, and the

maximum magnetostriction was as high as 400 9 10-6 at a

low applied magnetic field of *20 kA�m-1 along h100icrystal orientation [1–3]. More importantly, Fe–Ga alloy

has a strong mechanical strength of about 500 MPa [4],

which is not obtained in other energy transducer materials,

such as Terfenol-D and piezoelectric ceramic. The high

magnetostriction combined with good mechanical proper-

ties makes Fe–Ga alloy a unique material, and draws

considerable interest due to its potential applications in

sensors, actuators, and energy harvesting [5]. There are

several researches concerned with bulk alloys [3, 6] and

wires [7], and the ternary addition is of great importance

for the magnetic properties in the Fe–Ga alloys [8, 9].

Owing to their high conductivity, Fe–Ga alloys need to

be formed into thin sheets to avoid eddy current losses.

Since the maximum magnetostriction and easy axis occur

along the h001i orientation, it is desirable to obtain the g-

fiber, h100i//RD (rolling direction) in Fe–Ga sheets,

including Goss texture ({110}h001i) or cubic texture

({100}h001i). Efforts were made to fabricate g-fiber-tex-

tured Fe–Ga sheets by conventional rolling process [10–

18]. Secondary recrystallization, also called abnormal grain

growth (AGG), is widely used to obtain Goss-textured

sheets in electrical steel. In previous studies [15, 16], NbC

was reported to promote the AGG of Goss grains in Fe–Ga

alloy sheets in a process that was similar to the inhibition of

normal grain growth (NGG) due to the precipitation of

second-phase particles in Fe–Si steel, and single-like grain

growth of Goss grain was obtained combined with the

S-induced surface energy effect [17]. However, owing to

the low ductility of binary Fe–Ga alloy, the addition of Nb

C. Yuan, X.-X. Gao*, J.-H. Li, X.-Q. Bao

State Key Laboratory of Advanced Metals and Materials,

University of Science and Technology Beijing, Beijing 100083,

China

e-mail: gaox@skl.ustb.edu.cn

C. Yuan

e-mail: chaoyuan916@sina.com

123

Rare Met. RARE METALSDOI 10.1007/s12598-014-0284-5 www.editorialmanager.com/rmet

or NbC was as much as 1 at% (equal to *1.8 wt%) to

suppress the grain boundary fracture and improve the rol-

lability [14–18], compared with the much lower addition of

0.09 wt% Nb [19] as inhibitors in Fe–Si steel. The exces-

sive Nb would result in a large amount of Nb-rich pre-

cipitates, due to the low solubility of Nb in Fe–Ga alloy

[20]. As precipitates in magnetic material would restrain

the domain motion, causing higher coercivity and core

losses, their removal would be beneficial for improving the

efficiency of transducer. However, a few studies were

undertaken to reduce the addition of NbC and eliminate

Nb-rich precipitates during the annealing process, in order

to avoid the probable unfavorable effect on Fe–Ga ultra-

sonic transducer.

In this work, 0.27 mm Fe–Ga sheets were successfully

prepared by a rolling process with the low addition of NbC

(0.16 wt%). Compared with previous work [14–18], the

addition amount of 0.16 wt% was less than 1/10 of the

previous contents (*1.8 wt%), resulting in easy elimina-

tion of Nb-rich precipitates. Two-stage annealing process

was made to produce Goss-textured sheets. The preferred

Goss texture was obtained during first stage of S-annealing

in quartz ampoules. Hydrogen was introduced to promote

Goss grain growth and eliminate Nb-rich precipitates dur-

ing the second stage of high-temperature annealing.

Finally, strong secondary recrystallization of Goss texture

was obtained with few precipitates in the sheets.

2 Experimental

The 0.16 wt% NbC doped Fe82.5Ga17.5 ingots were pre-

pared from Fe (99.9 % purity), Ga (99.99 % purity) and the

master alloys of Fe–C and Nb–Fe by induction melting

under Ar atmosphere. The cast ingot was hot forged to a

thickness of 20 mm, followed by a rolling process of hot

rolling (1,100 �C) to 1.8 mm, intermediate annealing

(830 �C for 1 h), warm rolling (500 �C) to 0.6 mm, and

cold rolling to a final thickness of 0.27 mm. Two-stage

annealing process was made to fabricate Goss textured Fe–

Ga sheets. The first stage was made in quartz ampoules

with elemental S enclosed (*1 mg�cm-2) under Ar

(0.3 9 105 Pa), and a slow heating rate of 15 �C�h-1

between 900 and 1,080 �C was employed to promote the

selective growth of Goss grains. To investigate the effect of

S on textural evolution during the first stage, the same

annealing protocol was carried out for another ampoule

without the elemental S. To promote further grain growth

and eliminate precipitates in the S-annealed sheets, the

second stage of high-temperature annealing was performed

at 1,200 �C with various annealing time under H2/Ar-

mixed (25 vol% H2) gas flow, followed by furnace cooling

to around 400 �C and air cooling to room temperature. The

illustration of annealing process is shown in Fig. 1, and

digital photographs of final annealed sheets are shown in

Fig. 2.

The phase on the surface was detected by X-ray dif-

fraction (XRD), after polishing the surface with SiC sand

paper. Scanning electron microscopy (SEM) and energy

dispersive X-ray spectroscopy (EDX) were employed to

investigate the precipitates in the sheets. The X-ray pole

made on the sheets of dimension, 10 mm 9 20 mm, was

employed to obtain orientation distribution function (ODF)

to get the information of macrotexture. The magnetostric-

tion was measured by strain-gages positioned along the

RD, and the active area of strain-gages was

2.8 mm 9 2.0 mm. The saturation magnetostriction is the

value of (3/2)kS = k// - k\, where k// and k\ are

the magnetostriction when the magnetic fields parallel and

perpendicular to the RD are applied, respectively.

3 Results and discussion

The XRD patterns for the sheets after annealing are shown

in Fig. 3. As it can be seen, for the sheets after both first-

and second-stage annealing steps, the main phase is the a-

Fe (A2) phase, and no other phase related with Nb is found,

which may be attributed to its low content. Notably, much

higher diffraction intensity ratio of (110) grains to (200)

grains can be observed in the S-annealed than that in the

Ar-annealed sheets after the first-stage annealing, in

Fig. 3a. Although the preferred growth of (110) grains is

obtained in the S-annealed sheets, (110) grains are not

dominant after the first stage annealing, and weak patterns

for (200) and (211) grains are observed. After the second

stage of high-temperature annealing, the (110) grains in the

S-annealed sheets further develop, resulting in the domi-

nant patterns for (110) grains, as shown in Fig. 3b.

Fig. 1 Illustration of two-stage annealing process

C. Yuan et al.

123 Rare Met.

For the NGG in rolled Fe–Ga sheets, the average grain

size is\1 mm, and secondarily recrystallized grains could

be as large as several millimeters [15, 16]. In this work,

large secondarily recrystallized (110) grains of several

millimeters are obtained, as even clearly seen by naked

eyes in Fig. 2, especially for the sheets annealed for 6 h,

with clear grain boundary across the whole sample along

the RD. The black line in the 6 h annealed sheet (Fig. 2d)

shows a grain boundary across the sample along the RD,

indicating a large grain (*2 cm) in the sheet. It should be

noted that, though secondary recrystallization can be

observed in all the sheets, it is not uniform for the distri-

bution of large secondarily recrystallized grains, and this

inhomogeneity may cause the deviation of magnetostric-

tive performance. For instance, clear grain boundaries

indicative of large grains are observed on the right side of

the 2 h annealed sheet (in Fig. 2), while boundaries on the

left side are not clear due to smaller-sized grains.

SEM images for the surface and cross section are shown

in Fig. 4. As can be seen for the observation of surface and

cross section, though only 0.16 wt% NbC was added in the

sheets, several Nb-rich precipitates uniformly distribute in

the Ar-annealed sheets after the first-stage annealing.

However, for the S-annealed sheets, the S atoms spread

into the matrix and precipitates with Nb atoms, forming

into large amounts of fine S-rich and Nb-rich precipitates,

as shown in Table 1. Unlike the uniform distribution in the

Ar-annealed sheets, the distribution of precipitates in the

S-annealed sheets is similar to the sandwich structure, in

which precipitates tend to distribute on the two sides near

Fig. 2 Digital images of sheets after high-temperature annealing: a 1 h, b 2 h, c 4 h, d 6 h, e 8 h

Fig. 3 XRD patterns for sheets: a after first-stage annealing and b after second-stage annealing

Secondary recrystallization of Goss texture in magnetostrictive Fe–Ga-based sheets

123Rare Met.

the surface as shown in Fig. 4d. The chemical composition

of precipitates on the cross sections after the first-stage

annealing is shown in Table 1.

In previous studies with the S-induced selective grain

growth in Fe–Ga–B sheets, Na and Flatau [18] reported the

segregation of S on the surface in addition to obvious XRD

patterns for FeS, and the layer of S-segregation was only a

few nanometers by XPS and AES analysis. However, in

this work, no obvious XRD pattern for sulfide is found

(Fig. 3a), which may be due to the low content. Besides,

the S-rich and Nb-rich precipitates are found in several tens

of microns away from the surface, as shown in Fig. 4d and

Table 1.

Since the nonmagnetic particles would restrain the

domain motion, they cause core losses and other unfavor-

able effects on transducer; thus, hydrogen was introduced

to eliminate the Nb-rich precipitates. As expected, though

the Nb-rich precipitates are observed both in the Ar-

annealed and S-annealed sheets after the first-stage

annealing, they could hardly be found after annealing at

1,200 �C for 6 h under H2/Ar atmosphere, as shown in

Fig. 4e, f. The elimination of Nb-rich precipitates may be

similar to the behavior of AlN or MnS in Fe–Si steel during

high-temperature annealing in dry H2. Owing to the strong

reduction effect of H2, N or S atoms are eliminated by the

type of NH3 or H2S, and Al or Mn atoms dissolve into the

matrix, and thus, few precipitates remain in the sheets after

hydrogen annealing, avoiding the unfavorable effect of

precipitates on magnetic properties. As mentioned above in

Table 1, the S atoms are prone to precipitate with Nb

atoms. The S and C atoms could be eliminated due to the

strong reduction effect of H2, increasing the solubility of

Nb atoms, and thus Nb-rich precipitates uniformly dissolve

into the matrix, avoiding the unfavorable effect of precip-

itates on magnetic properties.

To maximize the magnetostrictive performance, the tex-

ture along the RD is also of great importance, and g-fiber

texture ({110}h001i or {100}h001i) is preferred. The ODF

plots (u2 = 45�) from the X-ray pole figure of the specimen

are shown in Fig. 5. As shown in Fig. 5a, the main texture for

Ar-annealed sheets after the first-annealing stage is the

rotating cubic texture ({001}h110i), accompanied with

strong c-fiber texture (h111i//ND), consisting of {111}h110iand {111}h112i. However, for the S-annealed sheets, the

main texture becomes the preferred Goss texture

({110}h001i) as shown in Fig. 5b, consistent with the pre-

ferred pattern for the (110) grains as found from the XRD

results (Fig. 3a).

It was reported that the S-induced surface energy effect

could play an important role in the selective grain growth

in Fe–Si electrical steel [21, 22], also in Fe–Ga-rolled

sheets [11, 17, 18]. Except for the surface energy effect, the

dispersion of NbC particles would play as inhibitors

causing the local pinning of non-Goss grain boundaries,

Fig. 4 SEM images after first-stage annealing: a surface and b cross section of Ar-annealed, c surface and d cross section of S-annealed; after

second-stage annealing for 6 h: e surface and f cross section

Table 1 Chemical composition of precipitates on cross section

(wt%)

Precipitates Fe Ga Nb S

Ar-annealed 9.24 2.51 88.25 –

S-annealed 34.38 5.87 33.69 26.06

C. Yuan et al.

123 Rare Met.

which promoted the preferred growth of Goss texture.

Compared with the most previous studies using much

higher content of NbC (1 at%, equal to *1.8 wt%) in Fe–

Ga sheets [15–18], only 0.16 wt% NbC was used in this

study, and this is much closer to the addition of only

0.09 wt% Nb as inhibitors in Fe–Si steel [19]. However,

the single effect of NbC could not effectively promote the

development of Goss texture in this work, as shown in

Fig. 5a. Thus, the selective growth of Goss texture during

the first-stage annealing may be attributed to the combined

effects of the dispersion of NbC particles and the S-induced

surface energy effect. However, further study is needed to

distinguish which one plays the primary role in the texture

evolution during the first-stage annealing.

As shown in Fig. 5b, although the preferred Goss texture

develops after the first stage of S-annealing, Goss texture is

not dominant, and other weak textures are found. Thus, high-

temperature annealing was made to promote the further

growth of Goss texture. As expected, Goss grains further

develop during high-temperature annealing under flow H2/

Ar atmosphere, and become dominant with a maximum

intensity value of 51 in the 6 h annealed sheets, as shown in

Fig. 5c. The ODF plot is also consistent with the XRD results

as shown in Fig. 3b, which shows dominant patterns for

(110) grains. It was reported that the preferred (110) grain

growth occurred under very clean surface condition because

of the lower surface energy compared with (100) and (111)

grains [21]. Hydrogen would purify the surface, causing the

constant surface with little contamination. Thus, the pre-

ferred (110) grain growth may be attributed to the low sur-

face energy, and unlike that in Ref. [10], the preferred (110)

grain growth does not vary with the annealing time, as shown

in Fig. 3b. Considering that there are only a few particles as

inhibitors (Fig. 4f), the surface energy effect is considered to

be the key factor in the development of (110) grain growth

during the second-stage annealing.

The measured magnetostriction data are shown in

Fig. 6. Although the thickness is 0.27 mm, the annealed

Fig. 5 ODF plots (u2 = 45�) after first-stage annealing: Ar-annealed a and S-annealed b; after second-stage annealing for 6 h c

Fig. 6 Magnetostriction versus applied magnetic field curves (k// and k\) after first-stage annealing a and measured saturation magnetostriction

(k// - k\) and average values (k// - k\) after second-stage annealing b

Secondary recrystallization of Goss texture in magnetostrictive Fe–Ga-based sheets

123Rare Met.

sheets are strong enough to keep straight in a field below

80 kA�m-1, and the measurement error range is below

2–3 9 10-6 for the same gage within several times of

measurement. Figure 6a gives the magnetostrictive curves

versus magnetic field after first stage annealing. Because of

the preferred Goss texture in Fig. 5b, a 51 % increase of

saturation magnetostriction (k// - k\) is observed, i.e.,

from 87 9 10-6 in the Ar-annealed sheet to 132 9 10-6 in

the S-annealed sheet. For the magnetostrictive measure-

ment after the second-stage annealing, owing to the inho-

mogeneity of development in the secondary

recrystallization in Fig. 2, three strain gages parallel to

each other along the RD were used to measure the mag-

netostrictive strain. The measured saturation magneto-

striction values (3/2)kS = k// - k\ as a function of

dwelling time at 1,200 �C are shown in Fig. 6b. The

measured maximum (3/2)kS is 203 9 10-6 for the 4 h

annealed sheet. The average values for (3/2)kS are above

165 9 10-6 for the 4–8 h annealed sheets, with a maxi-

mum of 180 9 10-6 for the 6 h annealed sheet. However,

large deviation for (3/2)kS is observed, especially for the

annealing times of 2 and 4 h, which may be due to the

inhomogeneity of secondary recrystallization. In addition,

other minor orientations could be found in the XRD pat-

terns (Fig. 3b) and the ODF plot (Fig. 5c), which would

also cause the deviation of magnetostriction. Further study

is needed to improve the homogeneity of magnetostriction,

and the homogeneity of secondary recrystallization may be

the key factor. Although there is a large deviation in sat-

uration magnetostriction, with the thickness \0.3 mm, the

average value for (3/2)kS above 165 9 10-6 is large

enough for ultrasonic application.

4 Conclusion

The Fe82.5Ga17.5 alloy sheets with low addition of

0.16 wt% NbC were successfully prepared with a final

thickness of 0.27 mm by a rolling process, and the two-

stage annealing process was used to fabricate the Goss

textured sheets. Sulfur annealing in quartz ampoules pro-

duced a preferred growth of Goss texture, and a 51 %

increase was observed in saturation magnetostriction, from

87 9 10-6 in the Ar-annealed sheets to 132 9 10-6 in the

S-annealed sheets. A large number of fine S-rich and Nb-

rich precipitates were found in the S-annealed sheets after

the first-stage annealing, and hydrogen during the high-

temperature annealing stage could eliminate precipitates.

Owing to the lower surface energy of (110) grains under

clean surface condition, the preferred (110) grain growth

was observed, and large grains (*2 cm) of secondary

recrystallization were obtained with the maximum average

values for magnetostriction of 180 9 10-6.

Acknowledgments This study was financially supported by the

National Basic Research Program of China (No. 2011CB606304), the

National Natural Science Foundation of China (No. 51271019), the

Fundamental Research Funds for the Central Universities (No. FRF-

SD-12-025A), and the National Natural Science Foundation for Post-

doctoral Scientists of China (No. 2011M500229).

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