Retaining the 〈001〉 Orientation from Initial Columnar Grains and Magnetostriction in Binary Fe Ga Alloy Sheets

Retaining the

©

001

ª

Orientation from Initial Columnar Grains

and Magnetostriction in Binary Fe-Ga Alloy Sheets

Jiheng Li, Chao Yuan, Wenlan Zhang, Xiaoqian Bao, Jie Zhu and Xuexu Gao

State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China

Sheets of binary Fe83Ga17alloy were prepared by a method which involved rolling of specimens with©100ªoriented columnar grains

arranged initially along rolling direction. The ductility of the binary Fe83Ga17alloy with©100ªoriented columnar grains fabricated via directional

solidification could be improved compared with that of the alloy with equiaxed grains. The orientation gradually rotated to a©110ªfrom the ©100ªdirection during the rolling process, and the final rolled texture was dominated by strong{100}©011ª. The sharp{110}©001ªGoss orientation was confirmed in thefinal annealed sheets. Excluding the effects of inhibitors and surface energy on textural evolution, it is thought that the©001ªorientation was obtained from the hereditary characteristics of the initial columnar grains. A high magnetostriction of 185 ppm without pre-stress was attributed to the strong Goss texture observed in thefinal recrystallized sheets. [doi:10.2320/matertrans.M2015282]

(Received July 9, 2015; Accepted September 8, 2015; Published October 23, 2015)

Keywords: magnetostriction, iron-gallium alloy, rolling, Goss texture, initial columnar-grains

1. Introduction

Magnetostrictive Fe-Ga alloys, also named “Galfenol”,

have received increasing attention as a new magnetostrictive smart material for actuator, sensor, and energy harvesting

applications.1,2) This interest stems from facts that unlike

existing smart material systems, Galfenol is thefirst to offer

the combination of good magnetostrictive properties and

mechanical properties. Magnetically,3) _{the addition of Ga}

increases the magnetostrictive capability of Fe over tenfold

up to 400 ppmð3₂­₁₀₀Þalong©100ªdirection in single crystal

material. Mechanically,4) _{Fe-Ga alloys are robust not}

exhibited by materials such as PZT, PMN, or Terfenol-D.

In addition, Fe-Ga alloys have high permeability (®r>

100),5) high Curie temperature (Tc>650°C)6) and low

temperature dependence (³0.5 ppm/°C).7)This combination

of magnetic and mechanical properties makes Fe-Ga alloy a unique material.

The high conductivity of Fe-Ga alloy requires its form of thin sheet to avoid eddy current losses. Efforts have been

made to get thin sheets by rolling process.8­10) However,

the studies of Cheng8) _{and Na}9) _{et al. indicated that the}

polycrystalline Fe81Ga19 binary alloy cracked and fractured

along grain boundaries during hot rolling and was too brittle to make thin sheets. Subsequently, it has been reported that addition of B, Mo, Nb and NbC could improved the ductility

of polycrystalline Fe-Ga alloys for plastic working.11) _But,

in almost all cases, ternary additions have decreased the

magnetostriction values from the binary Fe-Ga alloy.12) _On

the other hand, Cubic texture {100}©001ª or Goss texture

{110}©001ª was reported in previous work with ternary

addition, such as B13)or NbC,14)which was thought to act as

inhibitors, resulting in abnormal grain growth (AGG) in the Fe-Ga sheets. The texture in the Fe-Ga sheets is a key factor affecting magnetostrictive performance due to the anisotropic nature of the magnetostriction. The development of strong

©100ª (the magnetic easy axes)//RD (RD=rolling

direc-tion) texture is critical in order to achieve maximum

magnetostrictive strain. Unfortunately, high content up to

2.5 at% of ternary addition resulted in the large number of

island-like grains and precipitates,15) which would bring

some unfavorable influences on other magnetic properties, such as permeability.

The ©100ª orientation in bulk Fe-Ga alloys was usually

produced by a directional solidification process, meanwhile

coarse columnar grains was obtained after directional

solid-ification.16) _{Numerous studies reported that mechanical}

property of the alloy with columnar grains could be improved obviously compared with that of the alloy with equiaxed

grains.17,18)_{For Ni}

3Al alloy which exhibited grain boundary

embrittlement,17)_{the mechanical property of this alloy with}

columnar grains fabricated by directional solidification could be improved because of the low-energy grain boundary which could resist the intergranular cracks. The significant enhancement of the tensile ductility of the columnar-grained

Fe-6.5 mass%Si alloy at intermediate temperatures was

mainly ascribed to the formation of a great number of

homogeneous deformation twins.18)

For polycrystalline Fe-Ga binary alloy which exhibited grain boundary embrittlement, it is possible that the

direc-tional solidified binary alloy could be rolled to make thin

sheets. Meanwhile, the large magnetostriction could be

achieved in Fe-Ga binary alloy sheets if the ©001ª initial

orientation could be retained through microstructure heredi-tary of columnar grains. Therefore, we have been studying material design in order to prepare binary Fe-Ga alloy sheets

by rolling of the specimens with ©100ª oriented columnar

grains initially arranged along rolling direction. In this work,

The sheets of binary Fe83Ga17alloy were prepared, and Goss

texture was obtained in annealed Fe83Ga17 sheets, which

resulting in a high magnetostriction (­_//-­_¦) of 185 ppm.

2. Experimental Procedure

The Fe83Ga17 (at%) alloy was prepared from pure Fe

(99.95 mass% purity) and Ga (99.99 mass% purity) by

induction melting under argon atmosphere. The oriented

Fe83Ga17 alloy with 40 mm diameter and about 100 mm

+_{Corresponding author, E-mail: gaox@skl.ustb.edu.cn}

length was produced by directional solidification (DS). Figure 1 shows a schematic illustration of the directional solidification apparatus and a picture of directionally solidified rod. The raw material was molten in induction melting system, and then the molten metal was drawn into a quartz tube via the gating system. The quartz tube was placed in the holding furnace (thermal insulation system driven by resistance heating) with a given temperature gradient (about

55 K/cm). After pouring, the tube was moved down with rate

of 720 mm·hr¹1 by motor immediately. The microstructure

was observed by optical microscope. Static tensile tests at a

constant velocity of 1©10¹3_s¹1 _{were employed to}

inves-tigate the influence of columnar grains on the ductility. Fracture morphologies of specimens were observed by scanning electron microscope (SEM). The columnar-grained

slabs with a thickness of³18 mm were obtained from the DS

alloys by electrical discharge machining. The slabs were hot rolled along the growth direction of columnar grains at

1150°C to 2.1 mm with about 88% reduction, followed by

warm rolling at about 600°C to 1.1 mm. The final rolling

was undertaken at room temperature to the final thickness

of 0.3 mm. The Fe83Ga17 sheets, 12 mm©16 mm, were

enclosed in quartz ampoules with 0.3 atm argon protecting

gas. The heating rate at 0.25°C/min was employed from

900°C to 1080°C and followed by quenching without dwell at 1080°C. Final annealing process was undertaken at 1200°C for 6 h, and followed by furnace cooling to 800°C and then quenching to room temperature. The phase of sheets was examined using X-ray diffraction (XRD). The texture was analyzed using electron back-scattering diffraction

(EBSD). The EBSD analysis was carried out on a SUPRATM

55 field emission scanning electron microscope. The

magnetostriction was measured by strain gauge, and the gauges were positioned along the rolling direction. For the

magnetostriction measurement (­_//and­_¦), a magnetic field

parallel and perpendicular to the rolling direction was applied respectively.

3. Results and Discussion

In the columnar-grained specimens fabricated by

direc-tional solidification, some columnar grains are distributed

homogeneously with a width of³1000 µm which are nearly

parallel to each other, as shown in Fig. 2(a). The grain

morphology of the directionally solidified specimens is related to the direction of heat dissipation which is parallel to the drawing direction without transversal heat dissipation.

The grains with ©100ª crystallographic orientation grow

preferentially during directional solidification process, which

resulted in a strong©100ªfiber texture along axial direction of

rods, as shown in Fig. 2(b). Comparatively, the as-cast ingot was prepared by induction melting, and then was hot-forged to both reduce casting defects and improve the homoge-nization. It can be seen from Fig. 2(d) that many equiaxed

grains with a size of 100³600 µm are observed in the

hot-forged specimens, and those grains are without preferred orientation shown in Fig. 2(e). The directionally solidified and hot-forged alloys herein are called as columnar-grained and equiaxed-grained specimens, respectively. In order to determine the effect of oriented columnar grains on the

plasticity of Fe83Ga17alloy, tensile tests of columnar-grained

and equiaxed-grained specimens were carried out at 500°C. During the tests, the standard tensile specimens were heated

to 500°C at a heating rate of 5°C/min and held for 10 min

before tensile test. A maximum elongation of 30.3% is

observed for the columnar-grained specimen, while the

equiaxed-grained specimen has elongation of 7.5%, as shown

in Fig. 2(c) and (f ). The fracture morphology shown in the inset of Fig. 2(c) appears as dimple fracture for columnar-grained specimen, which suggests that a typical plastic fracture occurred in tensile testing. Although some slip bands indicative of plastic deformation are observed in equiaxed-grained specimen, the intergranular cleavage indicating a severe deterioration in ductility is mainly via fracture, as shown in the inset of Fig. 2(f ). These results indicate that columnar-grained specimens exhibit higher tensile ductility than that of the equiaxed-grained specimens. Grain boundary acts as potential source of cracks in Fe-Ga binary alloys which are prone to intergranular fracture. Directional solid-ification can decrease the density of transversal grain boundary, which could work as a method for improving the mechanical property of these alloys. As reported in previous

studies,19,20)the tensile strength of the equiaxed-grained

Fe-Ga (15­18 at%) alloy without preferred orientation was about

380³450 MPa at room temperature, and the fracture mode

was intergranular fracture. It is considered that the tensile strength depends on the grain boundary bonding strength. As

for ©001ª oriented single crystal Fe83Ga17 alloy, the tensile

Induction melting system Gating system

Thermal insulation system driven by resistance

heating Cooling system driven by flowing water

Drawing system

(a)

(b)

strength achieved 515 MPa at room temperature, and the

fracture mode was cleavage fracture.21)The breaking strength

of grains along ©001ª direction was larger than the grain

boundary bonding strength. In this work, the fracture mode

of Fe83Ga17 alloy with ©100ª oriented columnar grains is

transgranular fracture (cleavage fracture), because the size of columnar grains is very large, and the grain boundary is most parallel to the direction of tensile stress, which indicate

that the fracture behavior of the ©100ª oriented

columnar-grained sample is similar to that of single crystal samples. Comparatively, the intergranular cleavage indicates a severe deterioration in tensile strength of the equiaxed-grained sample. Therefore, the yield stress maybe higher in the columnar grained samples even though the grain size is

larger. On the other hand, A. E. Noltinget al.22)found that

the tensile strength was not decreased with increasing grain size in Galfenol steel and 4.6Al-Galfenol alloys, and they have pointed out that the change in mechanical properties was likely contributed to a complex strengthening mecha-nisms.

Fe83Ga17 binary alloy sheets with 0.3 mm thickness were

prepared by a rolling process, which was benefit from©100ª

oriented columnar grains initially arranged along rolling

direction. The textures evolution of©100ªoriented columnar

grains during rolling process were analyzed using EBSD, as shown in Fig. 3. All the three types of samples (hot-rolled, warm-rolled and cold-rolled) show severe through-thickness structure and texture gradients. The orientation has been indicated by a standard stereographic triangle. With the increase of the amount of deformation, the deformation of grains became more serious, leading to a large number of blind area (white area in Fig. 3(c)) in orientation map of

cold-rolled sheets. From inverse pole figure maps shown in

Fig. 3(a) and its inset, it can be seen that ©100ª orientation

was retained in hot rolled sheet from initial columnar grains. The texture of hot rolled sheets mainly featured strong

favorable {110}©001ª Goss texture and nearly {100}©001ª

Cubic texture, as shown in Fig. 3(d). It is believed that©100ª

orientation was retained from the initial Goss and Cubic oriented columnar grains without dynamic recrystallization

during hot rolling. Y. F. Gu et al.23) _{have pointed that}

dynamic recrystallization of directionally solidified alloy was mainly influenced by temperature and strain rate, especially the strain rate. It was found that dynamic recrystallization did

not take place in DS Ni3Al alloys deformed under a high

strain rate of 2.1©10¹2_S¹1 _{and a high temperature. In}

general, the shear texture is prone to form Goss texture after hot rolling in any initial orientation used. Goss orientation was formed not by recrystallization during and after hot rolling but by slip rotation near the surface due to constrained

deformation.24)Cubic texture was resulted from the heredity

of the initial cube orientation, which was consistent with the

results obtained by N. Tsuji et al. that {100}©001ª oriented

grains maintain their initial orientations even after 70%

rolling reduction.25)Figure 3(b) presents the grain orientation

in warm rolled sheet. There is an increase of the area of yellow and blue color, which indicates that after a further deformation at 600°C, the orientation of elongated grains

began to rotate from ©001ª to©011ª shown in the inset of

Fig. 3(b). The dominant texture of warm rolled sheet are

nearly{113}©210ª,{100}©001ªand{111}©112ª, as shown in

Fig. 3(e). From the Fig. 3(c) and its inset, it can be seen that yellow is dominant color, which indicates that the orientation has finished the rotation to ©011ª in the final cold rolled

sheet. The texture is mainly featured a strong {100}©011ª

component (rotate cubic texture) and a weak {111}©112ª

texture shown by white arrow (Fig. 3(f )). The rotation

around the direction of ©011ª was influenced by the initial

grain boundaries nearly parallel to the rolling direction due to the minimum grain boundary restriction. The initial coarse

columnar grains with ©100ªtexture prevented the formation

of a large quantity of small{111}grains. Deformation texture

of columnar-grained Fe-Ga alloy is significantly different

from£-fiber texture ({111}©uvwª) obtained easily by rolling

0 2 4 6 8 10

0 100 200 300

400 _{Equiaxed-grained specimens}

Stress (MPa)

Strain ε/%

0 5 10 15 20 25 30 35 0

100 200 300

400 Columnar-grained specimens

Stress (MPa)

Fig. 2 Microstructure, orientation and tension stress-strain curves of Fe83Ga17alloy, columnar-grained specimen: (a), (b), (c);

Non-oriented polycrystalline cubic metals, which would have different influence on evolution of recrystallization texture.

Phase transformation of Fe83Ga17 alloy sheets during heat

treatment was examined using X-ray diffraction, as shown in Fig. 4. It can be found that the phase is always body-centered cubic structure (bcc) (A2 phase) during heat treatment, which indicates that there is just recrystallization without phase transformation in annealing process. The change of relative diffraction intensity is mainly due to the texture evolution.

Figure 5 shows an orientation map and ODF of the final

annealed Fe83Ga17 alloy sheet. The results show that the

average grain size is about 800 µm, but the recrystallized microstructure is inhomogeneous, as shown in Fig. 5(a). The

red color area represents the grains with ©100ª orientations

along the rolling direction to within 15°. From the misorientation distribution curve shown in Fig. 5(b), it can be found that the majority misorientation is about at 7.5°, and

the area oriented in the rolling directions is 61.9%for fully

scanned area. The major texture component of the annealed

sheet shown in Fig. 5(c) is {110}©001ª Goss texture.

Generally, the texture of {100}©011ª (rotate cube texture)

is very stable, which is attributed to that the deformed

{100}©011ª grains cannot recrystallize. The primary

recrys-tallization is suppressed so that strong recovery prevails due to low deformation stored energy. On the other hand, it has

been reported that the {111}©112ª texture was formed after

cold rolling in Goss oriented Fe-Si single crystal alloy, but

Goss texture was obtained again after recrystallization.26)_The

possible explanation was given as follow. There were some micro zones with initial orientation in deformed grains, and these areas provided sites to nucleate for Goss oriented grains. The Goss grains have a high proportion of high-energy boundaries and thus have a larger critical size for growth than other orientations. Despite the fact that a high percentage of the Goss grains shrunk because they were smaller than their critical size during the initial phase of annealing, some Goss grains which survived in this initial

stage were selected to grow quickly and dominated thefinal

texture. Thus, it is considered that in as-rolled Fe83Ga17sheet,

Φ2=45° Φ2=45° Φ2=45°

RD

ND

RD RD RD

001

101 111

001

101 111

001

101 111

(a) (b) (c)

(d) (e) (f)

Fig. 3 Inverse polefigure (IPF) maps and ODFs of EBSD data displayed at¤2=45° section of Fe83Ga17alloy at different deformation

stages. (a) (d) hot rolling, (b) (e) warm rolling, (c) (f ) cold rolling. The insets are the corresponding IPFs.

30 40 50 60 70 80 90

Intensity (a.u.)

Diffraction angle 2θ/ °

after heating from 900°C to1080°C and quenching

Final annealed sheet at 1200°C

(211) (200)

(110)

bcc-Fe(Ga)

Fig. 4 XRD of the Fe83Ga17alloy sheets during the heat treatment.

10 20 30 (a)

(b) _(c)

2=45° ϕ

Fig. 5 Orientation map (a), its corresponding misorientation (b) and ODF of EBSD data displayed at¤2=45° section (c) of thefinal annealed

there could be some micro bands which provide sites for Goss grains to nucleate and grow. In other words, the strong

{110}©001ª Goss texture was obtained from the hereditary

characteristic of the initial columnar grains.

Magnetostriction of Fe83Ga17 alloy sheets varies with

texture evolution during prepare process, as shown in Fig. 6(a). It can be seen that there is an observed decrease in magnetostriction during rolling process. The magneto-strictive value decreased from 275 ppm in columnar-grained slabs to 36 ppm in cold-rolled sheet, which was mainly

ascribed to the deviation of orientation from©001ªdirection

and a lot of worsening deformed grains resulted by rolling

deformation. After final annealing, the magnetostriction of

Fe83Ga17alloy sheet has an increase up to185 ppm due to the

ideal Goss texture and the improvement in the crystal perfection after recrystallization. Figure 6(b) presents

mag-netostriction curves of columnar-grained slabs and final

annealed sheets. There is little hysteresis in the striction vs. H, resulting in an almost reversible magneto-striction.

4. Conclusions

In conclusion, sheets of binary Fe83Ga17 alloy were

prepared by a method with rolling of specimens with ©100ª

oriented columnar grains arranged initially along rolling

direction. The ductility of the binary Fe83Ga17 alloy with

©100ª oriented columnar grains fabricated by directional

solidification could be improved compared with that of the alloy with equiaxed grains. The orientation of columnar

grains gradually rotated to a ©110ª from the ©100ªdirection

during the rolling process. The rotation around the direction

of©110ªwas influenced by the initial grain boundaries nearly

parallel to the rolling direction. Strong {110}©001ª Goss

texture was confirmed in the final annealed sheets, which

were obtained from the hereditary characteristics of the initial Goss oriented columnar grains. A high magnetostriction

of 185 ppm without pre-stress was achieved in the final

recrystallized sheets due to the strong Goss texture observed.

Acknowledgments

This study was financially supported by the National

Basic Research Program of China (973 Program) (No.

2011CB606304), National Natural Science Foundation of China (No. 51271019, 51501006).

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0 50 100 150 200 250 300

{110}<001> {100}<011> {113}<210> {110}<001> <001> oriented

Annealed sheets Cold rolled

Warm rolled Hot rolled

Magnetostriction,

λ //-λ ⊥

(ppm)

Columnar-grained slabs 0 200 400 600 800 1000

-120 -80 -40 0 40 80 120 160 200

185ppm

λ_⊥ λ_//

Magnetostriction (ppm)

Magnetic field, H/Oe Columnar-grained slabs Final Annealed sheets

275ppm

(a) (b)