Large magnetostriction and structural characteristics of Fe83Ga17 wires

Physica B 407 (2012) 1186–1190

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Large magnetostriction and structural characteristics of Fe83Ga17 wires J.H. Li a, X.X. Gao b,n, J.X. Xie b,c, J. Zhu b, X.Q. Bao b, R.B. Yu a a b c

School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China Institute of Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 November 2011 Received in revised form 3 January 2012 Accepted 4 January 2012 Available online 10 January 2012

The columnar-grained structure induced by directional solidification was beneficial to improve the deformability of Fe83Ga17 alloy. Fe83Ga17 wires with diameter of 0.5  0.9 mm were prepared successfully by hot rotary swaging and warm drawing from the directional solidified rods. The magnetostriction and microstructure of the as-drawn and the annealed Fe83Ga17 wires with diameter of 0.6 mm were investigated. Results demonstrated that the magnetostriction of Fe83Ga17 wires depended on the microstructure and the fiber texture, which were controlled by heat treatment process. The maximum magnetostriction of 160 ppm was detected in the annealed wire, which has the ideal o 100 4 fiber texture. The phase mixture of A2 containing heterogeneous modified-DO3 phase has beneficial effect on magnetostriction. Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved.

Keywords: Fe-Ga alloy Wires Magnetostriction Modified-DO3 structure Texture

1. Introduction Fe-Ga alloys (Galfenol) have been received considerable interest due to potential applications for sensors and actuators [1–3]. Magnetically, the additions of Ga increase the magnetostrictive capability of Fe over tenfold up to 400 ppm [4]. Mechanically, Fe-Ga alloys have high strength above 500 MPa and robustness, which are not exhibited by other energy harvesting materials such as PZT, PMN, or Terfenol-D [5]. In addition, Fe-Ga alloys have high permeability (mr 4100) and Curie temperature (Tc 4650 1C) [6,7]. This combination of magnetic and mechanical properties makes FeGa alloy a unique material. Over the past several years, the research of Fe-Ga alloys focused on bulk materials, such as single crystal and oriented polycrystalline alloys [8–10]. Recently, the Fe-Ga thin sheets and wires have been drawn more attention [11,12]. The origin for the enhanced magnetostriction of Fe-Ga alloy is still an open question. Magnetostriction is generally an intrinsic more atomistic property, which doesn’t depend on the microstructure. But, there is growing evidence that this enhancement of Fe-Ga alloys is not an intrinsic property of a conventional homogeneous ferromagnetic phase [13–17]. Two possible origin of the huge increase magnetostriction have been offered. On one hand, it is ascribed to a lowering of the symmetry at Fe atoms with Ga near neighbors and thus a marked change in the local strain dependence of the magnetic anisotropy [14]. On the other

n

Corresponding author. Tel.:þ 86 01 62334343; fax: þ86 01 62334327. E-mail address: [email protected] (X.X. Gao).

hand, the Fe-Ga bcc solid solution near its solubility limit is a coarsening-resistant nanodispersion of a DO3 phase that is formed due to coherency lifting by excess vacancies, and this compositionally heterogeneous state undergoes a cubic-tetragonal displacive transformation [13,15]. In this model, tetragonal distortions present in the cubic lattice (A2) are easily deformed in a magnetic field due to their magnetic coupling to the matrix. The existence of tetragonal nanoclusters with o 1004 Ga-Ga pairs and a tenfold increase in the magnetostriction of the Fe-Ga atomic bond near a Ga-Ga environment was proved by measuring from DiffXAS on Fe81Ga19 alloy [17]. The magnetostrictive wires are materials used as waveguide wires in many technical fields. Tb-Dy-Fe alloys have magnetostriction on the order of 1000 ppm, but the intrinsic brittleness makes its use as a wire impossible. Therefore, it is meaningful to develop a new kind of large magnetostrictive wires. The Fe85Ga15 wires with 1.5 mm in diameter were prepared and the magnetostriction of 66 ppm was obtained in the work of Liu et al. [18]. Recently, it has been reported that the magnetostriction of 170 ppm was obtained in the Fe81Ga19 wires with diameter of 1.7 mm prepared using Taylor wire method [19]. The authors investigated the Wiedemann effect of Fe-Ga wires, and a large Wiedemann twist up to 24500 /cm was observed in the annealed Fe83Ga17 wires [12]. In present work, the Fe83Ga17 wires with diameter of 0.5  0.9 mm were prepared successfully by the directional solidification, hot rotary swaging and warm drawing. The effects of heat treatment on the microstructure and magnetostriction of Fe83Ga17 wires were investigated. A large magnetostriction of 160 ppm was obtained in the annealed Fe83Ga17

0921-4526/$ - see front matter Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2012.01.080

J.H. Li et al. / Physica B 407 (2012) 1186–1190

wires with 0.6 mm in diameter, and the structural origin was also discussed.

2. Experimental The cast alloy of Fe83Ga17 was prepared by vacuum induction melting from elements of Fe and Ga with 99.99 wt.% purity. Additional 2.0 wt.% Ga was added to make up for burning loss. The super high temperature gradient directional solidification device was employed to prepare the oriented rod samples with 12 mm in diameter. Subsequently, a reduction of diameter to 3.5 mm was achieved by hot rotary swaging at about 950 1C. Finally, the Fe83Ga17 wires with diameter of 0.50.9 mm were prepared by two time warm drawing at about 750 1C and 450 1C, respectively. The asdrawn wires with 0.6 mm in diameter were recrystallized at 1150 1C for 1 h and then quenched to room temperature. The final heat treatment was carried out at 650 900 1C at the interval of 50 1C for

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1 h and then quench cooling. In all case the impure purity argon was used as annealing atmosphere. Micrographs of the directional solidified rods, the as-drawn and annealed wires were observed with an optical microscope. The X-ray diffraction (XRD) patterns were taken to analyze the structure. The XRD was conducted on radial cross-section for the directional solidified rod samples and longitudinal section for the wire samples. X-ray diffraction examination was performed using the Cu Ka radiation in a Siemenss D5000 x-ray diffractometer. The fiber texture was analyzed using electron backscattering diffraction (EBSD). The EBSD analysis was carried out on a SUPRATM 55 filed emission scanning electron microscope equipped with automatic OIM (orientation imaging map) software from TSL. The longitudinal magnetostriction was measured using standard strain gauges, which were glued on a plane surface along axial direction of wire samples. The wires were not stressed during magnetostriction testing. The strain gage type is BX1200.5AA, and the size of sensing grid element is 0.5  0.5 mm.

Inde ensitty (a.u.)

110(A2)

As-directional solidified 211(A2)

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Diffraction angle 2θ (°)

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Magnetostriction λ (ppm)

Fig. 1. Optical micrograph of the directional solidified Fe83Ga17 rods (a), X-ray diffraction pattern of the as-cast and the directional solidified Fe83Ga17 rods (b).

120 100 80 60

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RT

650 C 700 C 750 C 800 C 850 C 900 C

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-20

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H (kAm-1) Fig. 2. Relationship between magnetostriction and heat treatment conditions (RT means recrystallization treatment at 1150 1C for 1 h; 650 1C means that the sample was annealed at 650 1C for 1 h after recrystallization at 1150 1C for 1 h, and then quenched to room temperature) (a); magnetostriction curve (b).

J.H. Li et al. / Physica B 407 (2012) 1186–1190

The Fe-Ga binary alloy was cracked and fractured along grain boundaries during hot deformation [20,21]. In order to improve the deformability of Fe-Ga alloy, the rod samples were prepared by the directional solidification. Because some previous works have indicated that improvements in tensile strength were obtained by changing the casting method from die cast to directional solidification in Al-Si alloys, and the ductility of Ni3Al alloys with columnar-grained structure was improved after directional solidification [22,23]. The microstructure of the directional solidified Fe83Ga17 rods shows many coarse columnar grains along the axial direction, as shown in Fig. 1(a). Compared to that of the as-cast sample, the higher relative intensity of (110) peak indicates a preference of (110) axis of the grains to be parallel to the axial direction for the directional solidified rods, as shown in Fig. 1(b). The intergranular brittle fracture was prone to occur in the as-cast Fe83Ga17 alloy in tensile testing at room temperature [24]. However, the single crystal Fe83Ga17 alloy with o1104 tensile axis orientation elongated at least 1.6%, and the ultimate tensile strength was up to 580 MPa [5]. Therefore, the microstructure characteristics consisting of the coarse columnar grains and the o1104 preferred orientation after directional solidification could provide a possible for Fe83Ga17 rods being made into wires by thermomechanical processing. The Fe83Ga17 wires with diameter of 0.5 0.9 mm were prepared by hot rotary swaging and warm drawing from the directional solidified rods. Ga concentration was measured by energy dispersion spectroscopy (EDS), and results showed that it approached the nominal composition. Fig. 2 shows the magnetostrictive values of the as-drawn and the annealed Fe83Ga17 wires with 0.6 mm in diameter. The measurement error was established using the method of averaging the values of three samples with the same heat treatment condition. Fig. 2(a) shows the curve of heat treatment condition dependence of magnetostriction. It can be seen that the magnetostrictive value of the asdrawn wire is only 28 ppm. The magnetostrictive value of 122 and 160 ppm is corresponding to the heat treatment condition of recrystallization at 1150 1C for 1 h and additional annealing at 800 1C for 1h after recrystallization, respectively. As shown in Fig. 2(b), there is little hysteresis in the magnetostriction against magnetic field, suggesting an almost reversible magnetostriction. It is well known that the magnetostriction of Fe-Ga alloy is sensitive to both the composition and thermal history [6,8,25,26]. Thermal process history controls the structural changes in the material. The influence of thermal history on magnetostriction thus requires an examination of the structural changes. Fig. 3 displays the optical micrographs of the as-drawn and the annealed Fe83Ga17 wires, and where the micrographs are taken

from the longitudinal section of the samples. A large number of fiber-like deformed grains are observed in the as-drawn wires, while coarse equiaxial grains appear in the annealed wires. The XRD patterns (Fig. 4(a)) demonstrate that the disordered A2 phase is the main phase in all samples, but some weak diffraction peaks are also observed in some of the annealed samples. As shown in Fig. 4(b), besides three main diffraction peaks from A2 phase, a weak diffraction peak at 30.361 and a small shoulder on the low

110

α−Fe(Ga) (A2)

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200 900°C

Indensity (a.u.)

3. Results and discussions

850°C 800°C 750°C 700°C 650°C

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Diffraction angle 2θ (°)

Indensity (a.u.)

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800°C

RT

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Diffraction angle 2θ (°) Fig. 4. XRD spectra of the Fe83Ga17 wires after different heat treatment (a); partially enlarged XRD spectrum (b).

Fig. 3. Optical micrographs of the as-drawn (a) and the same scale of the recrystallized wires (b).

J.H. Li et al. / Physica B 407 (2012) 1186–1190

angle side of (200) diffraction peak of A2 structure emerge in the recrystallizated wires. The angle for (200) superlattice reflection of ordered DO3 phase (Fe3Ga) is determined as 30.481. The peak shift of  0.121 and the small shoulder could be associated with the tetragonal deformation resulting from the Ga-Ga atom pairs along [001] direction in the modified-DO3 (or B2-like) structure. The modified-DO3 (or B2-like) structure, in which the Ga-Ga atom

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pairs located on the body-centered positions along [001] directions, could enhance magnetostriction in Fe-Ga alloys [8,27]. So the modified-DO3 phase within an A2 matrix could provide an important basis for the large magnetostriction of 122 ppm. The wire annealed at 8001C has the maximum magnetostriction of 160 ppm. The corresponding XRD pattern shows that a relative low intensity diffraction peak emerges at 59.601, as

Fig. 5. Inverse pole figures along drawing direction (DD) of the as-drawn wires (a) and the annealed wires, 700 1C (b), 750 1C (c), 800 1C (d), 850 1C (e).

Fig. 6. EBSD texture component map of the annealed wires at 800 1C for 1 h after recrystallization.

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J.H. Li et al. / Physica B 407 (2012) 1186–1190

shown in Fig. 4(b). A calculated power diffraction pattern for the modified-DO3 structure indicates that the diffraction peak of (321) is located at 59.601 [8], but the experimental evidence is lacking. The diffraction peak at 59.601 also maybe from impurity phases. Additional, a shoulder on the low angle side of (200) diffraction peak of A2 structure is observed. The identification of detailed structural characteristic needs a further study. The development of o1004 crystallographic texture is also critical in order to achieve maximum magnetostrictive strain due to the anisotropic nature of magnetostriction of Fe-Ga alloys. Fig. 5 shows the inverse pole figures along drawing direction of the wires. Texture shown in Fig. 5(a) exhibits a o1104 orientation along the axial direction of the as-drawn wires. It is noteworthy that a o100 4 fiber texture was obtained in the wires annealed at 800 1C after recrystallization, as shown in Fig. 5(d). In the corresponding texture map (Fig. 6), grains with o1004 orientations along axial direction to within 151 are highlighted in yellow, and the area oriented in the axis direction is 74.7% for fully scanned area. In other annealed samples, the texture (Fig. 5(b) (c) and (e)) deviated from the o100 4 direction in different degrees. In the case of the slip system of {110} o111 4 being considered only, the [111] slip direction itself is parallel to the maximum principal stress direction, which is transformed into [001] on recrystallization, since the (110) plane contains the maximum principal stress direction [111] and the minimum Young’s modulus direction [001], and the [110] rotation is taken for variant selection [28,29]. The maximum observed magnetostriction up to 160 ppm could be attributed to the ideal o1004 texture along the axial direction of wires. Compared to that of the bulk highly textured polycrystals of Fe83Ga17 alloy [30], the relative smaller magnetostriction of Fe83Ga17 wires could be attributed to that the microstructure is equiaxial grains in the annealed wires, while the bulk oriented alloys have columnar-grained structure, and the larger magnetostriction of bulk oriented alloys is achieved under compressive stress.

4. Conclusions The microstructure characteristics consisting of many coarse columnar grains and the o1104 orientation along axial direction after directional solidification was beneficial to improve the deformability of Fe83Ga17 rods. The Fe83Ga17 wires with diameter of 0.6 mm were prepared by hot rotary swaging and warm drawing from the directional solidified rods. The modified-DO3 phase within an A2 matrix could provide an important basis for the large magnetostriction of 122 ppm, which was obtained in the recrystallizatied wires at 1150 1C for 1 h. When the alloy was quenched from high temperatures, long-range ordering was suppressed; however, a tetragonal distortion of the A2 structure could occur. The maximum observed magnetostriction of 160 ppm was obtained in the wires annealed at 800 1C for 1 h.

Structural analysis indicated that they have the ideal o100 4 fiber texture along the axial direction of wires.

Acknowledgment This work was supported by a grant from the Major State Basic Research Development Program of China (973 Program) (no. 2011CB606304), China Postdoctoral Science Foundation funded project (no. 2011M500229) and Program for New Century Excellent Talents in University (no. NCET-09-02120).

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