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Ga-Al composite strip
Accepted Manuscript Effects of Annealing on Microstructure, Composition and Magnetic Properties of Rolled Fe/Ga-Al Composite Strip Yanwen Zheng, Zhihao Zhang, Yanbin Jiang PII: DOI: Reference:
S0304-8853(17)32884-6 https://doi.org/10.1016/j.jmmm.2017.12.091 MAGMA 63560
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
13 September 2017 9 November 2017 26 December 2017
Please cite this article as: Y. Zheng, Z. Zhang, Y. Jiang, Effects of Annealing on Microstructure, Composition and Magnetic Properties of Rolled Fe/Ga-Al Composite Strip, Journal of Magnetism and Magnetic Materials (2017), doi: https://doi.org/10.1016/j.jmmm.2017.12.091
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effects of Annealing on Microstructure, Composition and Magnetic Properties of Rolled Fe/Ga-Al Composite Strip Yanwen Zheng a，Zhihao Zhang a,b ,Yanbin Jiang a,b a Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China b Key Laboratory for Advanced Materials Processing(MOE), University of Science and Technology Beijing, Beijing 100083, China
Abstract：The Ga liquid and Al powder were mechanically mixed and poured into a hollow iron plate，after alloying, the composite plate was rolled at room temperature for preparing an Fe/Ga-Al composite strip. The effect of annealing conditions on the diffusion, microstructures and magnetostrictive properties of the strip were studied. The composite plate had good cold rolling formability. After annealing at 750~850 ℃ for 5 h of the cold-rolled sample with a reduction of 97%, the diffusion distance of Ga and Al in the Fe matrix increased with an increase of the annealing temperature. However, some holes appeared in the center of the sample annealed at a temperature of more than 830℃, which was detrimental to the subsequent rolling. The combination of the secondary cold rolling and annealing was beneficial to improve the composition homogeneity and magnetic properties of the sample. The magnetostriction coefficient (λ//) of the primary rolled sample was low, ~4×10-6. After annealing and secondary cold rolling, the λ// of the sample increased to 9×10-6 and the λ// of the sample conducted by further annealing at 820℃ for 20 h reached 27.5×10-6.
Key words：Fe-Ga-Al Alloy; Rolling compound; Diffusion annealing; Magnetostriction
* Corresponding author: Zhihao Zhang, Ph.D, Tel.: +86 010 62332253, Fax: +86 010 62332253, E-mail: [email protected]
1.Introduction Compared with the widely studied rare earth magnetostrictive material Tb-Dy-Fe (Terfornal-D), Fe-Ga alloys containing the Ga content of 19% and 27% have no obvious advantage in magnetostriction (the magnetostriction of single crystal Fe81Ga19 is about 1/4 of Terfornal-D), but they exhibit high strength, low saturation magnetic induction (about Terfornal-D 1/10) and high magnetic permeability (about 6~10 times as much as Terfornal-D), wide using temperature range, etc. The unique combination of magnetostrictive and mechanical properties in Fe-Ga alloys attracts a lot of attention for sensors and transducers applications [1-3]. Due to Al and Ga in the same main group, Al has similar atomic radius and valence electron structure to Ga, and also exists in Fe in the form of substitutional solid solution. [4]. In the Fe-Ga alloy, the substitution of a small amount of Al for Ga has little effect on the magnetic properties [5], such as the magnetostrictive properties of Fe80Ga15Al5 alloy and Fe80Ga20 alloy are basically same [6]. Since the price of Ga is much higher than that of Al, adding an appropriate amount of Al into Fe-Ga alloy not only can satisfy the requirement of magnetostrictive property, but also reduces its cost. At present, the preparation method of Fe-Ga alloy includes casting-rolling, directional solidification, powder metallurgy, the melt spinning rapid quenching etc [7]. The Fe-Ga alloy strip prepared by casting-rolling method has been applied in ultrasonic transducers. However, due to the great difference of melting point between Fe and Ga, the burning loss (the evaporation of Ga) of Ga is serious (usually more than 10%) in the process of casting, which increases the cost of the alloy and the difficulty to control chemical composition of the alloy. In addition, the Fe-Ga alloy prepared by the casting method has a low plasticity at room temperature. Although the addition of B, Y, Cr, V and Nb improved the plasticity of the alloy [8-10], the elongation only reached about 3.6% [11], which is much less than that required for the cold forming. This method needs multi-pass hot rolling or warm rolling [6], and multi-cutting off the edge crack results in a low yield. In this paper, a preparation method of FeGaAl alloy strip, which was cold-rolled and then subjected to diffusion annealing after mechanical mixing of Ga/Al into a hollow iron plate alloying process, was proposed, and the effect of annealing conditions on the composition, microstructure and magnetostriction properties of the composite strip
were studied.
2.Experimental Fe80Ga15Al5 alloy was prepared from pure Fe (99.9wt%), pure Ga (99.99wt%) and pure Al (99.9wt%). From the results of our previous experiments, Ga and Al losses were considered as 2wt% and 0.5wt%, respectively. In this work, the size of the iron plate was 10×50×60 mm and the size of the center hole was 5×40×50 mm. The Ga liquid and Al powder were mechanically mixed and poured into a hollow iron plate, and then the open end was sealed with 5×40×10mm iron block by welding and finally produced into a 10×50×60mm composite plate. The composite plate was annealed at 500℃ for 4h to achieve the alloying of Ga and Al. The composite plate was then subjected to multi-pass cold rolling at a reduction of 0.5 mm to obtain a thin strip with a thickness of 0.3 mm. A good surface quality and borderless thin strip was shown in Fig. 1(a). Fig.1(b) showed a cross section of the rolled sample. The core of the sample was a Ga-Al composite layer which was well bonded with the Fe matrix. Fig. 1(c) and Fig. 1(d) showed that Fe element existed in the Ga-Al composite layer because of the pre-alloying treatment, and the compositional distribution of Ga/ Al varies at different locations in the rolled sample. In order to homogenize the composition of the sample, the sample was further annealed. The annealing temperature was selected in the range of 750~850 ℃, and argon was used to avoid oxidation of the sample. The primary annealed sample with a thickness of 0.3 mm was subjected to secondary cold rolling to obtain a strip with a thickness of 0.2 mm, and the secondary cold-rolled strip was also annealed (Referred to as secondary annealing). The samples were polished and etched with a solution of 10% HNO 3 and 90% C2H5OH at room temperature. The microstructure of the sample was observed by metallographic microscope. The morphology of the sample was observed by scanning electron microscope (SEM), and the composition and the component distribution of the sample were measured by energy spectrum (EDS). The magnetostrictive coefficient of the sample was measured by JDAW-2011 type AC and DC magnetostrictive coefficient measuring instrument at room temperature and non-prestress through resistance strain gauge.
(a)
(b)
A1
A2
Fe
Ga-Al B1
B2
50μm
(d)
(c)
A1
Fe
B1
A2
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Fig. 1 Rolled strip, microstructure and composition distribution of the composite strip
3.Results and discussion 3.1 Effects of rolling and annealing conditions on the microstructure and composition of the sample Fig.2 showed the microstructure of the rolled composite strip annealed at different temperatures for 5 h. After annealing at 750 ℃ for 5 h. Recrystallization of the Fe matrix happened and coarse equiaxed grains formed. In addition, the diffusion layer of the sample extended equidistantly to the upper and lower surfaces, An obvious boundary between the Fe matrix and the diffusion layer formed. However, the color of the diffusion layer was also different due to the different diffusion rate between Ga and Al in the Fe matrix at 750℃. From the composition distribution diagram of the sample (Fig. 2 (b)), the diffusion layer of the sample was distinct from the composition of the Fe matrix and the composition of the diffusion layer was uneven. When annealing temperature was increased to 800 °C, the microstructure of the sample was similar to that of the sample annealed at 750°C, and the thickness of the diffusion layer was increased from 133μm to 190μm, as shown in Fig.2(c). In Fig.2(d),
the composition uniformity of the diffusion layer was greatly improved compared with that of the sample annealed at 750 °C. However, the Al content at the center of the diffusion layer was much larger than that in the other parts. Fig.2(e) showed the microstructure of the sample annealed at 820℃ for 5h. The thickness of the diffusion layer in the sample increased, and the Kirkendall voids in the diffusion layer increased. Fig.2(f) indicated that the distance of Ga and Al diffusion in Fe matrix increased, while the Al content in the center of diffusion layer decreased. Fig.2(g) showed that the microstructure of the sample annealed at 830℃ for 5h was similar to that of the sample annealed at 820℃, while the Kirkendall voids located at the diffusion layer increased, which had negative effect on the following cold rolling process. As shown in Fig. 2(h), the distribution of Ga and Al in the diffusion layer at 830℃ were more uniform than that at 820℃. The microstructure of the sample annealed at 850°C for 5 h was shown in Fig. 2(i). The thickness of the diffusion layer was further increased, and the diffusion layer of the sample was columnar crystal and the other parts were equiaxed crystals formed by recrystallization. However, the Kirkendall voids increased obviously in the center of the sample. With the increase of the annealing temperature, the diffusion coefficients of Ga and Al differed greatly, which led to more Kirkendall voids. Fig. 2 (j) showed the composition distribution of the samples after annealing at this temperature. It can be seen that with the increase of diffusion distance, the Ga content in the diffusion layer decreased and the uniformity of Al increased.
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A Fig.2 Rolled strips annealed at different temperatures for 5h: a) 750°C, c) 800°C, e) 820°C, g) 830℃, i) 850°C, b) d) f) h) j) composition distribution for 750°C,800°C,820°C,830°C,850°C
Fig.3 illustrated the diffusion layer thickness of the sample annealed at different temperatures for 5h. With the increase of annealing temperature, the thickness of diffusion layer increased gradually. For annealing at 850 ℃, the diffusion layer thickness reached 235μm, which was 4/5 of the cross section of the sample. However, Kirkendall holes in the central area of the sample were obvious, which were detrimental to the secondary cold rolling. However, some holes appeared in the center of the sample annealed at a temperature of more than 830℃, which was detrimental to the subsequent rolling. And the diffusion distance of the sample after annealing at 820℃ for 5h is closed to that of the sample annealed at 830℃, therefore, 820℃ was selected as the diffusion annealing temperature before the second cold rolling.
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Fig.3 The average diffusion distance of the cold-rolled sample at different temperatures for 5 h The samples were annealed at 820℃ for 5h, and then further cold rolled to 0.2mm (Secondary cold rolling). The samples were then secondary annealed at 820°C for 5~20 h in argon atmosphere. Fig.4(a) showed the SEM images of the secondary cold-rolled sample after annealing for 5h. Some fine microstructures appeared in the central region of the sample. Since the primary cold-rolled sample was annealed at 820°C for 5 h, FeGaAl compound formed in the central region of the sample. There are four kinds of stable compounds in Fe-Ga alloy, Fe3Ga, Fe6Ga5, Fe3Ga4 and FeGa3[9]. The formation of FeGaAl alloy by Al dissolving into Fe (Ga) increased the lattice distortion of the alloy, which increased the brittleness of the alloy. The FeGaAl compound broke easily during rolling because of its brittleness, and the fine microstructure formed in the broken compound during annealing. Fig.4(b) and (c) showed the SEM images of the secondary cold-rolled samples annealed at 820 ℃ for 10 h and 20 h. From Fig.4, the morphology of the fine microstructure in the central area didn’t change significantly with the increase of the annealing time. Fig.4(d) showed the Electron Back-Scattered Diffraction (EBSD) image of the secondary cold-rolled sample annealed at 820°C for 20 h. Fine grains formed in the center of the sample, and coarse grains formed at the edge of the sample without delamination.
(a)
(c)
Rolling direction
(b)
(d)
Fig. 4 The SEM images of the secondary cold rolled samples annealed at 820℃ for a)5 h, b)10 h and c)20 h, d) the EBSD image for the annealing time of 20 h
Fig.5 showed the composition of Ga and Al in the secondary cold-rolled sample after annealing at 820 °C for different times. For the annealing at 820℃ for 5h, Ga and Al content were 0 at the edge of the sample. At the region corresponding to a distance of 30 μm from the edge, the contents of Ga and Al in the Fe matrix began to increase. The content of Ga reached the maximum at 50 μm, while the content of Al continued to increase. At the distance of 90 μm, the content of Al reached the maximum and then began to decrease. In the distance range of 50 μm to 135 μm, the content of Ga was basically uniform. When the distance exceeded 135 μm, the contents of Ga and Al began to decrease, and were simultaneously reduced to 0 at 165μm. Fig.5 (b), (c) and (d) showed the Ga and Al composition distribution in the samples annealed at 820℃ for 10 h, 15 h and 20 h, respectively. With increasing the annealing time, diffusion of Ga and Al atoms in the Fe matrix proceeded. The Ga and Al content in the core of the diffusion layer reduced, and the component homogeneity in the secondary cold rolled sample gradually increased.
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Fig.5 The composition distribution of Ga and Al in the secondary cold-rolled samples annealed at 820 °C for different times: a)5h, b)10h, c)15h, d)20h
3.2 The effect of diffusion annealing on magnetostrictive properties of the samples The primary rolled strip consisted of a non-diffused Ga-Al composite layer and Fe, and its magnetostrictive properties was shown in Fig6. The magnetostriction λ// parallel to the rolling direction reached the saturation value of 4×10-6 when the magnetic field was 700Oe. When the magnetic field was perpendicular to the rolling direction, the magnetostriction λ⊥ didn’t reached the saturation value at 800Oe. The saturated magnetic field intensities of Fe81Ga13Al6 and Fe82Ga4.5Al13.5 are 150Oe and 300Oe, respectively. In the present work, the saturated magnetic field intensity of the Fe-Ga-Al alloy prepared by cold rolling and diffusion annealing was about 200Oe [12-13]. There was no saturated magnetostriction in the sample, which was greatly related to the anisotropy produced by rolling and the inhomogeneity of microstructure and composition. Because the microstructure and composition of the samples were
not uniform during the primary rolling, the magnetostriction coefficient of the sample was very low. The thickness of the secondary cold rolling sample was 0.2 mm. During the rolling, the recrystallized microstructure was elongated and the Ga-Al diffusion layer was broken, which had a significant effect on the magnetostrictive properties of the samples. Fig.6 showed the magnetostrictive curves of the secondary cold rolled sample annealed at 820 °C for 20 h. For the secondary rolled sample, the magnetostriction of λ// was 7×10-6, and the magnetostriction of λ⊥was -2×10-6, however, the magnetostriction coefficient of the sample didn’t reach the saturation value
at
800Oe. When the secondary rolling sample was annealed at 820 °C for 20 h, the λ // increased from 7×10-6 to 13.5×10-6, and λ⊥ decreased from -2×10-6 to -14×10-6, and the saturated magnetic field were 200Oe, which can be ascribe to that the uniformity of microstructure and composition of the sample was greatly improved with an
Magnetostrictive coefficient (×10-6)
increase of the annealing time. 15
1
10
2 5
λ// 0 -5
λ⊥
-10 -15
0
200
400
600
800
1000
Magnetic field strength(Oe) Fig.6 Magnetostrictive curves of the secondary cold-rolled samples (1- annealing for 20 h, 2- rolled state)
Fig.7 showed the changes in the magnetostriction coefficient of the sample annealed at different times (5h，10h，20h), and the performance data of the relevant studies were also given in Fig.8, where (3/2) λRD was the value of λ//-λ⊥, (3/2)λRD was the
magnetostriction coefficient of the sample along the rolling direction. λ // and λ⊥
was the maximum strain when magnetic field parallel and perpendicular to the RD were applied, respectively. Fig.7 showed that the magnetostriction coefficient of the sample was significantly improved with an increase of the annealing time. The (3/2)
λRD of the sample annealed for 20h was as high as 27.5×10-6, due to the great improvement of microstructure homogeneity and increase of the FeGaAl solid solution with an increase of the annealing time. After annealing for 20 h, the (3/2) λRD was 27.5×10-6, however, the magnetostriction was much lower than that of the binary Fe-Ga alloy. Because the solid solution ability of Al and Ga was stronger than that of Fe and Al, Al preferred to entering Ga-Ga clusters, which had a negative effect on magnetostriction of Fe-Ga alloy Cao et al. [14] prepared the Fe81Ga19B2 alloy by the spin-quenching, the magnetostriction coefficient of the Fe81Ga19B2 alloy was 29.6×10-6. Taheri P et al. [15] prepared Fe81Ga19 alloy by powder metallurgy, the magnetostriction coefficient of the Fe81Ga19 alloy was 14×10-6. Ding et al. [4] prepared the Fe83Ga15Al2 by directional solidification, the magnetostriction coefficient of the Fe83Ga15Al2 was 8×10-6. In the present work, the magnetostrictive properties of the FeGaAl alloy prepared by the method of composite rolling and diffusion annealing were approximately equal to or slightly higher than those of the above methods. The method had some advantages in yield, preparation efficiency and control of element burning loss.
(3/2) λRD,10-6
[14]
Fe81Ga19B2 Cast
[15]
Fe81Ga19 Sintering state Fe83Ga15Al2 Cast
[4]
Diffusion annealing time(h) Fig.7 The maximal magnetostriction coefficient of the secondary cold-rolled samples annealed at 820°C for the different times
4.conclusions (1) The results showed that the composite plate had good cold rolling formability. After annealing at 750~850 ℃ for 5 h of the cold-rolled sample with a reduction of 97%, the diffusion distance of Ga and Al in the Fe matrix increased with an increase of the annealing temperature. However, some holes appeared in the center of the sample annealed at a temperature of more than 830℃, which was detrimental to the subsequent rolling. (2) The sample annealed at 820 °C for 5 h was secondary cold rolled to 0.2 mm in thick and then subjected to the secondary annealing at 820 °C. With the increase of annealing time, the composition uniformity of the samples increased gradually, and no obvious holes appeared in the central area of the sample. (3) The combination of the secondary cold rolling and annealing were beneficial to improve the composition homogeneity and magnetic properties of the sample. The magnetostriction coefficient (λ//) of the rolled sample was low, ~4×10-6. The λ// of the cold rolled sample after annealing and secondary cold rolling increased to 9×10-6，and the λ// of the sample annealed at 820 ℃ for 20 h reached 27.5×10 -6, which was ascribe to the improvement of the composition homogeneity and the formation of more FeGaAl solid solution in the sample by annealing.
Acknowledgments This work was supported by The National Key Research and Development Program of China under contract number 2011CB0606300.
5. References [1] A. E. Clark, J.B. Restorff, Wun-Fogle, M, K.B. Hathaway. Magnetostriction of ternary Fe –Ga–X, (X= C, V, Cr, Mn, Co, Rh), alloys[J]. J. Appl. Phys. 101 (2007) 09C507-09C507-3.
[2] B. W. Wang, S. Y. Li, Y. Zhou, W. M. Huang, S. Y. Cao. Structure, magnetic properties and magnetostriction of Fe81Ga19 thin films[J]. J. Magn. Magn. Mater. 320 (2008) 769-773.
[3] T. A. Lograsso, E. M. Summers. Detection and quantification of D03, chemical order in Fe–Ga alloys using high resolution X-ray diffraction[J]. Mater. Sci. Eng., A. (416) 2006 240-245.
[4] Y. T. Ding, C. G. Zhou, Y. Hu. Structure and magnetostriction performance of directionally solidified Fe83Ga15Al2 [J]. Journal of Lanzhou University of Technology. 39 (2013) 1-5.
[5] Y. Zhou, B. W. Wang, S. Y. Li, Z.H. Wang, W. M. Huang, S. Y. Cao, W.P. Huang. The magnetostriction of Fe– (18− x) at% Ga– x at% Al (3≤ x ≤13.5) alloys[J]. J. Magn. Magn. Mater. 322(2010) 2104-2107.
[6] N. Srisukhumbowornchai, S. Guruswamy. Large magnetostriction in directionally solidified FeGa and FeGaAl alloys[J]. J. Appl. Phys. 90 (2001) 5680-5688.
[7] L. P. Jiang, L. Guo, H.B. Hao, G.R. Zhang. Application and research progress of Fe-Ga alloys [J]. Met. Funct. Mater. 21 (2014) 32-36.
[8] S.M. Na, A.B. Flatau. Magnetostriction and surface-energy-induced selective grain growth in rolled Galfenol doped with sulfur[J]. Proceedings of SPIE - The International Society for Optical Engineering. (2005) 192-199.
[9] S.M. Na, A.B. Flatau. Deformation behavior and magnetostriction of polycrystalline Fe–Ga–X (X= B,C,Mn,Mo,Nb,NbC) alloys[J]. J. Appl. Phys. 103 (2008) 8621.
[10] J. H. Li, X.X. Gao, J. Zhu. Texture and magnetostriction in rolled Fe-Ga alloy[J]. Acta Metall. Sin. 44 (2008) 1031-1034.
[11] J.H. Li, X.X. Gao, J. Zhu. Effect of boron and chromium on magnetostriction and mechanical properties of polycrystalline Fe83Ga17 alloy [J]. J. Univ. Sci. Technol. B. 31 (2009) 1281-1285.
[12] S.U. Jen, W.C. Cheng, Y.C. Lin, Y.Z. Chen, IS Golovin. Magnetic and magneto-mechanical properties of Fe55Co19Ga26, alloy[J]. Mater. Lett. 182 (2016) 72-74.
[13] Y. Liu, J.H. Li, X.X. Gao. Influence of intermediate annealing on abnormal Goss grain growth in the rolled columnar-grained Fe-Ga-Al alloys[J]. J. Magn. Magn. Mater. 435 (2017) 194-200.
[14] M. Cao, H. Liu, H. Wang. Effects of B-and In- doping on microstructure, magnetostriction and magnetic properties of melt-spun Fe81Ga19 ribbons [J]. Acta Phys.-Chim. Sin, 32 (2016) 1829-1838.
[15] P. Taheri, R. Barua, J. Hsu. Structure, magnetism, and magnetostrictive properties of mechanically alloyed Fe81Ga19[J]. J. Alloy. Compd. 661 (2016) 306-311.
Highlights
A new method for preparing FeGaAl magnetostrictive material was proposed.
Evolutions of microstructure and composition during preparing was determined.
The magnetostrictive properties equal or exceed the ones by other methods.
S0304-8853(17)32884-6 https://doi.org/10.1016/j.jmmm.2017.12.091 MAGMA 63560
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
13 September 2017 9 November 2017 26 December 2017
Please cite this article as: Y. Zheng, Z. Zhang, Y. Jiang, Effects of Annealing on Microstructure, Composition and Magnetic Properties of Rolled Fe/Ga-Al Composite Strip, Journal of Magnetism and Magnetic Materials (2017), doi: https://doi.org/10.1016/j.jmmm.2017.12.091
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effects of Annealing on Microstructure, Composition and Magnetic Properties of Rolled Fe/Ga-Al Composite Strip Yanwen Zheng a，Zhihao Zhang a,b ,Yanbin Jiang a,b a Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China b Key Laboratory for Advanced Materials Processing(MOE), University of Science and Technology Beijing, Beijing 100083, China
Abstract：The Ga liquid and Al powder were mechanically mixed and poured into a hollow iron plate，after alloying, the composite plate was rolled at room temperature for preparing an Fe/Ga-Al composite strip. The effect of annealing conditions on the diffusion, microstructures and magnetostrictive properties of the strip were studied. The composite plate had good cold rolling formability. After annealing at 750~850 ℃ for 5 h of the cold-rolled sample with a reduction of 97%, the diffusion distance of Ga and Al in the Fe matrix increased with an increase of the annealing temperature. However, some holes appeared in the center of the sample annealed at a temperature of more than 830℃, which was detrimental to the subsequent rolling. The combination of the secondary cold rolling and annealing was beneficial to improve the composition homogeneity and magnetic properties of the sample. The magnetostriction coefficient (λ//) of the primary rolled sample was low, ~4×10-6. After annealing and secondary cold rolling, the λ// of the sample increased to 9×10-6 and the λ// of the sample conducted by further annealing at 820℃ for 20 h reached 27.5×10-6.
Key words：Fe-Ga-Al Alloy; Rolling compound; Diffusion annealing; Magnetostriction
* Corresponding author: Zhihao Zhang, Ph.D, Tel.: +86 010 62332253, Fax: +86 010 62332253, E-mail: [email protected]
1.Introduction Compared with the widely studied rare earth magnetostrictive material Tb-Dy-Fe (Terfornal-D), Fe-Ga alloys containing the Ga content of 19% and 27% have no obvious advantage in magnetostriction (the magnetostriction of single crystal Fe81Ga19 is about 1/4 of Terfornal-D), but they exhibit high strength, low saturation magnetic induction (about Terfornal-D 1/10) and high magnetic permeability (about 6~10 times as much as Terfornal-D), wide using temperature range, etc. The unique combination of magnetostrictive and mechanical properties in Fe-Ga alloys attracts a lot of attention for sensors and transducers applications [1-3]. Due to Al and Ga in the same main group, Al has similar atomic radius and valence electron structure to Ga, and also exists in Fe in the form of substitutional solid solution. [4]. In the Fe-Ga alloy, the substitution of a small amount of Al for Ga has little effect on the magnetic properties [5], such as the magnetostrictive properties of Fe80Ga15Al5 alloy and Fe80Ga20 alloy are basically same [6]. Since the price of Ga is much higher than that of Al, adding an appropriate amount of Al into Fe-Ga alloy not only can satisfy the requirement of magnetostrictive property, but also reduces its cost. At present, the preparation method of Fe-Ga alloy includes casting-rolling, directional solidification, powder metallurgy, the melt spinning rapid quenching etc [7]. The Fe-Ga alloy strip prepared by casting-rolling method has been applied in ultrasonic transducers. However, due to the great difference of melting point between Fe and Ga, the burning loss (the evaporation of Ga) of Ga is serious (usually more than 10%) in the process of casting, which increases the cost of the alloy and the difficulty to control chemical composition of the alloy. In addition, the Fe-Ga alloy prepared by the casting method has a low plasticity at room temperature. Although the addition of B, Y, Cr, V and Nb improved the plasticity of the alloy [8-10], the elongation only reached about 3.6% [11], which is much less than that required for the cold forming. This method needs multi-pass hot rolling or warm rolling [6], and multi-cutting off the edge crack results in a low yield. In this paper, a preparation method of FeGaAl alloy strip, which was cold-rolled and then subjected to diffusion annealing after mechanical mixing of Ga/Al into a hollow iron plate alloying process, was proposed, and the effect of annealing conditions on the composition, microstructure and magnetostriction properties of the composite strip
were studied.
2.Experimental Fe80Ga15Al5 alloy was prepared from pure Fe (99.9wt%), pure Ga (99.99wt%) and pure Al (99.9wt%). From the results of our previous experiments, Ga and Al losses were considered as 2wt% and 0.5wt%, respectively. In this work, the size of the iron plate was 10×50×60 mm and the size of the center hole was 5×40×50 mm. The Ga liquid and Al powder were mechanically mixed and poured into a hollow iron plate, and then the open end was sealed with 5×40×10mm iron block by welding and finally produced into a 10×50×60mm composite plate. The composite plate was annealed at 500℃ for 4h to achieve the alloying of Ga and Al. The composite plate was then subjected to multi-pass cold rolling at a reduction of 0.5 mm to obtain a thin strip with a thickness of 0.3 mm. A good surface quality and borderless thin strip was shown in Fig. 1(a). Fig.1(b) showed a cross section of the rolled sample. The core of the sample was a Ga-Al composite layer which was well bonded with the Fe matrix. Fig. 1(c) and Fig. 1(d) showed that Fe element existed in the Ga-Al composite layer because of the pre-alloying treatment, and the compositional distribution of Ga/ Al varies at different locations in the rolled sample. In order to homogenize the composition of the sample, the sample was further annealed. The annealing temperature was selected in the range of 750~850 ℃, and argon was used to avoid oxidation of the sample. The primary annealed sample with a thickness of 0.3 mm was subjected to secondary cold rolling to obtain a strip with a thickness of 0.2 mm, and the secondary cold-rolled strip was also annealed (Referred to as secondary annealing). The samples were polished and etched with a solution of 10% HNO 3 and 90% C2H5OH at room temperature. The microstructure of the sample was observed by metallographic microscope. The morphology of the sample was observed by scanning electron microscope (SEM), and the composition and the component distribution of the sample were measured by energy spectrum (EDS). The magnetostrictive coefficient of the sample was measured by JDAW-2011 type AC and DC magnetostrictive coefficient measuring instrument at room temperature and non-prestress through resistance strain gauge.
(a)
(b)
A1
A2
Fe
Ga-Al B1
B2
50μm
(d)
(c)
A1
Fe
B1
A2
B2
Fig. 1 Rolled strip, microstructure and composition distribution of the composite strip
3.Results and discussion 3.1 Effects of rolling and annealing conditions on the microstructure and composition of the sample Fig.2 showed the microstructure of the rolled composite strip annealed at different temperatures for 5 h. After annealing at 750 ℃ for 5 h. Recrystallization of the Fe matrix happened and coarse equiaxed grains formed. In addition, the diffusion layer of the sample extended equidistantly to the upper and lower surfaces, An obvious boundary between the Fe matrix and the diffusion layer formed. However, the color of the diffusion layer was also different due to the different diffusion rate between Ga and Al in the Fe matrix at 750℃. From the composition distribution diagram of the sample (Fig. 2 (b)), the diffusion layer of the sample was distinct from the composition of the Fe matrix and the composition of the diffusion layer was uneven. When annealing temperature was increased to 800 °C, the microstructure of the sample was similar to that of the sample annealed at 750°C, and the thickness of the diffusion layer was increased from 133μm to 190μm, as shown in Fig.2(c). In Fig.2(d),
the composition uniformity of the diffusion layer was greatly improved compared with that of the sample annealed at 750 °C. However, the Al content at the center of the diffusion layer was much larger than that in the other parts. Fig.2(e) showed the microstructure of the sample annealed at 820℃ for 5h. The thickness of the diffusion layer in the sample increased, and the Kirkendall voids in the diffusion layer increased. Fig.2(f) indicated that the distance of Ga and Al diffusion in Fe matrix increased, while the Al content in the center of diffusion layer decreased. Fig.2(g) showed that the microstructure of the sample annealed at 830℃ for 5h was similar to that of the sample annealed at 820℃, while the Kirkendall voids located at the diffusion layer increased, which had negative effect on the following cold rolling process. As shown in Fig. 2(h), the distribution of Ga and Al in the diffusion layer at 830℃ were more uniform than that at 820℃. The microstructure of the sample annealed at 850°C for 5 h was shown in Fig. 2(i). The thickness of the diffusion layer was further increased, and the diffusion layer of the sample was columnar crystal and the other parts were equiaxed crystals formed by recrystallization. However, the Kirkendall voids increased obviously in the center of the sample. With the increase of the annealing temperature, the diffusion coefficients of Ga and Al differed greatly, which led to more Kirkendall voids. Fig. 2 (j) showed the composition distribution of the samples after annealing at this temperature. It can be seen that with the increase of diffusion distance, the Ga content in the diffusion layer decreased and the uniformity of Al increased.
(a)
A
(b)
Fe
Diffusion layer Ga
B
(c)
A
Al
(d)
Fe Al
B
Ga
(e)
A
(f)
Fe Al
B
(g)
Ga
A
(h)
Fe Al
B
(i)
Ga
A
(j)
Fe
Al
B
Ga
A Fig.2 Rolled strips annealed at different temperatures for 5h: a) 750°C, c) 800°C, e) 820°C, g) 830℃, i) 850°C, b) d) f) h) j) composition distribution for 750°C,800°C,820°C,830°C,850°C
Fig.3 illustrated the diffusion layer thickness of the sample annealed at different temperatures for 5h. With the increase of annealing temperature, the thickness of diffusion layer increased gradually. For annealing at 850 ℃, the diffusion layer thickness reached 235μm, which was 4/5 of the cross section of the sample. However, Kirkendall holes in the central area of the sample were obvious, which were detrimental to the secondary cold rolling. However, some holes appeared in the center of the sample annealed at a temperature of more than 830℃, which was detrimental to the subsequent rolling. And the diffusion distance of the sample after annealing at 820℃ for 5h is closed to that of the sample annealed at 830℃, therefore, 820℃ was selected as the diffusion annealing temperature before the second cold rolling.
B
270
Diffusion distance/μm
240 210 180 150 120
Cold rolling
90 60 725
750
775
800
825
850
A
temperature/℃
Fig.3 The average diffusion distance of the cold-rolled sample at different temperatures for 5 h The samples were annealed at 820℃ for 5h, and then further cold rolled to 0.2mm (Secondary cold rolling). The samples were then secondary annealed at 820°C for 5~20 h in argon atmosphere. Fig.4(a) showed the SEM images of the secondary cold-rolled sample after annealing for 5h. Some fine microstructures appeared in the central region of the sample. Since the primary cold-rolled sample was annealed at 820°C for 5 h, FeGaAl compound formed in the central region of the sample. There are four kinds of stable compounds in Fe-Ga alloy, Fe3Ga, Fe6Ga5, Fe3Ga4 and FeGa3[9]. The formation of FeGaAl alloy by Al dissolving into Fe (Ga) increased the lattice distortion of the alloy, which increased the brittleness of the alloy. The FeGaAl compound broke easily during rolling because of its brittleness, and the fine microstructure formed in the broken compound during annealing. Fig.4(b) and (c) showed the SEM images of the secondary cold-rolled samples annealed at 820 ℃ for 10 h and 20 h. From Fig.4, the morphology of the fine microstructure in the central area didn’t change significantly with the increase of the annealing time. Fig.4(d) showed the Electron Back-Scattered Diffraction (EBSD) image of the secondary cold-rolled sample annealed at 820°C for 20 h. Fine grains formed in the center of the sample, and coarse grains formed at the edge of the sample without delamination.
(a)
(c)
Rolling direction
(b)
(d)
Fig. 4 The SEM images of the secondary cold rolled samples annealed at 820℃ for a)5 h, b)10 h and c)20 h, d) the EBSD image for the annealing time of 20 h
Fig.5 showed the composition of Ga and Al in the secondary cold-rolled sample after annealing at 820 °C for different times. For the annealing at 820℃ for 5h, Ga and Al content were 0 at the edge of the sample. At the region corresponding to a distance of 30 μm from the edge, the contents of Ga and Al in the Fe matrix began to increase. The content of Ga reached the maximum at 50 μm, while the content of Al continued to increase. At the distance of 90 μm, the content of Al reached the maximum and then began to decrease. In the distance range of 50 μm to 135 μm, the content of Ga was basically uniform. When the distance exceeded 135 μm, the contents of Ga and Al began to decrease, and were simultaneously reduced to 0 at 165μm. Fig.5 (b), (c) and (d) showed the Ga and Al composition distribution in the samples annealed at 820℃ for 10 h, 15 h and 20 h, respectively. With increasing the annealing time, diffusion of Ga and Al atoms in the Fe matrix proceeded. The Ga and Al content in the core of the diffusion layer reduced, and the component homogeneity in the secondary cold rolled sample gradually increased.
15
15
（a）
Al Ga
12
content (at.%)
content (at.%)
12 9 6 3 0
The sample center 0
50
100
9 6 3 0
150
200
The sample center 0
50
Distance(μm)
100
150
200
Distance(μm)
15
15 Al Ga
（c） 12
（d）
Al Ga
12
content (at.%)
content (at.%)
Al Ga
（b）
9 6 3
The sample center
0 0
50
100
9 6 3 0
150
200
The sample center 0
Distance(μm)
50
100
150
200
Distance(μm)
Fig.5 The composition distribution of Ga and Al in the secondary cold-rolled samples annealed at 820 °C for different times: a)5h, b)10h, c)15h, d)20h
3.2 The effect of diffusion annealing on magnetostrictive properties of the samples The primary rolled strip consisted of a non-diffused Ga-Al composite layer and Fe, and its magnetostrictive properties was shown in Fig6. The magnetostriction λ// parallel to the rolling direction reached the saturation value of 4×10-6 when the magnetic field was 700Oe. When the magnetic field was perpendicular to the rolling direction, the magnetostriction λ⊥ didn’t reached the saturation value at 800Oe. The saturated magnetic field intensities of Fe81Ga13Al6 and Fe82Ga4.5Al13.5 are 150Oe and 300Oe, respectively. In the present work, the saturated magnetic field intensity of the Fe-Ga-Al alloy prepared by cold rolling and diffusion annealing was about 200Oe [12-13]. There was no saturated magnetostriction in the sample, which was greatly related to the anisotropy produced by rolling and the inhomogeneity of microstructure and composition. Because the microstructure and composition of the samples were
not uniform during the primary rolling, the magnetostriction coefficient of the sample was very low. The thickness of the secondary cold rolling sample was 0.2 mm. During the rolling, the recrystallized microstructure was elongated and the Ga-Al diffusion layer was broken, which had a significant effect on the magnetostrictive properties of the samples. Fig.6 showed the magnetostrictive curves of the secondary cold rolled sample annealed at 820 °C for 20 h. For the secondary rolled sample, the magnetostriction of λ// was 7×10-6, and the magnetostriction of λ⊥was -2×10-6, however, the magnetostriction coefficient of the sample didn’t reach the saturation value
at
800Oe. When the secondary rolling sample was annealed at 820 °C for 20 h, the λ // increased from 7×10-6 to 13.5×10-6, and λ⊥ decreased from -2×10-6 to -14×10-6, and the saturated magnetic field were 200Oe, which can be ascribe to that the uniformity of microstructure and composition of the sample was greatly improved with an
Magnetostrictive coefficient (×10-6)
increase of the annealing time. 15
1
10
2 5
λ// 0 -5
λ⊥
-10 -15
0
200
400
600
800
1000
Magnetic field strength(Oe) Fig.6 Magnetostrictive curves of the secondary cold-rolled samples (1- annealing for 20 h, 2- rolled state)
Fig.7 showed the changes in the magnetostriction coefficient of the sample annealed at different times (5h，10h，20h), and the performance data of the relevant studies were also given in Fig.8, where (3/2) λRD was the value of λ//-λ⊥, (3/2)λRD was the
magnetostriction coefficient of the sample along the rolling direction. λ // and λ⊥
was the maximum strain when magnetic field parallel and perpendicular to the RD were applied, respectively. Fig.7 showed that the magnetostriction coefficient of the sample was significantly improved with an increase of the annealing time. The (3/2)
λRD of the sample annealed for 20h was as high as 27.5×10-6, due to the great improvement of microstructure homogeneity and increase of the FeGaAl solid solution with an increase of the annealing time. After annealing for 20 h, the (3/2) λRD was 27.5×10-6, however, the magnetostriction was much lower than that of the binary Fe-Ga alloy. Because the solid solution ability of Al and Ga was stronger than that of Fe and Al, Al preferred to entering Ga-Ga clusters, which had a negative effect on magnetostriction of Fe-Ga alloy Cao et al. [14] prepared the Fe81Ga19B2 alloy by the spin-quenching, the magnetostriction coefficient of the Fe81Ga19B2 alloy was 29.6×10-6. Taheri P et al. [15] prepared Fe81Ga19 alloy by powder metallurgy, the magnetostriction coefficient of the Fe81Ga19 alloy was 14×10-6. Ding et al. [4] prepared the Fe83Ga15Al2 by directional solidification, the magnetostriction coefficient of the Fe83Ga15Al2 was 8×10-6. In the present work, the magnetostrictive properties of the FeGaAl alloy prepared by the method of composite rolling and diffusion annealing were approximately equal to or slightly higher than those of the above methods. The method had some advantages in yield, preparation efficiency and control of element burning loss.
(3/2) λRD,10-6
[14]
Fe81Ga19B2 Cast
[15]
Fe81Ga19 Sintering state Fe83Ga15Al2 Cast
[4]
Diffusion annealing time(h) Fig.7 The maximal magnetostriction coefficient of the secondary cold-rolled samples annealed at 820°C for the different times
4.conclusions (1) The results showed that the composite plate had good cold rolling formability. After annealing at 750~850 ℃ for 5 h of the cold-rolled sample with a reduction of 97%, the diffusion distance of Ga and Al in the Fe matrix increased with an increase of the annealing temperature. However, some holes appeared in the center of the sample annealed at a temperature of more than 830℃, which was detrimental to the subsequent rolling. (2) The sample annealed at 820 °C for 5 h was secondary cold rolled to 0.2 mm in thick and then subjected to the secondary annealing at 820 °C. With the increase of annealing time, the composition uniformity of the samples increased gradually, and no obvious holes appeared in the central area of the sample. (3) The combination of the secondary cold rolling and annealing were beneficial to improve the composition homogeneity and magnetic properties of the sample. The magnetostriction coefficient (λ//) of the rolled sample was low, ~4×10-6. The λ// of the cold rolled sample after annealing and secondary cold rolling increased to 9×10-6，and the λ// of the sample annealed at 820 ℃ for 20 h reached 27.5×10 -6, which was ascribe to the improvement of the composition homogeneity and the formation of more FeGaAl solid solution in the sample by annealing.
Acknowledgments This work was supported by The National Key Research and Development Program of China under contract number 2011CB0606300.
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Highlights
A new method for preparing FeGaAl magnetostrictive material was proposed.
Evolutions of microstructure and composition during preparing was determined.
The magnetostrictive properties equal or exceed the ones by other methods.