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Effect of yttrium on the mechanical and magnetostrictive properties of Fe83Ga17 alloy
JOURNAL OF RARE EARTHS, Vol. 33, No. 10, Oct. 2015, P. 1087
Effect of yttrium on the mechanical and magnetostrictive properties of Fe83Ga17 alloy LI Jiheng (李纪恒), XIAO Ximing (肖锡铭), YUAN Chao (袁 超), GAO Xuexu (高学绪)*, BAO Xiaoqian (包小倩) (State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China) Received 6 June 2015; revised 30 June 2015
Abstract: Polycrystalline rod samples of (Fe83Ga17)100–xYx (x=0, 0.16, 0.32, 0.48, 0.64) were prepared by induction melting under argon atmosphere. Effect of yttrium on the mechanical and magnetostrictive properties of Fe83Ga17 alloy was investigated. Small amount of yttrium (0.16 at.%) increased the tensile strength of as-cast Fe83Ga17 alloys to 674 MPa and improved the ductility with elongation of 4.2% at room temperature. The Y2Fe17−xGax (6≤x≤7) phase was formed in the Y-doped Fe83Ga17 alloy since yttrium was hardly dissolved into the α-Fe lattice. Y2(FeGa)17 secondary phase dispersed along the grain boundaries and inside the grains played an important role for the enhancement of mechanical property. The 0.64 at.% Y-doped alloy had magnetostriction of 133 ppm, which was thought to be associated with the alteration of the grain shape and <100> preferential orientation along the axial direction of rods. Keywords: yttrium; Fe-Ga alloy; magnetostriction; mechanical property; rare earths
In recent years, there has been an ever-increasing interest in the Fe-Ga (Galfenol) alloy as a new magnetostrictive material for actuator, sensor, and energy harvesting applications. The saturation magnetostriction can be as high as 400 ppm in single crystals[1]. The relatively low magnetostriction values for Fe-Ga alloys compared to giant magnetostrictive materials such as Terfenol-D alloy[2] (λs~2000 ppm) would beg the question “Why use Fe-Ga alloy?”. The answer to this question comes when we begin to understand Fe-Ga alloy’s mechanical properties. Mechanically, Fe-Ga alloys have high strength above 500 MPa and robustness[3], which is not exhibited by other energy harvesting materials such as PZT, PMN, or Terfenol-D[4]. In addition, Fe-Ga alloys have high permeability (µr>100) and Curie temperature (Tc>650 ºC)[5]. This combination of magnetic and mechanical properties makes Fe-Ga alloy aunique material. However, the polycrystalline Fe81.6Ga18.4 alloy exhibited tensile strengths only 370 MPa with elongation of 0.8% before failure at room temperature[6], and the studies of Na et al.[7] and Cheng et al.[8] indicated that the polycrystalline Fe-Ga binary alloy cracked and fractured along grain boundaries during hot rolling and was too brittle to make thin sheets which can avoid eddy current losses for device operation. Thereafter, it has been reported that addition of B, Cr, Nb and Mo could improved the ductility of polycrystalline Fe-Ga alloys for plastic working, but, has a little adverse influence on magnetostriction[9–11]. Takahashi et al.[12] have developed
Fe-Ga-Al alloys with high strength (>790 MPa) and high magnetostriction by means of the formation of fine carbide precipitation of intermetallic compound, but, the ductility performance was not presented in their work. Recently, there is an obvious improvement of magnetostriction of Fe-Ga alloy by small adding of rare earth element Tb or Dy[13–15]. The beneficial influence was attributed to the strong localized magnetocrystalline anisotropy of Tb and Dy. Therefore, we have been studying material design in order to enhance the mechanical property and magnetostriction of Galfenol with the addition of rare earth element. A study of the additions of small amounts of yttrium to Fe83Ga17 alloys was initiated. In this work, the 0.16 at.% Y-doped Fe83Ga17 as-cast alloy had tension strength of 674 MPa and elongation of 4.2% at room temperature, and also had an improved magnetostriction.
1 Experimental Polycrystalline rod samples of (Fe83Ga17)100–xYx (x=0, 0.16, 0.32, 0.48, 0.64) with dimension of 21 mm×120 mm, were prepared from Fe (99.95% purity), Ga (99.99% purity) and Y (99.9% purity) by induction melting under argon atmosphere. The samples for magnetostriction test were annealed at 1100 ºC for 1 h and then the specimens were furnace-cooled to 730 ºC for 3 h in an argon atmosphere and quenched into water. Magnetostriction were measured by means of a standard resistance strain
Foundation item: Project supported by the National Basic Research Program of China (973 Program) (2011CB606304), National Natural Science Foundation of China (51271019) and the Fundamental Research Funds for the Central Universities (FRF-SD-12-025A) * Corresponding author: GAO Xuexu (E-mail: [email protected]; Tel.: +86-10-62334343) DOI: 10.1016/S1002-0721(14)60530-5
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gauge technique, and the gages were pasted longitudinal the direction of the rod. The samples for tension test were hot forged at 1150 ºC to a diameter of 14 mm, and then annealed at 750 ºC for 4 h. The rod samples were cut into standard tensile specimen. Static tensile tests were carried out using MTS 810 Materials Testing System in constant velocity of 1×10–3 s–1 at room temperature. Vibrating sample magnetometer (VSM) was employed to detect the magnetic properties. The X-ray diffraction (XRD) patterns were taken to analyze the phase structure. X-ray diffraction examination was performed using the Cu Kα radiation in a Siemens D5000® X-ray diffractometer. The fracture surface of the tensile sample was observed by a Cambridge-S250 scanning electron microscope (SEM) and the microstructure was observed by an optical microscope and SEM. The fiber texture was analyzed using electron back-scattering diffraction (EBSD). The EBSD analysis was carried out on a SUPRATM 55 filed emission scanning electron microscope equipped with automatic OIM (orientation imaging map) soft-ware from TSL.
to the Y2(FeGa)17 structure (2:17R). On the other hand, it was reported that the solubility of Ga in Y2Fe17 is large and the maximum value of x could reache 8 in Y2Fe17–xGax alloy, and there is a slight change of Y2(FeGa)17 structure due to different contents of Ga[16–18]. The structure of Y2Fe17–xGax is hexagonal Th2Ni17 structure if x≤4, and when Ga content is higher (4
2 Results and discussion The crystal structure of the samples was checked by X-ray diffractions, as shown in Fig. 1. The main diffraction peaks are observed for all the alloys, which are indexed as (110), (200) and (211) from the body-centered cubic (bcc) structure, as shown in Fig. 1(a). This demonstrates that the matrix of all the Y-doped Fe83Ga17 alloys possesses the same bcc structure as that of the binary Fe83Ga17 alloy. But some weak diffraction peaks, as shown in Fig. 1(b), are also observed in the samples for x=0.64 alloy, which means that there is Y-rich precipitation in the alloy. The peak of 30.760° could be corresponding to Y2O3. As a rare earth element, yttrium has high chemical activity depending on the character of their electro-construction, and forms an oxide very easily. According to the Fe-Y binary phase diagram and combining with the XRD results, it is suggested that these peaks of 35.663°, 41.416° and 75.535° are corresponding
Table 1 Components of the different zones in micro-region BSE image for x=0.64 alloy (at.%) Element
Matrix area
White area
Fe
79.4±3.0
53.5±1.2
Black area 6.9±0.0
Ga
20.6±2.8
34.3±0.8
1.4±1.4
Y
–
12.2±1.7
27.2±1.5
O
–
–
64.5±2.2
Fig. 1 X-ray diffraction patterns of (Fe83Ga17)100–xYx (x=0, 0.16, 0.32, 0.48, 0.64) alloys (a) and the enlarged XRD of x=0.64 alloy (b)
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Fig. 2 Cross-section images of (Fe83Ga17)100–xYx (x=0, 0.16, 0.32, 0.64) alloys, (a)–(d) are optical images, (e) and (f) are BSE images (The inset in (f) is the micro-region BSE image for x=0.64 alloy) (a) x=0.0; (b) x=0.16; (c) x=0.32; (d) x=0.64; (e) x=0.16; (f) x=0.64
Fig. 2(f) and combining with the EDS results (Table 1), it can be found that there is some Y2Fe17−xGax (6≤x≤7) phase (the white area) and Y2O3 phase particles (the black area) inside the grains. On the other hand, although the precipitated Y-rich phase was not detected by X-ray diffraction due to small quantity in (Fe83Ga17)100–xYx (x=0.16, 0.32, 0.48) alloys shown in Fig. 1(a), it seemed to be reasonable to believe that some of Y2Fe17–xGax (6≤x≤7) phase and a little of Y2O3 particles distribute along grain boundaries or inside the grains of those alloys due to small solid solubility of Y atoms in A2 matrix, as shown in Fig. 2(b) and (c). Fig. 3 exhibits the inverse pole figures of the samples with different yttrium additions. It can be clearly seen that the <100> preferential orientation along axis direction of rods was obtained for x=0.32 and 0.64 alloys, as shown in Fig. 3(c) and (d), and the degree of orientation increased with the addition of yttrium. It should be attributed to developed dendrite structure, where the Y2Fe17−xGax (6≤x
Fig. 3 Inverse pole figures of (Fe83Ga17)100–xYx (x=0, 0.16, 0.32, 0.64) alloys (a) x=0.0; (b) x=0.16; (c) x=0.32; (d) x=0.64
≤7) phase distributes preferentially along the grain boundary and causes the preferred orientation of grains. The effect of yttrium additions on the room temperature mechanical properties of polycrystalline Fe83Ga17 alloy are shown in Fig. 4(a). For the Fe83Ga17 binary alloy, there was only elastic deformation, and an ultimate tensile strength of 476 MPa was observed with fracture occurring in tensile testing. Notably, there is an evident increase of the tensile strength and ductility for x=0.16 alloy, and the performance is up to 674 MPa and 4.2% respectively. But, the beneficial effect of yttrium on the mechanical properties of Fe83Ga17 alloy became weak with increase of addition amount. The excessive Y addition led to a drop in tensile strength and elongation of (Fe83Ga17)100–xYx alloys. Fig. 4(b)–(e) present scanning electron microscopy (SEM) fractographs of (Fe83Ga17)100–xYx alloys. From the Fig. 4(b), it can be found that Fe83Ga17 alloy exhibited intergranular fracture with smooth facets, which is a typical brittle fracture indicating a severe deterioration in ductility. The poor ductility of Fe83Ga17 alloys could be attributed to the grain boundary weakness. As shown in Fig. 4(c)–(e), the addition of yttrium changed the fracture of intergranular into transgranular cleavage. According to the fractographs and combining with the microstructure analysis, it is obvious that tiny Y2Fe17−xGax (6≤x≤7) precipitates dispersed in alloy matrix play an important role for the enhancement of mechanical property of x=0.16 alloy. The precipitates increased the grain boundary cohesion and eliminated local boundary stress, so the intergranular fracture was inhibited. In addition, grains were fined, leading to an increase of the area of grain boundaries, which causes the rising of resistance to crack propagation. Therefore, the ductility of x=0.16 alloy was improved.
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Fig. 4 Tensile test curves (a) and SEM fractographs (b−e) for (Fe83Ga17)100–xYx (x=0, 0.16, 0.32, 0.48, 0.64) alloys at room temperature
Furthermore, the precipitates distributed inside the grains hinder the movement of dislocations of matrix alloy, resulting in improvement of tensile strength. Thus, a small addition of yttrium is advantageous to improve the room temperature mechanical properties of polycrystalline Fe83Ga17 alloy. In order to explore the reason why an excessive Y addition leads to a drop in mechanical properties of (Fe83Ga17)100–xYx alloys, the fractography of x=0.16 and 0.64 alloys was analyzed in detail, as shown in Fig. 5 and Table 2. From the fractograph for x=0.16 alloy shown in Fig. 5(a), it can be seen that the fracture morphology appears as cleavage steps with varying patterns (river- like, linear and simple "λ" shapes) and the fracture mechanism belongs to cleavage fracture. By comparison, there are three typical characteristics on the fracture surface for x=0.64 alloy, as shown in Fig. 5(b), where their corresponding components is displayed in Table 1. The area “A” exhibits granular fracture with smooth facets indicating a severe deterioration in ductility, and the feature of transgranular fracture appears in region “C”. It is considered that the area “A” and “C” exhibit the fracture characteristics of matrix alloy. The fracture feature of Y2Fe17−xGax (6≤x≤7) phase is shown in area “B”, where
intermetallic compound with Th2Zn17 structure is brittle, because of the complex crystal structure and few slip systems. A large number of brittle Y2Fe17−xGax (6≤x≤7) phase continuously distributed along grain boundary resulted in the mechanical brittleness for x=0.64 alloy. The saturation magnetostriction λ of (Fe83Ga17)100–xYx alloy rods under 15 MPa compressive stress is given in Fig. 6(a). The un-doped Fe83Ga17 alloy shows 40 ppm, which is comparable to previously reported values of as-cast polycrystalline Fe-Ga alloys[19]. Magnetostriction of Fe83Ga17 alloy was improved universally by adding yttrium. As the amount of yttrium increases to 0.64 at.%, the Y-doped Fe83Ga17 alloy approaches a maximal magnetostriction coefficient of 133 ppm. It has been verified that yttrium is able to change the preferred orientation of grains through residing preferentially in the grain boundary region, and <100> preferential orientation along the Table 2 Components of the different zones on fracture for x=0.64 alloy (at.%) Element
A
B
C
Fe
82.9±3.3
58.9±0.9
83.4±1.1
Ga
17.1±3.2
31.8±1.8
16.6±2.1
Y
–
9.3±0.4
–
Fig. 5 SEM fractographs for (Fe83Ga17)100–xYx (x=0.16, 0.64) alloys (a) x=0.16; (b) x=0.64
LI Jiheng et al., Effect of yttrium on the mechanical and magnetostrictive properties of Fe83Ga17 alloy
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Fig. 6 Saturation magnetostriction λ under 15 MPa compressive stress (a), saturation magnetization Ms (b) and coercivity Hc (c) for (Fe83Ga17)100–xYx (x=0, 0.16, 0.32, 0.48, 0.64) alloys (magnetostriction versus applied magnetic field under different compressive stresses for x=0.32 alloy (d))
axial direction of rods are obtained in x=0.32 and x=0.64 alloys, as shown in Fig. 3(c) and (d). Sato et al.[20,21] and Emdadi et al.[22] found that the magnetostriction of copper mold-cast polycrystalline alloys depends on the thermal gradient and the value in the direction parallel to thermal gradient can reach 90 ppm, and the improvement of magnetostriction was attributed to the crystallographic texture and the particular microstructure composed of large and elongated grains. The saturation magnetization (Ms) is presented in Fig. 6(b). The Ms of (Fe83Ga17)100–xYx alloy rods decreases gradually, from 175 emu/g for Fe83Ga17 to a minimum 169 emu/g for x=0.64 alloy. The decrease in Ms with addition of yttrium is due to the reduction of Fe-Fe interactions and the Y-rich precipitation. The coercivity Hc decreases appreciably first and then increases sharply as the yttrium content increases to 0.64 at.%, as shown in Fig. 6(c). The magnetic moment of the secondary phase is different from the matrix, which will impede the movement of the domain wall and increase the coercivity. The increase of the coercivity is not proportional to the number of the secondary phases because of their different sizes and distributions. Fig. 6(d) shows the slope of the magnetostriction versus applied magnetic field under different compressive stresses for x=0.32 alloy. A “jump effect” and narrow hysteresis of magnetostriction is observed.
3 Conclusions In summary, the yttrium addition could improve the room temperature mechanical properties and magnetostriction of as-cast polycrystalline Fe83Ga17 alloys. Small amount of yttrium (0.16 at.%) increased the tensile strength of Fe83Ga17 alloys to 674 MPa and improved the
ductility with elongation of 4.2% at room temperature. The 0.64 at.% Y-doped Fe83Ga17 as-cast alloy had magnetostriction coefficient of 133 ppm under 15 MPa compressive stress. The Y2Fe17−xGax (6≤x≤7) phase was formed in the Y-doped Fe83Ga17 alloy since yttrium was hardly dissolved into the α-Fe lattice. Appropriate amount of Y2Fe17−xGax (6≤x≤7) phase dispersed along the grain boundaries or inside the grains played an important role for the enhancement of mechanical properties. The improvement of magnetostriction of Fe83Ga17 as-cast alloy with addition of yttrium could be attributed to the alteration of the grain shape and the formation of columnar dendrites which resulted in the formation of <100> preferential direction along the axial direction of rods.
References: [1] Clark A E, Hathaway K B, Wun-Fogle M, Restorff J B, Lograsso T A, Keppens V M, Petculescu G, Taylor R A. Extraordinary magnetoelasticity and lattice softening in bcc Fe-Ga alloys. J. Appl. Phys., 2003, 93: 8621. [2] Zhang S Q, Zhang M C, Qiao Y, Liu M X, Wang H C, Xu Y Z. Magnetostriction, composition and microstructure of Tb0.29(Dy1–xPrx)0.71Fe1.97 alloys with heat treatment. J. Rare Earths, 2013, 31(6): 577. [3] Kellogg R A, Russell A M, Lograsso T A, Flatau A B, Clark A E, Wun-Fogle M. Tensile properties of magnetostrictive iron-gallium alloy. Acta. Mater., 2004, 52: 5043. [4] Kaleta J, Wiewiórski P. Magnetovisual method for monitoring thermal demagnetization of permanent magnets used in magnetostrictive actuators. J. Rare Earths, 2014, 32(3): 236. [5] Clark A E, Wun-Fogle M, Restorf J B, Lograsso T A, Cullen J R. Effect of quenching on the magnetostriction of
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Fe1–xGax (0.13
Fe83Ga17Dyx alloys. J. Rare Earths, 2010, 28(1): 409. [15] Zhang S X, Wu W, Zhu X X, Liu J H, Jiang C B. Microstructure and magnetostrictive properties of Tb doped FeGa bulk alloys prepared by melt quenching. Rare Met. 2014, 33(3): 309. [16] Shen B G, Cheng Z H, Gong H Y, Liang B, Yan Q W, Wang F W. Effects of Ga substitution in Y2Fe17 compounds on the magnetocrystalline anisotropy. J. Alloys Compd., 1995, 226: 51. [17] Wang F W, Zhang P L, Wang J Y, Sun X D, Yan Q W, Shen B G, Zhang L G. Fe-Fe bond length and exchange interaction in Y2Fe17–xGax (x=0–8) compounds. J. Magn. Magn. Mater., 1999, 192: 485. [18] Maruyama F, Nagai H. Magnetic properties of R2Fe17–xGax (R=Y and Gd) compounds. Solid State Commun., 2005, 135: 424. [19] Srisukhumbowornchai N, Guruswamy S. Large magnetostriction in directionally solidified FeGa and FeGaAl alloys. J. Appl. Phys., 2001, 90(11): 5680. [20] Sato Turtelli R, Bormio-Nunes C, Sinnecker J P, Grossinger R. Effect of grain shape on magnetostrictive polycrystalline Fe-Ga alloys. Physica B, 2006, 384: 265. [21] Bormio-Nunes C, Sato Turtelli R, Mueller H, Grossinger R, Sassik H, Tirelli M A. Magnetostriction and structural characterization of Fe-Ga-X (X=Co, Ni, Al) mold-cast bulk. J. Magn. Magn. Mater., 2005, 290-291: 820. [22] Emdadi A A, Nedjad S H, Ghavifekr H B. Effect of solidification texture on the magnetostrictive behavior of galfenol. Metal. Mater. Trans. A, 2014, 45(2): 906.
Effect of yttrium on the mechanical and magnetostrictive properties of Fe83Ga17 alloy LI Jiheng (李纪恒), XIAO Ximing (肖锡铭), YUAN Chao (袁 超), GAO Xuexu (高学绪)*, BAO Xiaoqian (包小倩) (State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China) Received 6 June 2015; revised 30 June 2015
Abstract: Polycrystalline rod samples of (Fe83Ga17)100–xYx (x=0, 0.16, 0.32, 0.48, 0.64) were prepared by induction melting under argon atmosphere. Effect of yttrium on the mechanical and magnetostrictive properties of Fe83Ga17 alloy was investigated. Small amount of yttrium (0.16 at.%) increased the tensile strength of as-cast Fe83Ga17 alloys to 674 MPa and improved the ductility with elongation of 4.2% at room temperature. The Y2Fe17−xGax (6≤x≤7) phase was formed in the Y-doped Fe83Ga17 alloy since yttrium was hardly dissolved into the α-Fe lattice. Y2(FeGa)17 secondary phase dispersed along the grain boundaries and inside the grains played an important role for the enhancement of mechanical property. The 0.64 at.% Y-doped alloy had magnetostriction of 133 ppm, which was thought to be associated with the alteration of the grain shape and <100> preferential orientation along the axial direction of rods. Keywords: yttrium; Fe-Ga alloy; magnetostriction; mechanical property; rare earths
In recent years, there has been an ever-increasing interest in the Fe-Ga (Galfenol) alloy as a new magnetostrictive material for actuator, sensor, and energy harvesting applications. The saturation magnetostriction can be as high as 400 ppm in single crystals[1]. The relatively low magnetostriction values for Fe-Ga alloys compared to giant magnetostrictive materials such as Terfenol-D alloy[2] (λs~2000 ppm) would beg the question “Why use Fe-Ga alloy?”. The answer to this question comes when we begin to understand Fe-Ga alloy’s mechanical properties. Mechanically, Fe-Ga alloys have high strength above 500 MPa and robustness[3], which is not exhibited by other energy harvesting materials such as PZT, PMN, or Terfenol-D[4]. In addition, Fe-Ga alloys have high permeability (µr>100) and Curie temperature (Tc>650 ºC)[5]. This combination of magnetic and mechanical properties makes Fe-Ga alloy aunique material. However, the polycrystalline Fe81.6Ga18.4 alloy exhibited tensile strengths only 370 MPa with elongation of 0.8% before failure at room temperature[6], and the studies of Na et al.[7] and Cheng et al.[8] indicated that the polycrystalline Fe-Ga binary alloy cracked and fractured along grain boundaries during hot rolling and was too brittle to make thin sheets which can avoid eddy current losses for device operation. Thereafter, it has been reported that addition of B, Cr, Nb and Mo could improved the ductility of polycrystalline Fe-Ga alloys for plastic working, but, has a little adverse influence on magnetostriction[9–11]. Takahashi et al.[12] have developed
Fe-Ga-Al alloys with high strength (>790 MPa) and high magnetostriction by means of the formation of fine carbide precipitation of intermetallic compound, but, the ductility performance was not presented in their work. Recently, there is an obvious improvement of magnetostriction of Fe-Ga alloy by small adding of rare earth element Tb or Dy[13–15]. The beneficial influence was attributed to the strong localized magnetocrystalline anisotropy of Tb and Dy. Therefore, we have been studying material design in order to enhance the mechanical property and magnetostriction of Galfenol with the addition of rare earth element. A study of the additions of small amounts of yttrium to Fe83Ga17 alloys was initiated. In this work, the 0.16 at.% Y-doped Fe83Ga17 as-cast alloy had tension strength of 674 MPa and elongation of 4.2% at room temperature, and also had an improved magnetostriction.
1 Experimental Polycrystalline rod samples of (Fe83Ga17)100–xYx (x=0, 0.16, 0.32, 0.48, 0.64) with dimension of 21 mm×120 mm, were prepared from Fe (99.95% purity), Ga (99.99% purity) and Y (99.9% purity) by induction melting under argon atmosphere. The samples for magnetostriction test were annealed at 1100 ºC for 1 h and then the specimens were furnace-cooled to 730 ºC for 3 h in an argon atmosphere and quenched into water. Magnetostriction were measured by means of a standard resistance strain
Foundation item: Project supported by the National Basic Research Program of China (973 Program) (2011CB606304), National Natural Science Foundation of China (51271019) and the Fundamental Research Funds for the Central Universities (FRF-SD-12-025A) * Corresponding author: GAO Xuexu (E-mail: [email protected]; Tel.: +86-10-62334343) DOI: 10.1016/S1002-0721(14)60530-5
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gauge technique, and the gages were pasted longitudinal the direction of the rod. The samples for tension test were hot forged at 1150 ºC to a diameter of 14 mm, and then annealed at 750 ºC for 4 h. The rod samples were cut into standard tensile specimen. Static tensile tests were carried out using MTS 810 Materials Testing System in constant velocity of 1×10–3 s–1 at room temperature. Vibrating sample magnetometer (VSM) was employed to detect the magnetic properties. The X-ray diffraction (XRD) patterns were taken to analyze the phase structure. X-ray diffraction examination was performed using the Cu Kα radiation in a Siemens D5000® X-ray diffractometer. The fracture surface of the tensile sample was observed by a Cambridge-S250 scanning electron microscope (SEM) and the microstructure was observed by an optical microscope and SEM. The fiber texture was analyzed using electron back-scattering diffraction (EBSD). The EBSD analysis was carried out on a SUPRATM 55 filed emission scanning electron microscope equipped with automatic OIM (orientation imaging map) soft-ware from TSL.
to the Y2(FeGa)17 structure (2:17R). On the other hand, it was reported that the solubility of Ga in Y2Fe17 is large and the maximum value of x could reache 8 in Y2Fe17–xGax alloy, and there is a slight change of Y2(FeGa)17 structure due to different contents of Ga[16–18]. The structure of Y2Fe17–xGax is hexagonal Th2Ni17 structure if x≤4, and when Ga content is higher (4
2 Results and discussion The crystal structure of the samples was checked by X-ray diffractions, as shown in Fig. 1. The main diffraction peaks are observed for all the alloys, which are indexed as (110), (200) and (211) from the body-centered cubic (bcc) structure, as shown in Fig. 1(a). This demonstrates that the matrix of all the Y-doped Fe83Ga17 alloys possesses the same bcc structure as that of the binary Fe83Ga17 alloy. But some weak diffraction peaks, as shown in Fig. 1(b), are also observed in the samples for x=0.64 alloy, which means that there is Y-rich precipitation in the alloy. The peak of 30.760° could be corresponding to Y2O3. As a rare earth element, yttrium has high chemical activity depending on the character of their electro-construction, and forms an oxide very easily. According to the Fe-Y binary phase diagram and combining with the XRD results, it is suggested that these peaks of 35.663°, 41.416° and 75.535° are corresponding
Table 1 Components of the different zones in micro-region BSE image for x=0.64 alloy (at.%) Element
Matrix area
White area
Fe
79.4±3.0
53.5±1.2
Black area 6.9±0.0
Ga
20.6±2.8
34.3±0.8
1.4±1.4
Y
–
12.2±1.7
27.2±1.5
O
–
–
64.5±2.2
Fig. 1 X-ray diffraction patterns of (Fe83Ga17)100–xYx (x=0, 0.16, 0.32, 0.48, 0.64) alloys (a) and the enlarged XRD of x=0.64 alloy (b)
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Fig. 2 Cross-section images of (Fe83Ga17)100–xYx (x=0, 0.16, 0.32, 0.64) alloys, (a)–(d) are optical images, (e) and (f) are BSE images (The inset in (f) is the micro-region BSE image for x=0.64 alloy) (a) x=0.0; (b) x=0.16; (c) x=0.32; (d) x=0.64; (e) x=0.16; (f) x=0.64
Fig. 2(f) and combining with the EDS results (Table 1), it can be found that there is some Y2Fe17−xGax (6≤x≤7) phase (the white area) and Y2O3 phase particles (the black area) inside the grains. On the other hand, although the precipitated Y-rich phase was not detected by X-ray diffraction due to small quantity in (Fe83Ga17)100–xYx (x=0.16, 0.32, 0.48) alloys shown in Fig. 1(a), it seemed to be reasonable to believe that some of Y2Fe17–xGax (6≤x≤7) phase and a little of Y2O3 particles distribute along grain boundaries or inside the grains of those alloys due to small solid solubility of Y atoms in A2 matrix, as shown in Fig. 2(b) and (c). Fig. 3 exhibits the inverse pole figures of the samples with different yttrium additions. It can be clearly seen that the <100> preferential orientation along axis direction of rods was obtained for x=0.32 and 0.64 alloys, as shown in Fig. 3(c) and (d), and the degree of orientation increased with the addition of yttrium. It should be attributed to developed dendrite structure, where the Y2Fe17−xGax (6≤x
Fig. 3 Inverse pole figures of (Fe83Ga17)100–xYx (x=0, 0.16, 0.32, 0.64) alloys (a) x=0.0; (b) x=0.16; (c) x=0.32; (d) x=0.64
≤7) phase distributes preferentially along the grain boundary and causes the preferred orientation of grains. The effect of yttrium additions on the room temperature mechanical properties of polycrystalline Fe83Ga17 alloy are shown in Fig. 4(a). For the Fe83Ga17 binary alloy, there was only elastic deformation, and an ultimate tensile strength of 476 MPa was observed with fracture occurring in tensile testing. Notably, there is an evident increase of the tensile strength and ductility for x=0.16 alloy, and the performance is up to 674 MPa and 4.2% respectively. But, the beneficial effect of yttrium on the mechanical properties of Fe83Ga17 alloy became weak with increase of addition amount. The excessive Y addition led to a drop in tensile strength and elongation of (Fe83Ga17)100–xYx alloys. Fig. 4(b)–(e) present scanning electron microscopy (SEM) fractographs of (Fe83Ga17)100–xYx alloys. From the Fig. 4(b), it can be found that Fe83Ga17 alloy exhibited intergranular fracture with smooth facets, which is a typical brittle fracture indicating a severe deterioration in ductility. The poor ductility of Fe83Ga17 alloys could be attributed to the grain boundary weakness. As shown in Fig. 4(c)–(e), the addition of yttrium changed the fracture of intergranular into transgranular cleavage. According to the fractographs and combining with the microstructure analysis, it is obvious that tiny Y2Fe17−xGax (6≤x≤7) precipitates dispersed in alloy matrix play an important role for the enhancement of mechanical property of x=0.16 alloy. The precipitates increased the grain boundary cohesion and eliminated local boundary stress, so the intergranular fracture was inhibited. In addition, grains were fined, leading to an increase of the area of grain boundaries, which causes the rising of resistance to crack propagation. Therefore, the ductility of x=0.16 alloy was improved.
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Fig. 4 Tensile test curves (a) and SEM fractographs (b−e) for (Fe83Ga17)100–xYx (x=0, 0.16, 0.32, 0.48, 0.64) alloys at room temperature
Furthermore, the precipitates distributed inside the grains hinder the movement of dislocations of matrix alloy, resulting in improvement of tensile strength. Thus, a small addition of yttrium is advantageous to improve the room temperature mechanical properties of polycrystalline Fe83Ga17 alloy. In order to explore the reason why an excessive Y addition leads to a drop in mechanical properties of (Fe83Ga17)100–xYx alloys, the fractography of x=0.16 and 0.64 alloys was analyzed in detail, as shown in Fig. 5 and Table 2. From the fractograph for x=0.16 alloy shown in Fig. 5(a), it can be seen that the fracture morphology appears as cleavage steps with varying patterns (river- like, linear and simple "λ" shapes) and the fracture mechanism belongs to cleavage fracture. By comparison, there are three typical characteristics on the fracture surface for x=0.64 alloy, as shown in Fig. 5(b), where their corresponding components is displayed in Table 1. The area “A” exhibits granular fracture with smooth facets indicating a severe deterioration in ductility, and the feature of transgranular fracture appears in region “C”. It is considered that the area “A” and “C” exhibit the fracture characteristics of matrix alloy. The fracture feature of Y2Fe17−xGax (6≤x≤7) phase is shown in area “B”, where
intermetallic compound with Th2Zn17 structure is brittle, because of the complex crystal structure and few slip systems. A large number of brittle Y2Fe17−xGax (6≤x≤7) phase continuously distributed along grain boundary resulted in the mechanical brittleness for x=0.64 alloy. The saturation magnetostriction λ of (Fe83Ga17)100–xYx alloy rods under 15 MPa compressive stress is given in Fig. 6(a). The un-doped Fe83Ga17 alloy shows 40 ppm, which is comparable to previously reported values of as-cast polycrystalline Fe-Ga alloys[19]. Magnetostriction of Fe83Ga17 alloy was improved universally by adding yttrium. As the amount of yttrium increases to 0.64 at.%, the Y-doped Fe83Ga17 alloy approaches a maximal magnetostriction coefficient of 133 ppm. It has been verified that yttrium is able to change the preferred orientation of grains through residing preferentially in the grain boundary region, and <100> preferential orientation along the Table 2 Components of the different zones on fracture for x=0.64 alloy (at.%) Element
A
B
C
Fe
82.9±3.3
58.9±0.9
83.4±1.1
Ga
17.1±3.2
31.8±1.8
16.6±2.1
Y
–
9.3±0.4
–
Fig. 5 SEM fractographs for (Fe83Ga17)100–xYx (x=0.16, 0.64) alloys (a) x=0.16; (b) x=0.64
LI Jiheng et al., Effect of yttrium on the mechanical and magnetostrictive properties of Fe83Ga17 alloy
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Fig. 6 Saturation magnetostriction λ under 15 MPa compressive stress (a), saturation magnetization Ms (b) and coercivity Hc (c) for (Fe83Ga17)100–xYx (x=0, 0.16, 0.32, 0.48, 0.64) alloys (magnetostriction versus applied magnetic field under different compressive stresses for x=0.32 alloy (d))
axial direction of rods are obtained in x=0.32 and x=0.64 alloys, as shown in Fig. 3(c) and (d). Sato et al.[20,21] and Emdadi et al.[22] found that the magnetostriction of copper mold-cast polycrystalline alloys depends on the thermal gradient and the value in the direction parallel to thermal gradient can reach 90 ppm, and the improvement of magnetostriction was attributed to the crystallographic texture and the particular microstructure composed of large and elongated grains. The saturation magnetization (Ms) is presented in Fig. 6(b). The Ms of (Fe83Ga17)100–xYx alloy rods decreases gradually, from 175 emu/g for Fe83Ga17 to a minimum 169 emu/g for x=0.64 alloy. The decrease in Ms with addition of yttrium is due to the reduction of Fe-Fe interactions and the Y-rich precipitation. The coercivity Hc decreases appreciably first and then increases sharply as the yttrium content increases to 0.64 at.%, as shown in Fig. 6(c). The magnetic moment of the secondary phase is different from the matrix, which will impede the movement of the domain wall and increase the coercivity. The increase of the coercivity is not proportional to the number of the secondary phases because of their different sizes and distributions. Fig. 6(d) shows the slope of the magnetostriction versus applied magnetic field under different compressive stresses for x=0.32 alloy. A “jump effect” and narrow hysteresis of magnetostriction is observed.
3 Conclusions In summary, the yttrium addition could improve the room temperature mechanical properties and magnetostriction of as-cast polycrystalline Fe83Ga17 alloys. Small amount of yttrium (0.16 at.%) increased the tensile strength of Fe83Ga17 alloys to 674 MPa and improved the
ductility with elongation of 4.2% at room temperature. The 0.64 at.% Y-doped Fe83Ga17 as-cast alloy had magnetostriction coefficient of 133 ppm under 15 MPa compressive stress. The Y2Fe17−xGax (6≤x≤7) phase was formed in the Y-doped Fe83Ga17 alloy since yttrium was hardly dissolved into the α-Fe lattice. Appropriate amount of Y2Fe17−xGax (6≤x≤7) phase dispersed along the grain boundaries or inside the grains played an important role for the enhancement of mechanical properties. The improvement of magnetostriction of Fe83Ga17 as-cast alloy with addition of yttrium could be attributed to the alteration of the grain shape and the formation of columnar dendrites which resulted in the formation of <100> preferential direction along the axial direction of rods.
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