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Preparation of Terfenol-D with precise 〈110〉 orientation and observation of the oriented growth crystal morphology
Journal of Alloys and Compounds 333 (2002) 291–295
L
www.elsevier.com / locate / jallcom
Preparation of Terfenol-D with precise k110l orientation and observation of the oriented growth crystal morphology Chengchang Ji, Jianguo Li*, Weizeng Ma, Yaohe Zhou School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200030, PR China Received 17 May 2001; accepted 26 June 2001
Abstract In a super high temperature gradient directional solidification equipment developed by the authors, f7 mm rare earth giant magnetostrictive rods with precise k110l orientations were manufactured successfully without any seed crystal. A dispersion of only 38 exists between the k110l oriented growth direction and the axis of the rod. During the directional solidification process, there exists a certain relationship between the crystal growth morphology and the crystalline orientation degree. The crystallites’ orientation degree can be determined conveniently through the observation of the crystal morphology. Under the condition of the temperature gradient of 800 K / cm and the withdrawal velocity of 6 mm / s|10 mm / s, Tb 0.27 Dy 0.73 Fe 1.9 rod with k110l oriented crystals can be obtained when the solidified part of the rod reaches 20 mm. When the length of the solidified part is at least 30 mm, the k110l oriented crystallines can grow in a stable way. 2002 Elsevier Science B.V. All rights reserved. Keywords: Rare earth compounds; Transition metal compounds; Casting; Microstructure; Metallography
1. Introduction The rare earth giant magnetostrictive material, Tb 0.27 Dy 0.73 Fe 1.9 , as an advanced functional material, is of significant technological interest [1–3] because of its high energy intensity and high response velocity at low frequency as well as its giant magnetostriction at room temperature. Wide application fields have been identified since the material was exploited [4,5]. Because of the strong anisotropy of the magnetostrictive strain, l111 .. l100 , the preparation of defect-free single crystals with k111l orientation is desirable for this materials. The directional solidification method, including the Czochralski method, the Modified Bridgman method and the vertical float zoning method, is mainly employed to manufacture and investigate this rare earth magnetostrictive material, and different oriented crystals were achieved. Verhoeven et al. [6] and Clark et al. [7] reported that Terfenol-D dendrites grow as sheets parallel to h111j planes having k112l growth directions. Bonino et al. [8] reported that Terfenol-D with k110l orientation was obtained by casting *Corresponding author. Tel.: 186-21-6293-3074; fax: 186-21-62932026. E-mail address: [email protected] (J. Li).
in a strong unidirectional gradient mould, and the property analysis revealed that the magnetostriction of the k110l oriented rod is not less than that of the k112l oriented rod. This indicates that Tb–Dy–Fe alloy has many crystal growth directions under different solidification conditions, and that Terfenol-D with k110l orientation can also possess as high a magnetostrictive property as that with k112l orientation. The magnetostrictive property, therefore, should be determined by many factors, such as crystal orientation, orientation degree, microstructure, crystal defects, and so on. Terfenol-D rods with oriented grown crystalline can be obtained by using the corresponding seed crystals [9] or changing the solidification conditions [10], such as the temperature gradient at the solid–liquid interface and the crystal growth velocity. The authors prepared successfully the Terfenol-D rods with precise k110l orientation, only 38 dispersion to the rod axis, in a super high temperature gradient equipment by changing the directional solidification conditions. From the viewpoint of solidification, the crystal morphologies and the crystalline orientation degree, ranging from free oriented grains to the stable growth of the oriented crystals, were analyzed. The experiments also indicate that the entire process from the melting and casting of the Tb–Dy–Fe master alloy rod to the resulting
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01734-0
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C. Ji et al. / Journal of Alloys and Compounds 333 (2002) 291 – 295
Fig. 1. Illustration of the specimens’ location cut from the solidified rod.
directional solidification can be fulfilled in the super high temperature gradient installation. The equipment is an ideal one to investigate and prepare rare earth giant magnetostrictive materials.
2. Experimental Tb 0.27 Dy 0.73 Fe 1.9 master rods with the dimension of f7 mm3120 mm were melted and cast in the super high temperature gradient equipment. The starting materials are Tb 99.53%, Dy 99.38% and Fe 99.869% [11], and consequently the master rods were directionally solidified in the same equipment after changing the heating coil. The master alloy rod is contained in a high purity and thin wall quartz crucible, of internal diameter 7.1 mm and length 120 mm, and fixed in the middle of a pancake induction coil. High purity argon gas was filled into the chamber after the vacuum system was evacuated to 2310 23 Pa, and the high purity argon atmosphere in the chamber is 0.08 MPa. An approximately 5 mm long molten zone is formed by heating with the pancake coil driven by a 175 KHz current. After the zone is molten fully, the quartz crucible is lowered in the velocity of 6.3 mm / s, 9.2 mm / s and 10.3 mm / s respectively, the temperature gradient is maintained about 800 K / cm. Specimens were cut from the solidified rod, as shown in Fig. 1, for metallographic examination and crystal orientation detection. A forms the start of the directional solidifi-
cation. Microstructure and morphology observations were carried out using both a LECO image analysis instrument and a scanning electron microscopy (SEM). Some specimens were etched with 4% Nital (4% nitric in ethanol) or Vilella’s reagent (1 g picric acid in 5 ml HCl plus 95 ml methanol) for an intensive metallographic examination. X-ray diffraction and the Laue X-ray technique were employed to examine the crystal orientation and the orientation degree.
3. Results and discussion
3.1. Crystal orientation All rods directionally solidified in the super high temperature equipment had k110l oriented crystallites. The results analyzed in this paper were obtained at a zone rate of 9.2 mm / s. The transverse sections of the specimens at the locations B and C were employed to determine the crystal orientations by X-ray diffraction, as shown in Fig. 2a and b, respectively. Fig. 2a indicates that the k110l orientation degree is not very strong when the length of the solidified part is 15 mm, and crystallites with k110l and k113l orientations exist together. When the length of the solidified part reaches 22 mm, the diffraction intensity of the (022) peak is markedly stronger than that of the (113) peak, as shown in Fig. 2b. This indicates that the k110l oriented crystallites have dominated the crystal growth direction at location C. With Laue X-ray technique it was detected that the crystallite orientation at the transverse section E of the solidified part of the rod is precisely k110l, and a dispersion of only 38 exists between the crystal growth direction and the rod axis, as shown in Fig. 3. In other words, when the solidified part of the rod reaches 45 mm, crystallites with precise k110l orientation can be obtained. Fig. 4 shows that the RFe 2 (R5TbDy) matrix phase and the rare earth-rich phase are distributed along the directional solidification direction after the k110l
Fig. 2. X-ray diffraction patterns of transverse sections at location B and C in Fig. 1.
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Fig. 3. Laue X-ray diffraction pattern of the transverse section at location E in Fig. 1.
oriented crystallites grow stably. The morphology is consistent with the result of the Laue X-ray diffraction. Connecting a series of experimental results, it is sure that, in the super high temperature gradient equipment, the precise k110l oriented crystallites of Terfenol-D can be obtained under the conditions of a temperature gradient of 800 K / cm and a withdrawal velocity of 6|10 mm / s. The crystallite orientation is mainly in the k110l direction when the solidified part of the rod arrives at 20 mm. When the length of the solidified part is about 30 mm, the crystallites still grow unstably along the k110l direction. Crystallites with precise k110l orientation can grow stably when the solidified part becomes longer. If a seed crystal is used, the unstable growth course can be greatly shortened.
3.2. The crystal morphologies Previous investigations [12–14] of the microstructure are always focused on the texture obtained when the crystallites grow stably, and studies of the microstructure in the non-stable state are rare. Fig. 5 shows the crystal growth morphologies of the transverse sections of A, B, C, D and E, and displays the different morphology of the k110l oriented crystal in the different crystal growth courses. Fig. 5A shows the free aligned grains in the as-cast master rod. Generally there exist k111l, k110l, k112l and k113l randomly oriented grains [15], and they do not have any crystallographic alignment. After the directional solidification began, more k110l oriented crystals approach the direction of the temperature gradient, and gradually dominate the crystal growth direction. The number of the k110l aligned grains is increased obviously as the solidified part reaches 15 mm, as shown in Fig. 5B. When the solidified part is 22 mm, the k110l oriented crystallites, as shown in Fig. 5C, have dominated the crystals growth direction. As the solidified part reaches 30 mm, the transverse section of the rod is almost taken up by k110l
Fig. 4. Optical micrograph of the longitudinal cross section along the EF direction.
aligned grains, as shown in Fig. 5D. However, the crystal growth is not stable, some other oriented grains can be found locally. When the rod length reaches 45 mm, the crystallites have grown steadily along the k110l direction, and become ‘single’ Terfenol-D crystals as shown in Fig. 5E. From the longitudinal micrograph, as shown in Fig. 4, it is clear that the phases arrange themselves uniformly and regularly. In fact, during the directional solidification, crystals with different orientations compete with each other, and the preferred growth of the crystals will wash out other growth directions and gradually take up the whole intersection. The metallographic examination of the transverse sections of the specimens shows that the crystals of TerfenolD grow with a planar or a cellular morphology under this solidification condition. Compared with the k112l oriented crystal growing with a dendritic morphology [6,7], it is
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C. Ji et al. / Journal of Alloys and Compounds 333 (2002) 291 – 295
Fig. 5. Optical morphologies of transverse sections at locations A, B, C, D, and E in Fig. 1.
obviously that the solidification condition is very important for the crystal orientation and the growth morphology of the Terfenol-D. This experiment also verified that the solidification characteristics of Tb–Dy–Fe alloy possesses a strong crystallographic anisotropy.
3.3. The crystal morphologies and the crystallines orientation degree Patterns of the X-ray diffraction and the morphology, as shown in Fig. 2 and Fig. 5 respectively, reveal clearly that
there exists a certain relationship between the crystal growth morphology and the orientation degree of the k110l oriented crystallites. There are two main groups of oriented grains in Fig. 5B, and their relative area is almost equal. Comparing with Fig. 2a, only peaks of (022) and (113) exist, and their diffraction intensity is nearly equal, too. In Fig. 5C, the main phase (k110l aligned grains) possesses approximately 75% of the area of the transverse section, the ratio of the main phase to the second phase being about 3. From Fig. 2b it is apparent that the ratio of I ( 022) / I (113 ) , I ( 022) and I( 113) is the diffraction intensity of (022) peak
C. Ji et al. / Journal of Alloys and Compounds 333 (2002) 291 – 295
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and (113) peak respectively, is 3.2, which nearly equals the value of the area ratio in Fig. 5C. Fig. 5E shows that the transverse section of the specimen is fully occupied by the k110l oriented crystallites, and the analysis of the Laue X-ray diffraction photo, shown in Fig. 3, indicates that the crystal orientation at location E is precise the k110l direction. The crystal morphology is completely in agreement with the crystalline orientation degree. Therefore, the crystalline orientation degree can be determined conveniently by the observation of the crystal morphologies. On the basis of the relationship of the crystal morphology and the crystallites’ orientation degree, the crystallite orientation shown in Fig. 5D should be exactly the k110l direction.
2. Terfenol-D with precise k110l orientation, only 38 off the rod axis, can be prepared in the super high temperature gradient equipment. 3. The crystal morphology is completely consistent to the crystallines orientation degree, and the orientation degree can be determined conveniently by the crystal morphologies. 4. Under the conditions of a temperature gradient of 800 K / cm and a zone rate of 6 mm / s|10 mm / s, Tb 0.27 Dy 0.73 Fe 1.9 rods with mainly k110l orientation can be obtained when the solidified part of the rod is over 20 mm. When solidified part is at least 30 mm, the k110l oriented crystal can grow stably.
3.4. Phase spacing
Acknowledgements
The phase spacing of the directionally solidified Terfenol-D rod with k110l orientation was also examined because the spacing distribution will affect the microstructure inconsistency and the property inconsistency [16]. In all specimens, there exists a phase spacing between the plate-shape phases. From Fig. 5B to Fig. 5E it is obvious that the phase spacing is almost equal except that in Fig. 5D, in which the spacing is greater. It possibly results from the unstable growth of the crystallites because the phase spacing can adjust automatically during the solidification course. The metallographic examination shows that the increase of the phase spacing occurs in the same location of the specimens. The average phase spacing in Fig. 5B, C, E is approximate 35|40 mm, and the average phase spacing in Fig. 5D is about 70|80 mm, the latter being almost two times of the former. However, they are smaller than those reported by Mei [16], because the spacing is more sensitive to the temperature gradient for the Tb–Dy–Fe alloy. The origin of the phase spacing transformation needs intensive study. In addition, the RFe 3 phase has not been observed in all specimens etched with 4% Nital or Vilella’s regent by both optical miocroscopy and SEM. The microstructure is composed of the main RFe 2 matrix phase and the second phase is arranged parallel to the crystal growth direction as demonstrated by the vertical dark bands.
The authors would like to thank Professor Qi Rui for her analyses of the Laue photos, and acknowledge helpful discussions with Professor Chen Shipu. This work is supported by the National Science Foundation of China.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
4. Conclusions [16]
1. Terfenol-D crystals can grow along k112l and k110l orientations under different solidification conditions.
J.R. Cullen, Scripta Metall. Mater. 33 (1995) 1849. F.R. De Boer, K.H.J. Buschow, J. Alloys Comp. 258 (1997) 1. D.C. Jiks, J. Phys. D: Appl. Phys. 27 (1994) 1. O.D. McMasters, J. Rare Earth 13 (4) (1995) 295. S. Ashley, Mech. Eng. (Am.) 120 (6) (1998) 68. J.D. Verhoeven, E.D. Gibson, O.D. McMasters, H.H. Baker, Metall. Trans. A 18A (1987) 223. A.E. Clark, J.D. Verhoeven, O.D. McMasters, E.D. Gibson, IEEE Trans. Magn. 22 (5) (1986) 973. O. Bonino, P. De Rango, R. Tournier, J. Magn. Magn. Mater. 212 (2000) 225. W. Guangheng, Z. Xuegen, W. Jinghua, L. Jingyuan, J. Kechang, Z. Wenshan, Appl. Phys. Lett. 67 (14) (1995) 2005. O.D. McMasters, J.D. Verhoeven, E.D. Gibson, J. Magn. Magn. Mater. 54–57 (1986) 849. J. Chengchang, M. Weizeng, L. Jianguo, Z. Yaohe, J. Alloys Comp. (in press). J.D. Verhoeven, E.D. Gibson, O.D. McMasters, J.E. Ostenson, Metall. Trans. A 21A (1990) 2249. J.D. Verhoeven, J.E. Ostenson, E.D. Gibson, O.D. McMasters, J. Appl. Phys. 66 (2) (1989) 772. P. Westwood, J.S. Abell, A.E. Clark, K.C. Pitman, J. Appl. Phys. 64 (10) (1988) 5414. S. Zhengxing, C. Zhongmin, W. Xianhui, L. Menghua, Z. Yunmeng, J. Alloys Comp. 258 (1997) 30. W. Mei, T. Okane, T. Umeda, Z. Shouzheng, J. Alloys Comp. 248 (1997) 151.
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www.elsevier.com / locate / jallcom
Preparation of Terfenol-D with precise k110l orientation and observation of the oriented growth crystal morphology Chengchang Ji, Jianguo Li*, Weizeng Ma, Yaohe Zhou School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200030, PR China Received 17 May 2001; accepted 26 June 2001
Abstract In a super high temperature gradient directional solidification equipment developed by the authors, f7 mm rare earth giant magnetostrictive rods with precise k110l orientations were manufactured successfully without any seed crystal. A dispersion of only 38 exists between the k110l oriented growth direction and the axis of the rod. During the directional solidification process, there exists a certain relationship between the crystal growth morphology and the crystalline orientation degree. The crystallites’ orientation degree can be determined conveniently through the observation of the crystal morphology. Under the condition of the temperature gradient of 800 K / cm and the withdrawal velocity of 6 mm / s|10 mm / s, Tb 0.27 Dy 0.73 Fe 1.9 rod with k110l oriented crystals can be obtained when the solidified part of the rod reaches 20 mm. When the length of the solidified part is at least 30 mm, the k110l oriented crystallines can grow in a stable way. 2002 Elsevier Science B.V. All rights reserved. Keywords: Rare earth compounds; Transition metal compounds; Casting; Microstructure; Metallography
1. Introduction The rare earth giant magnetostrictive material, Tb 0.27 Dy 0.73 Fe 1.9 , as an advanced functional material, is of significant technological interest [1–3] because of its high energy intensity and high response velocity at low frequency as well as its giant magnetostriction at room temperature. Wide application fields have been identified since the material was exploited [4,5]. Because of the strong anisotropy of the magnetostrictive strain, l111 .. l100 , the preparation of defect-free single crystals with k111l orientation is desirable for this materials. The directional solidification method, including the Czochralski method, the Modified Bridgman method and the vertical float zoning method, is mainly employed to manufacture and investigate this rare earth magnetostrictive material, and different oriented crystals were achieved. Verhoeven et al. [6] and Clark et al. [7] reported that Terfenol-D dendrites grow as sheets parallel to h111j planes having k112l growth directions. Bonino et al. [8] reported that Terfenol-D with k110l orientation was obtained by casting *Corresponding author. Tel.: 186-21-6293-3074; fax: 186-21-62932026. E-mail address: [email protected] (J. Li).
in a strong unidirectional gradient mould, and the property analysis revealed that the magnetostriction of the k110l oriented rod is not less than that of the k112l oriented rod. This indicates that Tb–Dy–Fe alloy has many crystal growth directions under different solidification conditions, and that Terfenol-D with k110l orientation can also possess as high a magnetostrictive property as that with k112l orientation. The magnetostrictive property, therefore, should be determined by many factors, such as crystal orientation, orientation degree, microstructure, crystal defects, and so on. Terfenol-D rods with oriented grown crystalline can be obtained by using the corresponding seed crystals [9] or changing the solidification conditions [10], such as the temperature gradient at the solid–liquid interface and the crystal growth velocity. The authors prepared successfully the Terfenol-D rods with precise k110l orientation, only 38 dispersion to the rod axis, in a super high temperature gradient equipment by changing the directional solidification conditions. From the viewpoint of solidification, the crystal morphologies and the crystalline orientation degree, ranging from free oriented grains to the stable growth of the oriented crystals, were analyzed. The experiments also indicate that the entire process from the melting and casting of the Tb–Dy–Fe master alloy rod to the resulting
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01734-0
292
C. Ji et al. / Journal of Alloys and Compounds 333 (2002) 291 – 295
Fig. 1. Illustration of the specimens’ location cut from the solidified rod.
directional solidification can be fulfilled in the super high temperature gradient installation. The equipment is an ideal one to investigate and prepare rare earth giant magnetostrictive materials.
2. Experimental Tb 0.27 Dy 0.73 Fe 1.9 master rods with the dimension of f7 mm3120 mm were melted and cast in the super high temperature gradient equipment. The starting materials are Tb 99.53%, Dy 99.38% and Fe 99.869% [11], and consequently the master rods were directionally solidified in the same equipment after changing the heating coil. The master alloy rod is contained in a high purity and thin wall quartz crucible, of internal diameter 7.1 mm and length 120 mm, and fixed in the middle of a pancake induction coil. High purity argon gas was filled into the chamber after the vacuum system was evacuated to 2310 23 Pa, and the high purity argon atmosphere in the chamber is 0.08 MPa. An approximately 5 mm long molten zone is formed by heating with the pancake coil driven by a 175 KHz current. After the zone is molten fully, the quartz crucible is lowered in the velocity of 6.3 mm / s, 9.2 mm / s and 10.3 mm / s respectively, the temperature gradient is maintained about 800 K / cm. Specimens were cut from the solidified rod, as shown in Fig. 1, for metallographic examination and crystal orientation detection. A forms the start of the directional solidifi-
cation. Microstructure and morphology observations were carried out using both a LECO image analysis instrument and a scanning electron microscopy (SEM). Some specimens were etched with 4% Nital (4% nitric in ethanol) or Vilella’s reagent (1 g picric acid in 5 ml HCl plus 95 ml methanol) for an intensive metallographic examination. X-ray diffraction and the Laue X-ray technique were employed to examine the crystal orientation and the orientation degree.
3. Results and discussion
3.1. Crystal orientation All rods directionally solidified in the super high temperature equipment had k110l oriented crystallites. The results analyzed in this paper were obtained at a zone rate of 9.2 mm / s. The transverse sections of the specimens at the locations B and C were employed to determine the crystal orientations by X-ray diffraction, as shown in Fig. 2a and b, respectively. Fig. 2a indicates that the k110l orientation degree is not very strong when the length of the solidified part is 15 mm, and crystallites with k110l and k113l orientations exist together. When the length of the solidified part reaches 22 mm, the diffraction intensity of the (022) peak is markedly stronger than that of the (113) peak, as shown in Fig. 2b. This indicates that the k110l oriented crystallites have dominated the crystal growth direction at location C. With Laue X-ray technique it was detected that the crystallite orientation at the transverse section E of the solidified part of the rod is precisely k110l, and a dispersion of only 38 exists between the crystal growth direction and the rod axis, as shown in Fig. 3. In other words, when the solidified part of the rod reaches 45 mm, crystallites with precise k110l orientation can be obtained. Fig. 4 shows that the RFe 2 (R5TbDy) matrix phase and the rare earth-rich phase are distributed along the directional solidification direction after the k110l
Fig. 2. X-ray diffraction patterns of transverse sections at location B and C in Fig. 1.
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Fig. 3. Laue X-ray diffraction pattern of the transverse section at location E in Fig. 1.
oriented crystallites grow stably. The morphology is consistent with the result of the Laue X-ray diffraction. Connecting a series of experimental results, it is sure that, in the super high temperature gradient equipment, the precise k110l oriented crystallites of Terfenol-D can be obtained under the conditions of a temperature gradient of 800 K / cm and a withdrawal velocity of 6|10 mm / s. The crystallite orientation is mainly in the k110l direction when the solidified part of the rod arrives at 20 mm. When the length of the solidified part is about 30 mm, the crystallites still grow unstably along the k110l direction. Crystallites with precise k110l orientation can grow stably when the solidified part becomes longer. If a seed crystal is used, the unstable growth course can be greatly shortened.
3.2. The crystal morphologies Previous investigations [12–14] of the microstructure are always focused on the texture obtained when the crystallites grow stably, and studies of the microstructure in the non-stable state are rare. Fig. 5 shows the crystal growth morphologies of the transverse sections of A, B, C, D and E, and displays the different morphology of the k110l oriented crystal in the different crystal growth courses. Fig. 5A shows the free aligned grains in the as-cast master rod. Generally there exist k111l, k110l, k112l and k113l randomly oriented grains [15], and they do not have any crystallographic alignment. After the directional solidification began, more k110l oriented crystals approach the direction of the temperature gradient, and gradually dominate the crystal growth direction. The number of the k110l aligned grains is increased obviously as the solidified part reaches 15 mm, as shown in Fig. 5B. When the solidified part is 22 mm, the k110l oriented crystallites, as shown in Fig. 5C, have dominated the crystals growth direction. As the solidified part reaches 30 mm, the transverse section of the rod is almost taken up by k110l
Fig. 4. Optical micrograph of the longitudinal cross section along the EF direction.
aligned grains, as shown in Fig. 5D. However, the crystal growth is not stable, some other oriented grains can be found locally. When the rod length reaches 45 mm, the crystallites have grown steadily along the k110l direction, and become ‘single’ Terfenol-D crystals as shown in Fig. 5E. From the longitudinal micrograph, as shown in Fig. 4, it is clear that the phases arrange themselves uniformly and regularly. In fact, during the directional solidification, crystals with different orientations compete with each other, and the preferred growth of the crystals will wash out other growth directions and gradually take up the whole intersection. The metallographic examination of the transverse sections of the specimens shows that the crystals of TerfenolD grow with a planar or a cellular morphology under this solidification condition. Compared with the k112l oriented crystal growing with a dendritic morphology [6,7], it is
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C. Ji et al. / Journal of Alloys and Compounds 333 (2002) 291 – 295
Fig. 5. Optical morphologies of transverse sections at locations A, B, C, D, and E in Fig. 1.
obviously that the solidification condition is very important for the crystal orientation and the growth morphology of the Terfenol-D. This experiment also verified that the solidification characteristics of Tb–Dy–Fe alloy possesses a strong crystallographic anisotropy.
3.3. The crystal morphologies and the crystallines orientation degree Patterns of the X-ray diffraction and the morphology, as shown in Fig. 2 and Fig. 5 respectively, reveal clearly that
there exists a certain relationship between the crystal growth morphology and the orientation degree of the k110l oriented crystallites. There are two main groups of oriented grains in Fig. 5B, and their relative area is almost equal. Comparing with Fig. 2a, only peaks of (022) and (113) exist, and their diffraction intensity is nearly equal, too. In Fig. 5C, the main phase (k110l aligned grains) possesses approximately 75% of the area of the transverse section, the ratio of the main phase to the second phase being about 3. From Fig. 2b it is apparent that the ratio of I ( 022) / I (113 ) , I ( 022) and I( 113) is the diffraction intensity of (022) peak
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and (113) peak respectively, is 3.2, which nearly equals the value of the area ratio in Fig. 5C. Fig. 5E shows that the transverse section of the specimen is fully occupied by the k110l oriented crystallites, and the analysis of the Laue X-ray diffraction photo, shown in Fig. 3, indicates that the crystal orientation at location E is precise the k110l direction. The crystal morphology is completely in agreement with the crystalline orientation degree. Therefore, the crystalline orientation degree can be determined conveniently by the observation of the crystal morphologies. On the basis of the relationship of the crystal morphology and the crystallites’ orientation degree, the crystallite orientation shown in Fig. 5D should be exactly the k110l direction.
2. Terfenol-D with precise k110l orientation, only 38 off the rod axis, can be prepared in the super high temperature gradient equipment. 3. The crystal morphology is completely consistent to the crystallines orientation degree, and the orientation degree can be determined conveniently by the crystal morphologies. 4. Under the conditions of a temperature gradient of 800 K / cm and a zone rate of 6 mm / s|10 mm / s, Tb 0.27 Dy 0.73 Fe 1.9 rods with mainly k110l orientation can be obtained when the solidified part of the rod is over 20 mm. When solidified part is at least 30 mm, the k110l oriented crystal can grow stably.
3.4. Phase spacing
Acknowledgements
The phase spacing of the directionally solidified Terfenol-D rod with k110l orientation was also examined because the spacing distribution will affect the microstructure inconsistency and the property inconsistency [16]. In all specimens, there exists a phase spacing between the plate-shape phases. From Fig. 5B to Fig. 5E it is obvious that the phase spacing is almost equal except that in Fig. 5D, in which the spacing is greater. It possibly results from the unstable growth of the crystallites because the phase spacing can adjust automatically during the solidification course. The metallographic examination shows that the increase of the phase spacing occurs in the same location of the specimens. The average phase spacing in Fig. 5B, C, E is approximate 35|40 mm, and the average phase spacing in Fig. 5D is about 70|80 mm, the latter being almost two times of the former. However, they are smaller than those reported by Mei [16], because the spacing is more sensitive to the temperature gradient for the Tb–Dy–Fe alloy. The origin of the phase spacing transformation needs intensive study. In addition, the RFe 3 phase has not been observed in all specimens etched with 4% Nital or Vilella’s regent by both optical miocroscopy and SEM. The microstructure is composed of the main RFe 2 matrix phase and the second phase is arranged parallel to the crystal growth direction as demonstrated by the vertical dark bands.
The authors would like to thank Professor Qi Rui for her analyses of the Laue photos, and acknowledge helpful discussions with Professor Chen Shipu. This work is supported by the National Science Foundation of China.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
4. Conclusions [16]
1. Terfenol-D crystals can grow along k112l and k110l orientations under different solidification conditions.
J.R. Cullen, Scripta Metall. Mater. 33 (1995) 1849. F.R. De Boer, K.H.J. Buschow, J. Alloys Comp. 258 (1997) 1. D.C. Jiks, J. Phys. D: Appl. Phys. 27 (1994) 1. O.D. McMasters, J. Rare Earth 13 (4) (1995) 295. S. Ashley, Mech. Eng. (Am.) 120 (6) (1998) 68. J.D. Verhoeven, E.D. Gibson, O.D. McMasters, H.H. Baker, Metall. Trans. A 18A (1987) 223. A.E. Clark, J.D. Verhoeven, O.D. McMasters, E.D. Gibson, IEEE Trans. Magn. 22 (5) (1986) 973. O. Bonino, P. De Rango, R. Tournier, J. Magn. Magn. Mater. 212 (2000) 225. W. Guangheng, Z. Xuegen, W. Jinghua, L. Jingyuan, J. Kechang, Z. Wenshan, Appl. Phys. Lett. 67 (14) (1995) 2005. O.D. McMasters, J.D. Verhoeven, E.D. Gibson, J. Magn. Magn. Mater. 54–57 (1986) 849. J. Chengchang, M. Weizeng, L. Jianguo, Z. Yaohe, J. Alloys Comp. (in press). J.D. Verhoeven, E.D. Gibson, O.D. McMasters, J.E. Ostenson, Metall. Trans. A 21A (1990) 2249. J.D. Verhoeven, J.E. Ostenson, E.D. Gibson, O.D. McMasters, J. Appl. Phys. 66 (2) (1989) 772. P. Westwood, J.S. Abell, A.E. Clark, K.C. Pitman, J. Appl. Phys. 64 (10) (1988) 5414. S. Zhengxing, C. Zhongmin, W. Xianhui, L. Menghua, Z. Yunmeng, J. Alloys Comp. 258 (1997) 30. W. Mei, T. Okane, T. Umeda, Z. Shouzheng, J. Alloys Comp. 248 (1997) 151.