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Effect of a high magnetic field on the microstructure in directionally solidified Al–12 wt%Ni alloy
ARTICLE IN PRESS
Journal of Crystal Growth 306 (2007) 187–194 www.elsevier.com/locate/jcrysgro
Effect of a high magnetic field on the microstructure in directionally solidified Al–12 wt%Ni alloy Xi Lia,b,, Yves Fautrelleb, Zhongming Rena a
Department of Material Science and Engineering, Shanghai University, Shanghai, 200072, PR China b EPM-Madylam, ENSHMG BP 38402 St Martin d’Heres Cedex, France Received 13 February 2006; received in revised form 12 February 2007; accepted 13 April 2007 Communicated by M.E. Glicksman Available online 24 April 2007
Abstract Influence of an axial high magnetic field (up to 12 T) on the microstructure of the hypereutectic Al–12 wt%Ni alloy has been investigated. Results show that the magnetic field has affected the dendritic arrays significantly. As a consequence, Al3Ni phases are aligned in such a way that the planes of the primary Al3Ni phase are perpendicular to the magnetic field direction. Further, X-ray diffraction (XRD) results show that the /0 0 1S-crystal direction of the Al3Ni crystal orients along the magnetic field direction. During directional solidification under an axial magnetic field, the regular sandwich structure between the primary Al3Ni phase and the eutectic Al/Al3Ni phase is formed, and with the increase of growth speed, the size of the Al3Ni phase and the layer spacing decreases. The above experimental results have been discussed from magnetic and growth anisotropy of the crystal. Moreover, a method to control the crystallization process by means of magnetic field during directional solidification is proposed. r 2007 Elsevier B.V. All rights reserved. Keywords: A1. High magnetic field; A1. Orientation; A2. Directional solidification; B1. Al–Ni alloy; B2. Al3Ni phase
1. Introduction Solidification in a high magnetic field is an interesting topic and has attracted much attention of researchers ever since. The experimental researches have demonstrated that a high magnetic field presents significant influences on solidification structure. Mikelson and Karklin [1] obtained aligned solidification structure in Al–Cu and Cd–Zn alloys in a magnetic field of 1.5 T. Savitsky et al. [2] found that the MnBi phase in the alloy aligned along the magnetic field direction during solidification of Bi–Mn alloy in a magnetic field. Rango et al. [3] extended the investigation to the solidification of paramagnetic YBa2Cu3O7 material and obtained texture crystal structure in a magnetic field. Katsuki et al. [4] reported that diamagnetic benzophenone crystallized from n-hexane and KCl, and BaCl2 crystallized from solution aligned in a magnetic field of 10 T. This Corresponding author. EPM-Madylam, ENSHMG BP 38402 St Martin d’Heres Cedex, France. Tel.: +33 4 76 82 52 62. E-mail address: [email protected] (X. Li).
0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.04.036
alignment behavior has been investigated intensively by several researchers [5–7]. Asai [5] found perpendicular alignment in Al–Si–Fe alloy during solidification in a high magnetic field. Texture crystal growth of Bi–2201 [8] and Bi (Pb)2212 [9] has also been obtained in a high magnetic field. During directional solidification, magnetic field is always applied to damp the convection; and recently the effect of thermoelectromagnetic convection has been investigated [10–12]. However, there are hitherto few papers to investigate the effect of magnetic field on crystal orientation during directional solidification. The author has studied effects of a high magnetic field on the growth of a-Al and MnBi crystal in directionally solidified Al–4.5 wt%Cu and Bi–0.82 wt%Mn alloys, respectively [13,14]; and found that an axial magnetic field causes the /1 1 1S-crystal direction of a-Al crystal to orient along the solidification direction instead of the /1 0 0S-crystal direction. For MnBi crystal, the magnetic field has enhanced the growth of the /0 0 1S-crystal direction along the solidification direction. In this paper, the crystal orientation during directional solidification in a high
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magnetic field is extended by introducing the orientation of the Al3Ni crystal in Al–12 wt%Ni alloy. The purpose is to report on the orientation behavior of Al3Ni phase and propose a method to control the crystal orientation by means of Bridgman growth in a high magnetic field. 2. Experimental Al–12 wt%Ni alloy used in this study was prepared with high-purity Al (99.99 wt%) and Ni (99.9 wt%) in an induction furnace. The alloy, being put in a high-purity graphite crucible of 10 cm diameter, was heated to 900 1C, magnetically stirred for 0.5 h, and poured into a graphite mold to cast specimens with a diameter of 3 and 10 mm, respectively. The cast specimen with a diameter of 10 mm was enveloped in a tube of high-purity graphite with an inner diameter of 10 mm and a length of 30 mm. The sample was heated to 750 1C and held for 30 min, and then cooled below 500 1C at a cool rate of 18 K/min without and with a magnetic field of 10 T. The cast specimen with a diameter of 3 mm was enveloped in a tube of high-purity corundum with an inner diameter of 3 mm and a length of 200 mm for directional solidification. The directional solidification apparatus is shown in Fig. 1. It consists of a static superconductor magnet, a Bridgman furnace, and a growth velocity and temperature controller. The superconductor magnet could produce a vertical static magnetic field with the adjustable intensity up to 14 T. The thermocouple temperature used to control the temperature was 70.1 1C. A water-cooled cylinder containing liquid Ga–In–Sn metal (LMC) was used to cool down the
Heating elements Thermal insulation
Liquid
B
Center plane
Superconductor magnet
Solid
Thermal insulation disc
Crucible
LMC
Cold water
Water input
Water output
Crucible displacement Fig. 1. Schematic illustration of the Bridgman apparatus in a superconducting magnet.
specimen. The temperature gradient in the specimen was controlled by adjusting the temperature of the furnace’s hot zone, which was insulated from the LMC by a refractory disc. To determine the temperature gradient, a thermocouple was placed in the sample during directional solidification. The ratio of liquid temperature near the liquid–solid interface to the distance was used to measure the temperature gradient. The apparatus was designed such that the specimen moved downward while the furnace remains stationary. During the experiment, the specimens contained in the corundum crucibles were melted and directionally solidified in the Bridgman furnace by drawing the crucible assembly at various speeds into the LMC cylinder. The longitudinal (parallel to the growth direction) and transverse microstructures were examined in the etched condition by optical microscope, scanning electron microscopy and X-ray diffraction (XRD) with Cu-Ka. Layer spacing was evaluated by a computer-aided WT-MiVnt analyzer system. 3. Results In order to investigate the magnetic anisotropy of the Al3Ni phase, Al–12 wt%Ni alloy was solidified from 750 1C at a rate of 18 K/min with a magnetic field of 10 T. Fig. 2 shows the microstructures without and with a magnetic field of 10 T, respectively; and the black phases are the primary Al3Ni ones. Fig. 2(a) and (b) shows the microstructures on the sections parallel and perpendicular to the magnetic field direction, respectively. It can be observed that the Al3Ni phase was aligned in such a way that the plane of the Al3Ni phase is perpendicular to the magnetic field direction. Fig. 2(c) shows the microstructure solidified without the influence of the field and it can be observed that the microstructure has the usual equiaxed structure. Further, the XRD was used to investigate the orientation of the Al3Ni crystal. The XRD results (Fig. 3) show that on the section perpendicular to the magnetic field direction, the peak that indicates the (0 0 4)-crystal plane increases; and on the section parallel to the magnetic field direction, the peaks that indicate the (h k 0)-crystal planes increase by the application of the magnetic field. This indicates that the /0 0 1S-crystal direction orients along the magnetic field direction and that the Al3Ni phase has a remarkable magnetic anisotropy. Fig. 4 shows the microstructures of Al–12 wt%Ni alloy directionally solidified at 100 mm/s and a temperature gradient in the liquid (GT) of 68 K/cm without and with a magnetic field. Fig. 4(a) and (c) shows the microstructures on the sections parallel and perpendicular to the solidification direction, respectively. It can be seen that the Al3Ni dendrite aligns along the solidification direction. When a magnetic field of 12 T is imposed, comparing with the microstructures in the case of no field, the microstructures [Fig. 4(b) and (d)] are changed significantly, as a consequence, the Al3Ni phases are oriented in such a way that the plane of the Al3Ni phase is perpendicular to the
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Intensity (a.u.)
Fig. 2. Equiaxed microstructures solidified from 750 1C at a rate of 18 K/min: (a) longitudinal section, 10 T; (b) transverse section, 10 T; (c) 0 T.
10
20
30
40
50 2θ (deg)
60
70
80
90
Fig. 3. XRD of the samples in Fig. 2: (a) longitudinal section, 10 T; (b) transverse section, 10 T; (c) without magnetic field.
solidification direction and form the regular sandwich structure. Fig. 5(a) and (b) indicates the three-dimensional schematic drawing and photomicrograph of the above sample, respectively. Structures solidified at 100 mm/s under various magnetic field intensities (Fig. 6) have been investigated and the result is shown Fig. 6. It can be observed that when a magnetic field 2 T is imposed, the aligned structure begins to deviate from the solidification direction, and when magnetic field intensity increases to 4 T, the deviation increases. When the magnetic field intensity exceeds 8 T, the longer axis of the Al3Ni phase
(i.e., the preferred growth direction) orients perpendicular to the solidification direction and forms the sandwich structure at last. The structure transformation process with the increase of the magnetic field intensity indicates clearly that the preferred growth direction of the Al3Ni crystal deviates from the solidification direction gradually and forms the planes perpendicular to the solidification direction (i.e., the magnetic field direction). The structures solidified at various growth speeds in a magnetic field of 10 T have also been investigated and Fig. 7 shows the experimental results. It can be observed that the sandwich structure is formed at various
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Fig. 4. Effect of a high magnetic field on the microstructures directionally solidified at a growth speed of 100 mm/s and a temperature gradient of 68 K/cm: (a) longtudinal section, 0 T; (b) longtudinal section, 12 T; (c) transverse section, 0 T; (d) transverse section, 12 T.
Fig. 5. Three-dimensional diagram of the sandwich structure. (a) Schematic drawing; (b) photomicrograph of the sample directionally solidified at 100 mm/s and a temperature gradient of GT ¼ 68 K/cm under a magnetic field of 12 T.
growth speeds; however, the layer spacing and the size of the Al3Ni phase are changed at various growth speeds. It can be learned that with the increase in the growth speed, the layer spacing and the size of the Al3Ni phase decreases. Fig. 8 shows the layer spacing at various growth speeds under a magnetic field of 10 T, and it seems that there is a hyperbolic relationship between speed and spacing.
4. Discussion The magnetic property of the Al3Ni crystal was investigated by measuring the magnetization force of the Al3Ni crystal in a gradient magnetic field. According to the phase diagram of Al–Ni alloy [15], phases in Al–40 wt%Ni alloy are almost Al3Ni ones; therefore, the magnetic
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Fig. 6. Longitudinal structure solidified at 100 mm/s under various magnetic field intensities. (a)0 T; (b)2 T; (c)4 T; (d)6 T; (e)8 T; (f) 10 T; (g)12 T.
Fig. 7. Longitudinal structure directionally solidified at various growth speeds in a magnetic field of 10 T (G ¼ 68 K/cm). (a) 20 mm/s; (b) 50 mm/s; (c) 100 mm/s; (d) 200 mm/s.
property of the Al3Ni crystal may be determined by measuring one of Al–40 wt%Ni alloys. In our experiments, the typical field and force profile of the magnet obtained are presented in Fig. 9. The magnetization force can be expressed as F z ¼ ðw=m0 ÞBz dBz =dz,
(1)
where w is the susceptibility of the substance, m0 is the permeability of free space, Bz is the magnetic field
induction or magnetic flux density on the vertical direction. In SI. units, F is in N/kg, and w in m3/kg, and the value of m0 is 4p 107 H=m. The measuring result indicates that the magnetization force of the sample is downward at the position z40. Therefore, the magnetic property of the Al3Ni phase may be confirmed as paramagnetism. It is well known that when an anisotropic grain with a volume V is placed under a magnetic field H, it will be rotated by the field to its lowest energy state. The driving
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c
700
Distance between lays (μm)
600
10T, G = 68K/cm
B
500
ab
400 300 200
Fig. 10. An anisotropic grain under a magnetic field. B is the magnetic field direction, c and ab are the crystallographic axes, and y is the angle between the magnetic field and the c axis.
100 0 0
50
100
150
200
250
Growth speed R(μm/s)
12
600
10
400
8
200 0
6
-200
4
GZ (T2/m)
BZ (T)
Fig. 8. Layer spacing of the aligned Al3Ni phase vs the growth speed during directional solidification under a magnetic field of 10 T.
-400
2
-600 0 -24 -20 -16 -12 -8
-4
0 4 z(cm)
8
12 16 20 24
Fig. 9. Field profiles in the magnetic field operating at 12 T. Bz is the vertical component of the field on the axis of the coil. Gz is the vertical component of grad (B2a/2) on the axis of the coil (i.e. BzdBz/dz), z is the distance above the coil.
force is given by the anisotropic magnetic energy of the grains and the magnetic energy of a grain can be written as E m ðy; HÞ ¼ ðxab þ Dx cos2 yÞVH 2 =2,
(2)
where y is the angle between the magnetic field and the c-axis of the grain (Fig. 10), and Dx ¼ xc xab is the difference in the volume susceptibilities of the grains between the c-axis and the ab-axis. Hence, when y ¼ 0, we have E m ð0; HÞ ¼ Vxc H 2 =2,
(3)
whereas when y ¼ p=2, we have E m ðp=2; HÞ ¼ Vxab H 2 =2.
(4)
For the cases where xc 4xab , then E m ð0; HÞoE m ðp=2; HÞ and this means that the magnetic field will tend to rotate the grains to y ¼ 0, a lower energy state. On the other hand,
when xc oxab and E m ð0; HÞ4E m ðp=2; HÞ the magnetic field will tend to rotate the grains to y ¼ p=2. This means that for a paramagnetic crystal, the easy magnetic axis tends to turn along the magnetic field direction. The above experimental results indicate that the magnetic field does not affect the morphology of the Al3Ni phase. Therefore, the orientation of the Al3Ni crystal should be attributed to the rotation of crystals caused by the magnetic field. As the /0 0 1S-crystal direction of the Al3Ni crystal orients along the magnetic field direction, it can be deduced that the /0 0 1S-crystal direction is the easy magnetization axis of the Al3Ni crystal. During directional solidification under a magnetic field, an anisotropic crystal will grow along the preferred growth direction and at the same time the field tends to rotate the easy axis of the crystal to the magnetic field direction. Thus, when the easy axis of a crystal is same as the preferred growth direction, an axial magnetic field enhances the growth along the solidification direction during the directional solidification process. As the /0 0 1S-crystal direction of the MnBi crystal is not only an easy magnetization axis but also a preferred growth direction [16], the MnBi crystal in the hypereutectic Bi–0.82 wt%Mn alloy is chosen as an example. Fig. 11(a) shows the longitudinal microstructure solidified at a growth speed of 5 mm/s and a temperature gradient of 50 K/cm under an axial magnetic field of 10 T. It can be observed that the field has enhanced the growth of the MnBi crystal along the solidification direction. However, if there exists an angle between the easy magnetization axis and the preferred growth direction, the preferred growth direction will deviate from the solidification direction while the easy magnetization axis rotates to the magnetic field direction. For example, Al crystal, whose /0 0 1S- and /1 1 1S-crystal directions are the preferred growth direction and the easy magnetization, respectively, is orientated with the /1 1 1S-crystal direction along the solidification direction instead of the /1 0 0S-crystal direction in directionally solidified hypoeutectic Al–4.5 wt%Cu alloy under an axial magnetic field of 10 T (Fig. 11(b)) [13]. Especially, when the easy magnetization axis of a crystal is perpendicular to its preferred growth direction, the magnetic field will cause the preferred growth direction to turn to the plane perpendicular
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Fig. 11. Schematic illustration of crystal orientation with a corresponding example for different crystal structure during directional solidification in an axial magnetic field. (a) Easy magnetization axis is parallel to preferred growth direction, with an example of MnBi crystal in Bi-0.82 wt%Mn alloy; (b) there exists a certain angle between easy magnetization axis and preferred growth direction, with an example of Al crystal in Al–4.5 wt%Cu alloy; (c) easy magnetization axis is perpendicular to preferred growth direction, with an example of Al3Ni crystal in Al–12 wt%Ni alloy.
to the solidification direction; as a consequence, the layer structure is formed. The orientation of the Al3Ni crystal in this experiment should work in this way. This means that the crystallization process can be controlled by means of directional solidification in a magnetic field. The orientation direction may be adjusted by changing the angle between the solidification and magnetic field directions. Thus, any oriented material can be obtained by adjusting the magnetic direction during directional solidification. 5. Conclusions Microstructures solidified in a high magnetic field (up to 12 T) were investigated and it was found that the Al3Ni crystal oriented in such a way that the planes of Al3Ni phases were perpendicular to the magnetic field direction and formed the sandwich structure. XRD patterns indicate that the /0 0 1S-crystal direction of the Al3Ni crystal orients along the magnetic field direction. During directional solidification under an axial magnetic field, the regular sandwich structure was formed, and the size of Al3Ni phases and the layer spacing can be controlled by adjusting the growth speed. Moreover, the magnetic property of the Al3Ni crystal was determined as paramagnetism by means of measuring the magnetization force in a gradient magnetic field. From magnetic and growth anisotropy of the crystal, the solidification behavior in a magnetic field was analyzed and a method to control the
crystallization process by means of magnetic field during directional solidification was proposed. Acknowledgments This work is supported by the European Space Agency through the IMPRESS project and Natural Science Foundation of China (Nos. 50234020, 50225416 and 59871026) and the Science and Technology Committee of Shanghai and one of the authors (LX) is also grateful for an Egide/Eiffel Doctorate Scholarship. References [1] A.E. Mikelson, Y.K. Karklin, J. Crystal Growth 52 (1981) 524. [2] E.M. Savitisky, R.S. Torchinova, S.A.J. Turanoy, J. Crystal Growth 52 (1981) 519. [3] P.D. Rango, M. Lee, P. Lejay, A. Sulpice, et al., Nature 349 (1991) 770. [4] A. Katsuki, R. Tokunaga, S.I. Watanabe, Chem. Lett. 8 (1996) 607. [5] H. Wang, Z.M. Ren, K. Deng, Acta Metall, Sinica 38 (2002) 41. [6] H. Yasuda, I. Ohnaka, Y. Yamamoto, Mater. Trans. JIM 44 (2003) 2550. [7] H. Morikawa, K. Sassa, S. Asai, Mater. Trans. JIM 139 (1998) 814. [8] X.Y. Lu, A. Nagata, K. Watanabe, T. Nojima, K. Sugawara, S. Hannda, S. Kamada, Physica C 412 (2002) 602. [9] W.P. Chen, H. Maeda, K. Kakimoto, P.X. Zhang, K. Watanabe, M. Motokawa, H. Kumakura, K. Itoh, J. Crystal Growth 204 (1999) 69. [10] A. Croll, F.R. Szofran, P. Dold, K.W. Brez, S.L. Lehoczky, J. Crystal Growth 183 (1998) 554.
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[11] T. Alboussiere, R. Moreau, D. Camel, C.R. Acad. Sci. Paris 313/II (1991) 749. [12] R. Moreau, O. Laskar, M. Tanaka, D. Camel, Mater. Sci. Eng. A 173 (1993) 93. [13] X. Li, Z.M. Ren, Y. Fautrelle, J. Crystal Growth 290 (2006) 571.
[14] X. Li, Z.M. Ren, Trans. Chinese Non-ferrous Metals Soc., accepted. [15] W.G. Moffatt, The Handbook of Binary Phase Diagrams, Genium, USA, 1984, p. 83. [16] M.R. Notis, D.M. Shah, S.P. Young, C.D. Graham, IEEE 75 (1979) 957.
Journal of Crystal Growth 306 (2007) 187–194 www.elsevier.com/locate/jcrysgro
Effect of a high magnetic field on the microstructure in directionally solidified Al–12 wt%Ni alloy Xi Lia,b,, Yves Fautrelleb, Zhongming Rena a
Department of Material Science and Engineering, Shanghai University, Shanghai, 200072, PR China b EPM-Madylam, ENSHMG BP 38402 St Martin d’Heres Cedex, France Received 13 February 2006; received in revised form 12 February 2007; accepted 13 April 2007 Communicated by M.E. Glicksman Available online 24 April 2007
Abstract Influence of an axial high magnetic field (up to 12 T) on the microstructure of the hypereutectic Al–12 wt%Ni alloy has been investigated. Results show that the magnetic field has affected the dendritic arrays significantly. As a consequence, Al3Ni phases are aligned in such a way that the planes of the primary Al3Ni phase are perpendicular to the magnetic field direction. Further, X-ray diffraction (XRD) results show that the /0 0 1S-crystal direction of the Al3Ni crystal orients along the magnetic field direction. During directional solidification under an axial magnetic field, the regular sandwich structure between the primary Al3Ni phase and the eutectic Al/Al3Ni phase is formed, and with the increase of growth speed, the size of the Al3Ni phase and the layer spacing decreases. The above experimental results have been discussed from magnetic and growth anisotropy of the crystal. Moreover, a method to control the crystallization process by means of magnetic field during directional solidification is proposed. r 2007 Elsevier B.V. All rights reserved. Keywords: A1. High magnetic field; A1. Orientation; A2. Directional solidification; B1. Al–Ni alloy; B2. Al3Ni phase
1. Introduction Solidification in a high magnetic field is an interesting topic and has attracted much attention of researchers ever since. The experimental researches have demonstrated that a high magnetic field presents significant influences on solidification structure. Mikelson and Karklin [1] obtained aligned solidification structure in Al–Cu and Cd–Zn alloys in a magnetic field of 1.5 T. Savitsky et al. [2] found that the MnBi phase in the alloy aligned along the magnetic field direction during solidification of Bi–Mn alloy in a magnetic field. Rango et al. [3] extended the investigation to the solidification of paramagnetic YBa2Cu3O7 material and obtained texture crystal structure in a magnetic field. Katsuki et al. [4] reported that diamagnetic benzophenone crystallized from n-hexane and KCl, and BaCl2 crystallized from solution aligned in a magnetic field of 10 T. This Corresponding author. EPM-Madylam, ENSHMG BP 38402 St Martin d’Heres Cedex, France. Tel.: +33 4 76 82 52 62. E-mail address: [email protected] (X. Li).
0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.04.036
alignment behavior has been investigated intensively by several researchers [5–7]. Asai [5] found perpendicular alignment in Al–Si–Fe alloy during solidification in a high magnetic field. Texture crystal growth of Bi–2201 [8] and Bi (Pb)2212 [9] has also been obtained in a high magnetic field. During directional solidification, magnetic field is always applied to damp the convection; and recently the effect of thermoelectromagnetic convection has been investigated [10–12]. However, there are hitherto few papers to investigate the effect of magnetic field on crystal orientation during directional solidification. The author has studied effects of a high magnetic field on the growth of a-Al and MnBi crystal in directionally solidified Al–4.5 wt%Cu and Bi–0.82 wt%Mn alloys, respectively [13,14]; and found that an axial magnetic field causes the /1 1 1S-crystal direction of a-Al crystal to orient along the solidification direction instead of the /1 0 0S-crystal direction. For MnBi crystal, the magnetic field has enhanced the growth of the /0 0 1S-crystal direction along the solidification direction. In this paper, the crystal orientation during directional solidification in a high
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magnetic field is extended by introducing the orientation of the Al3Ni crystal in Al–12 wt%Ni alloy. The purpose is to report on the orientation behavior of Al3Ni phase and propose a method to control the crystal orientation by means of Bridgman growth in a high magnetic field. 2. Experimental Al–12 wt%Ni alloy used in this study was prepared with high-purity Al (99.99 wt%) and Ni (99.9 wt%) in an induction furnace. The alloy, being put in a high-purity graphite crucible of 10 cm diameter, was heated to 900 1C, magnetically stirred for 0.5 h, and poured into a graphite mold to cast specimens with a diameter of 3 and 10 mm, respectively. The cast specimen with a diameter of 10 mm was enveloped in a tube of high-purity graphite with an inner diameter of 10 mm and a length of 30 mm. The sample was heated to 750 1C and held for 30 min, and then cooled below 500 1C at a cool rate of 18 K/min without and with a magnetic field of 10 T. The cast specimen with a diameter of 3 mm was enveloped in a tube of high-purity corundum with an inner diameter of 3 mm and a length of 200 mm for directional solidification. The directional solidification apparatus is shown in Fig. 1. It consists of a static superconductor magnet, a Bridgman furnace, and a growth velocity and temperature controller. The superconductor magnet could produce a vertical static magnetic field with the adjustable intensity up to 14 T. The thermocouple temperature used to control the temperature was 70.1 1C. A water-cooled cylinder containing liquid Ga–In–Sn metal (LMC) was used to cool down the
Heating elements Thermal insulation
Liquid
B
Center plane
Superconductor magnet
Solid
Thermal insulation disc
Crucible
LMC
Cold water
Water input
Water output
Crucible displacement Fig. 1. Schematic illustration of the Bridgman apparatus in a superconducting magnet.
specimen. The temperature gradient in the specimen was controlled by adjusting the temperature of the furnace’s hot zone, which was insulated from the LMC by a refractory disc. To determine the temperature gradient, a thermocouple was placed in the sample during directional solidification. The ratio of liquid temperature near the liquid–solid interface to the distance was used to measure the temperature gradient. The apparatus was designed such that the specimen moved downward while the furnace remains stationary. During the experiment, the specimens contained in the corundum crucibles were melted and directionally solidified in the Bridgman furnace by drawing the crucible assembly at various speeds into the LMC cylinder. The longitudinal (parallel to the growth direction) and transverse microstructures were examined in the etched condition by optical microscope, scanning electron microscopy and X-ray diffraction (XRD) with Cu-Ka. Layer spacing was evaluated by a computer-aided WT-MiVnt analyzer system. 3. Results In order to investigate the magnetic anisotropy of the Al3Ni phase, Al–12 wt%Ni alloy was solidified from 750 1C at a rate of 18 K/min with a magnetic field of 10 T. Fig. 2 shows the microstructures without and with a magnetic field of 10 T, respectively; and the black phases are the primary Al3Ni ones. Fig. 2(a) and (b) shows the microstructures on the sections parallel and perpendicular to the magnetic field direction, respectively. It can be observed that the Al3Ni phase was aligned in such a way that the plane of the Al3Ni phase is perpendicular to the magnetic field direction. Fig. 2(c) shows the microstructure solidified without the influence of the field and it can be observed that the microstructure has the usual equiaxed structure. Further, the XRD was used to investigate the orientation of the Al3Ni crystal. The XRD results (Fig. 3) show that on the section perpendicular to the magnetic field direction, the peak that indicates the (0 0 4)-crystal plane increases; and on the section parallel to the magnetic field direction, the peaks that indicate the (h k 0)-crystal planes increase by the application of the magnetic field. This indicates that the /0 0 1S-crystal direction orients along the magnetic field direction and that the Al3Ni phase has a remarkable magnetic anisotropy. Fig. 4 shows the microstructures of Al–12 wt%Ni alloy directionally solidified at 100 mm/s and a temperature gradient in the liquid (GT) of 68 K/cm without and with a magnetic field. Fig. 4(a) and (c) shows the microstructures on the sections parallel and perpendicular to the solidification direction, respectively. It can be seen that the Al3Ni dendrite aligns along the solidification direction. When a magnetic field of 12 T is imposed, comparing with the microstructures in the case of no field, the microstructures [Fig. 4(b) and (d)] are changed significantly, as a consequence, the Al3Ni phases are oriented in such a way that the plane of the Al3Ni phase is perpendicular to the
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Intensity (a.u.)
Fig. 2. Equiaxed microstructures solidified from 750 1C at a rate of 18 K/min: (a) longitudinal section, 10 T; (b) transverse section, 10 T; (c) 0 T.
10
20
30
40
50 2θ (deg)
60
70
80
90
Fig. 3. XRD of the samples in Fig. 2: (a) longitudinal section, 10 T; (b) transverse section, 10 T; (c) without magnetic field.
solidification direction and form the regular sandwich structure. Fig. 5(a) and (b) indicates the three-dimensional schematic drawing and photomicrograph of the above sample, respectively. Structures solidified at 100 mm/s under various magnetic field intensities (Fig. 6) have been investigated and the result is shown Fig. 6. It can be observed that when a magnetic field 2 T is imposed, the aligned structure begins to deviate from the solidification direction, and when magnetic field intensity increases to 4 T, the deviation increases. When the magnetic field intensity exceeds 8 T, the longer axis of the Al3Ni phase
(i.e., the preferred growth direction) orients perpendicular to the solidification direction and forms the sandwich structure at last. The structure transformation process with the increase of the magnetic field intensity indicates clearly that the preferred growth direction of the Al3Ni crystal deviates from the solidification direction gradually and forms the planes perpendicular to the solidification direction (i.e., the magnetic field direction). The structures solidified at various growth speeds in a magnetic field of 10 T have also been investigated and Fig. 7 shows the experimental results. It can be observed that the sandwich structure is formed at various
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Fig. 4. Effect of a high magnetic field on the microstructures directionally solidified at a growth speed of 100 mm/s and a temperature gradient of 68 K/cm: (a) longtudinal section, 0 T; (b) longtudinal section, 12 T; (c) transverse section, 0 T; (d) transverse section, 12 T.
Fig. 5. Three-dimensional diagram of the sandwich structure. (a) Schematic drawing; (b) photomicrograph of the sample directionally solidified at 100 mm/s and a temperature gradient of GT ¼ 68 K/cm under a magnetic field of 12 T.
growth speeds; however, the layer spacing and the size of the Al3Ni phase are changed at various growth speeds. It can be learned that with the increase in the growth speed, the layer spacing and the size of the Al3Ni phase decreases. Fig. 8 shows the layer spacing at various growth speeds under a magnetic field of 10 T, and it seems that there is a hyperbolic relationship between speed and spacing.
4. Discussion The magnetic property of the Al3Ni crystal was investigated by measuring the magnetization force of the Al3Ni crystal in a gradient magnetic field. According to the phase diagram of Al–Ni alloy [15], phases in Al–40 wt%Ni alloy are almost Al3Ni ones; therefore, the magnetic
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Fig. 6. Longitudinal structure solidified at 100 mm/s under various magnetic field intensities. (a)0 T; (b)2 T; (c)4 T; (d)6 T; (e)8 T; (f) 10 T; (g)12 T.
Fig. 7. Longitudinal structure directionally solidified at various growth speeds in a magnetic field of 10 T (G ¼ 68 K/cm). (a) 20 mm/s; (b) 50 mm/s; (c) 100 mm/s; (d) 200 mm/s.
property of the Al3Ni crystal may be determined by measuring one of Al–40 wt%Ni alloys. In our experiments, the typical field and force profile of the magnet obtained are presented in Fig. 9. The magnetization force can be expressed as F z ¼ ðw=m0 ÞBz dBz =dz,
(1)
where w is the susceptibility of the substance, m0 is the permeability of free space, Bz is the magnetic field
induction or magnetic flux density on the vertical direction. In SI. units, F is in N/kg, and w in m3/kg, and the value of m0 is 4p 107 H=m. The measuring result indicates that the magnetization force of the sample is downward at the position z40. Therefore, the magnetic property of the Al3Ni phase may be confirmed as paramagnetism. It is well known that when an anisotropic grain with a volume V is placed under a magnetic field H, it will be rotated by the field to its lowest energy state. The driving
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c
700
Distance between lays (μm)
600
10T, G = 68K/cm
B
500
ab
400 300 200
Fig. 10. An anisotropic grain under a magnetic field. B is the magnetic field direction, c and ab are the crystallographic axes, and y is the angle between the magnetic field and the c axis.
100 0 0
50
100
150
200
250
Growth speed R(μm/s)
12
600
10
400
8
200 0
6
-200
4
GZ (T2/m)
BZ (T)
Fig. 8. Layer spacing of the aligned Al3Ni phase vs the growth speed during directional solidification under a magnetic field of 10 T.
-400
2
-600 0 -24 -20 -16 -12 -8
-4
0 4 z(cm)
8
12 16 20 24
Fig. 9. Field profiles in the magnetic field operating at 12 T. Bz is the vertical component of the field on the axis of the coil. Gz is the vertical component of grad (B2a/2) on the axis of the coil (i.e. BzdBz/dz), z is the distance above the coil.
force is given by the anisotropic magnetic energy of the grains and the magnetic energy of a grain can be written as E m ðy; HÞ ¼ ðxab þ Dx cos2 yÞVH 2 =2,
(2)
where y is the angle between the magnetic field and the c-axis of the grain (Fig. 10), and Dx ¼ xc xab is the difference in the volume susceptibilities of the grains between the c-axis and the ab-axis. Hence, when y ¼ 0, we have E m ð0; HÞ ¼ Vxc H 2 =2,
(3)
whereas when y ¼ p=2, we have E m ðp=2; HÞ ¼ Vxab H 2 =2.
(4)
For the cases where xc 4xab , then E m ð0; HÞoE m ðp=2; HÞ and this means that the magnetic field will tend to rotate the grains to y ¼ 0, a lower energy state. On the other hand,
when xc oxab and E m ð0; HÞ4E m ðp=2; HÞ the magnetic field will tend to rotate the grains to y ¼ p=2. This means that for a paramagnetic crystal, the easy magnetic axis tends to turn along the magnetic field direction. The above experimental results indicate that the magnetic field does not affect the morphology of the Al3Ni phase. Therefore, the orientation of the Al3Ni crystal should be attributed to the rotation of crystals caused by the magnetic field. As the /0 0 1S-crystal direction of the Al3Ni crystal orients along the magnetic field direction, it can be deduced that the /0 0 1S-crystal direction is the easy magnetization axis of the Al3Ni crystal. During directional solidification under a magnetic field, an anisotropic crystal will grow along the preferred growth direction and at the same time the field tends to rotate the easy axis of the crystal to the magnetic field direction. Thus, when the easy axis of a crystal is same as the preferred growth direction, an axial magnetic field enhances the growth along the solidification direction during the directional solidification process. As the /0 0 1S-crystal direction of the MnBi crystal is not only an easy magnetization axis but also a preferred growth direction [16], the MnBi crystal in the hypereutectic Bi–0.82 wt%Mn alloy is chosen as an example. Fig. 11(a) shows the longitudinal microstructure solidified at a growth speed of 5 mm/s and a temperature gradient of 50 K/cm under an axial magnetic field of 10 T. It can be observed that the field has enhanced the growth of the MnBi crystal along the solidification direction. However, if there exists an angle between the easy magnetization axis and the preferred growth direction, the preferred growth direction will deviate from the solidification direction while the easy magnetization axis rotates to the magnetic field direction. For example, Al crystal, whose /0 0 1S- and /1 1 1S-crystal directions are the preferred growth direction and the easy magnetization, respectively, is orientated with the /1 1 1S-crystal direction along the solidification direction instead of the /1 0 0S-crystal direction in directionally solidified hypoeutectic Al–4.5 wt%Cu alloy under an axial magnetic field of 10 T (Fig. 11(b)) [13]. Especially, when the easy magnetization axis of a crystal is perpendicular to its preferred growth direction, the magnetic field will cause the preferred growth direction to turn to the plane perpendicular
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Fig. 11. Schematic illustration of crystal orientation with a corresponding example for different crystal structure during directional solidification in an axial magnetic field. (a) Easy magnetization axis is parallel to preferred growth direction, with an example of MnBi crystal in Bi-0.82 wt%Mn alloy; (b) there exists a certain angle between easy magnetization axis and preferred growth direction, with an example of Al crystal in Al–4.5 wt%Cu alloy; (c) easy magnetization axis is perpendicular to preferred growth direction, with an example of Al3Ni crystal in Al–12 wt%Ni alloy.
to the solidification direction; as a consequence, the layer structure is formed. The orientation of the Al3Ni crystal in this experiment should work in this way. This means that the crystallization process can be controlled by means of directional solidification in a magnetic field. The orientation direction may be adjusted by changing the angle between the solidification and magnetic field directions. Thus, any oriented material can be obtained by adjusting the magnetic direction during directional solidification. 5. Conclusions Microstructures solidified in a high magnetic field (up to 12 T) were investigated and it was found that the Al3Ni crystal oriented in such a way that the planes of Al3Ni phases were perpendicular to the magnetic field direction and formed the sandwich structure. XRD patterns indicate that the /0 0 1S-crystal direction of the Al3Ni crystal orients along the magnetic field direction. During directional solidification under an axial magnetic field, the regular sandwich structure was formed, and the size of Al3Ni phases and the layer spacing can be controlled by adjusting the growth speed. Moreover, the magnetic property of the Al3Ni crystal was determined as paramagnetism by means of measuring the magnetization force in a gradient magnetic field. From magnetic and growth anisotropy of the crystal, the solidification behavior in a magnetic field was analyzed and a method to control the
crystallization process by means of magnetic field during directional solidification was proposed. Acknowledgments This work is supported by the European Space Agency through the IMPRESS project and Natural Science Foundation of China (Nos. 50234020, 50225416 and 59871026) and the Science and Technology Committee of Shanghai and one of the authors (LX) is also grateful for an Egide/Eiffel Doctorate Scholarship. References [1] A.E. Mikelson, Y.K. Karklin, J. Crystal Growth 52 (1981) 524. [2] E.M. Savitisky, R.S. Torchinova, S.A.J. Turanoy, J. Crystal Growth 52 (1981) 519. [3] P.D. Rango, M. Lee, P. Lejay, A. Sulpice, et al., Nature 349 (1991) 770. [4] A. Katsuki, R. Tokunaga, S.I. Watanabe, Chem. Lett. 8 (1996) 607. [5] H. Wang, Z.M. Ren, K. Deng, Acta Metall, Sinica 38 (2002) 41. [6] H. Yasuda, I. Ohnaka, Y. Yamamoto, Mater. Trans. JIM 44 (2003) 2550. [7] H. Morikawa, K. Sassa, S. Asai, Mater. Trans. JIM 139 (1998) 814. [8] X.Y. Lu, A. Nagata, K. Watanabe, T. Nojima, K. Sugawara, S. Hannda, S. Kamada, Physica C 412 (2002) 602. [9] W.P. Chen, H. Maeda, K. Kakimoto, P.X. Zhang, K. Watanabe, M. Motokawa, H. Kumakura, K. Itoh, J. Crystal Growth 204 (1999) 69. [10] A. Croll, F.R. Szofran, P. Dold, K.W. Brez, S.L. Lehoczky, J. Crystal Growth 183 (1998) 554.
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