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Crystal orientation and grain alignment in a hypoeutectic Mn–Sb alloy under high magnetic field conditions
Journal of Alloys and Compounds 481 (2009) 755–760
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Crystal orientation and grain alignment in a hypoeutectic Mn–Sb alloy under high magnetic field conditions Tie Liu, Qiang Wang ∗ , Ao Gao, Chao Zhang, Donggang Li, Jicheng He Key Laboratory of Electromagnetic Processing of Materials (Ministry of Education), Northeastern University, Shenyang 110004, China
a r t i c l e
i n f o
Article history: Received 30 August 2008 Received in revised form 9 March 2009 Accepted 14 March 2009 Available online 25 March 2009 Keywords: Metals and alloys Crystal growth Microstructure High magnetic fields
a b s t r a c t Mn–89.7 wt.%Sb alloy specimens were solidified in various magnetic fields with different holding times. The influence of the high magnetic field on the orientation of both the primary and eutectic MnSb crystals and on the alignment of the primary MnSb grains in the alloys has been investigated. It was found that the primary MnSb crystals were oriented with their c-plane parallel to the imposed magnetic field direction, but the eutectic MnSb from a eutectic MnSb alloy could not be oriented by the high magnetic field. The primary MnSb dendrites were aligned along the magnetic field direction. This alignment may be attributed to combined effects of crystal orientation, crystal growth feature and heat flow. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Crystal orientation and grain alignment, which are different in nature from each other with that the former is defined in terms of crystallization behavior and the later is characterized by macrostructural morphology, under high magnetic field conditions have been paid much attention recently. A great number of studies have been focused on the experimental and theoretical analyses of crystal oriented and grain aligned structures using colloidal [1,2], sintering [3,4] and solidification [5,6] processes with the aid of a high magnetic field. Especially, the solidification process, during which the crystalline materials with an anisotropic magnetic property can be driven to rotate freely by the imposed magnetic field for the reduction of the system energy, provides a promising method for the in situ fabrication of crystal oriented or grain aligned materials. In 1981, Mikelson and Karklin [7] firstly found that the oriented structures could be obtained when Al–Cu and Cd–Zn alloys solidified in 0.5–1.5 T magnetic fields. de Rango et al. [8] obtained crystal oriented YBa2 Cu3 O7 by solidification in a magnetic field and discussed the possibility of crystal orientation at a high temperature using a magnetic field. On the basis of magnetic anisotropy and magnetization energy, several researches have been carried out to theoretically analyze the crystal orientation [9,10]. On the other hand, Morikawa et al. [11] observed the alignment phenomena of
∗ Corresponding author. Tel.: +86 24 83681726; fax: +86 24 83681758. E-mail address: [email protected] (Q. Wang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.03.090
the precipitated primary MnBi and the AlFeSi intermetallic compound from a Bi–Mn alloy and an Al–Si–Fe alloy, respectively, by imposing a high magnetic field. Wang et al. [6] obtained primary Al3 Ni aligned structures in various hypoeutectic Al–Ni alloys during nondirectional solidification process in high magnetic fields. They attributed this alignment to the combined effects of magnetic orientation, crystal growth and the effects of the magnetic field on mass transport during the solidification. Li et al. [12] also obtained aligned structures of clustered primary Al3 Ni grains in hypereutectic Al–Ni alloys under high magnetic field conditions. They argued that this alignment of the primary Al3 Ni phase was the result of the orientation of the primary Al3 Ni crystals caused by the high magnetic field. Although those previous works have tried to discuss the forming mechanism of grain aligned structures, it is not clear yet due to the complexity of mechanism and multiplicity of phenomena for various materials. Especially, the grain alignment seems strongly dependent on the solidification condition as well as the application of the magnetic field. The purpose of this study is to investigate the mechanism for formation of the aligned structure from the view points of crystal orientation, crystal growth and heat flow. An Mn–89.7 wt.%Sb alloy was used in the experiments. Because MnSb is ferromagnetic below Tc = 587 K [13] and proposed to be a promising magnetic material, due to its strong magnetic–optical effect, high magnetic anisotropy and complicated magnetic phase transition [14]. Meanwhile, the primary phase of Mn–89.7wt.%Sb alloy, i.e. MnSb, has been observed to generally show a dendritic morphology after the soldification [15] and could be oriented by the high magnetic field during the semi-solid state process[16], it can serve as a good candi-
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Table 1 Experimental conditions for Mn–89.7wt.%Sb alloy. Magnetic flux density, B (T) Holding time (min)
0 30
4.4 30
11.5 30
11.5 60
date for characterizing the relationships among crystal orientation, crystal growth feature and heat flow. The alloy specimens were solidified in various magnetic fields. In the obtained structures of treated alloys, aligned primary MnSb dendrites were observed. With the aid of X-ray diffraction (XRD) analysis, the mechanism about this phenomenon was discussed. 2. Experimental An ingot of Mn–89.7 wt.%Sb was produced by induction-melting 99.99% Mn and 99.99% Sb under vacuum in a graphite crucible. From the obtained ingot, specimens were machined to cylinders 5 mm in diameter and 5 mm in length. A specimen was placed into an alumina crucible (inner diameter 6 mm; outer diameter 9 mm), heated under an argon atmosphere to 1123 K which is above the liquidus temperature at a heating rate of 5 K min−1 and held for 30 min to ensure its homogeneity. The specimen was cooled to 673 K at a cooling rate of approximately 1 K min−1 , and then cooled to room temperature by turning off the DC power source. The alloys were also solidified in the presence of various magnetic fields with different holding times. Details of the experimental apparatus have been reported previously [17]. The experimental conditions are summarized in Table 1. The magnetic fields were imposed and removed when the temperature of the charge was approximately 90 K higher than the Curie temperature of the MnSb phase in the alloys. The magnetic field-treated specimens were cut along longitudinal and transverse sections which were parallel and perpendicular to the magnetic field direction, respectively, and polished for metallographic analysis. No etchants were necessary for microstructures. The as-solidified structures of the specimens were examined by optical microscopy and XRD (CuK␣ radiation) analysis on these two sections. Energy dispersive X-ray (EDX) analysis was performed to characterize the chemical compositions of phases in the specimens.
3. Results and discussion Fig. 1. shows the typical microstructure of Mn–89.7 wt.%Sb master alloy. As checked by the EDX technique, there are two phases, i.e. MnSb (dark one) and Sb (bright one). The primary MnSb with a fine dendritic morphology is distributed homogeneously in MnSb/Sb eutectics which serve as a matrix. After remelting and solidifying under various magnetic field conditions, the phase pattern remains unaltered as confirmed by EDX analysis. But the primary MnSb dendrites become coarse obviously compared with that of the master alloy, because of the rather lower cooling rate of 1 K min−1 adopted in this work (Figs. 2 and 3). At 0 T, due to the difference in density between Mn and Sb elements, the primary MnSb dendrites segregate mainly in the upper region of the longitudinal section (Fig. 2(a), upside), while they are distributed uniformly on the transverse section (Fig. 2(a), downside).
Fig. 1. Microstructure of the Mn–89.7 wt.%Sb master alloy.
In the case of high magnetic field, the segregation of the primary MnSb dendrites is found to be inhibited throughout the longitudinal section (Fig. 2(b)–(d), upside). With increasing magnetic flux density and holding time, an increasing number of columnar-like dendrites with their main dendrite arm aligned highly parallel to the applied magnetic field direction are observed (Figs. 2 and 3). Especially for the case of a holding time of 60 min at 1.5 T, a columnar-like primary MnSb dendrite is found to grow almost throughout the longitudinal section of the specimen. For determination of the orientation of MnSb crystals, XRD measurement was performed on both the longitudinal and transverse sections which are parallel and perpendicular to the imposed magnetic field direction, respectively. The XRD patterns for the specimens after the solidification process in various magnetic fields with different holding times are shown in Fig. 4, together with that of the specimens without magnetic field for comparison. In the figures, the diffraction peaks are generally relevant to two phases, i.e. MnSb and Sb, but, in this work, only these diffraction peaks originating from MnSb are taken into account and indexed. The XRD patterns of the specimens solidified at 0 T reveal that there are only three reflections of (1 0 1), (1 0 3) and (3 0 2), corresponding to the longitudinal (Fig. 4(a)) and transverse (Fig. 4(a)) sections, strong enough to be detected. This is mainly due to the relatively lower cooling rate in this work. When imposed upon high magnetic fields, the intensities of the (h 0 l) i.e. (0 0 2) and (2 0 3) peaks obtained on the longitudinal sections are markedly enhanced (Fig. 4(a)). While on the transverse sections, both the (1 0 1) and (3 0 2) peaks are suppressed systematically and the (h k 0) i.e. (1 1 0) and (3 0 0) peaks are remarkably enhanced (Fig. 4(b)). Furthermore, a longer holding time gives no additional effect on the diffraction peaks. Considering the fact that MnSb has a NiAs-type crystal structure, i.e. a hexagonal structure, it can be concluded that the MnSb crystal is oriented with its c-plane parallel to the magnetic field direction. This result apparently indicates that the high magnetic field with a magnetic flux density above 4.4 T is high enough to induce the orientation of the MnSb phase in hypoeutectic Mn–Sb alloys, although the XRD pattern of the 4.4 T case on the transverse section exhibits a limited increase in (h k 0) peaks. However, due to the hypoeutectic composition of the alloy adopted in this work, the structures of all cases exhibit a coexistence of both the primary and eutectic MnSb phases as illustrated in Figs. 2 and 3. The diffraction peaks of the MnSb crystals mentioned above, thus, do not wholly originate from primary ones. One cannot distinguish the difference in crystallization behavior of each phase and detect the relationship between the crystallization processes of these two phases from the XRD measurement results. Therefore, additional solidification experiments using alloys with a eutectic composition to be the initial materials under similar conditions described above were carried out. It is because that the application of high magnetic fields on Mn–Sb alloy system could induce a slight shift of the eutectic point to lower Mn concentrations. That is, the alloy having a completely eutectic structure at 0 T, however, showed a hypereutectic solidified structure under high magnetic field conditions (this phenomenon will be discussed elsewhere). A hypereutectic Mn–Sb alloy was, therefore, adopted to do the solidification experiment under high magnetic field condition, of which there was no any primary phase to be detected. XRD measurement was also performed on the longitudinal and transverse sections of solidified specimens. The as-solidified eutectic microstructures of the alloys solidified at 0 T and 11.5 T with a holding time of 30 min are shown in Fig. 5(a) and (b), respectively. The XRD patterns corresponding to these two alloys are shown in Fig. 6. The figures indicate that although the intensities of some peaks are suppressed and some others are enhanced by the magnetic field, these changes are dissimilar to those obtained from the hypoeutectic case and corresponding to a randomly oriented structure (Fig. 4). This sug-
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Fig. 2. Longitudinal (upside) and transverse (downside) macrographs of Mn–89.7 wt.%Sb specimens solidified in various magnetic fields with different holding times. (a) 0 T, 30 min; (b) 4.4 T, 30 min; (c) 11.5 T, 30 min; and (d) 11.5 T, 60 min.
gests that high magnetic field cannot induce the orientation of the MnSb phase in a eutectic MnSb alloy. It can, thus, be concluded that the application of a high magnetic field can induce the orientation of the primary MnSb crystals of a hypoeutectic Mn–Sb alloy. The crystal orientation of materials under magnetic field conditions has been well investigated either experimentally or theoretically by researchers. A model known as rotation to orientation mechanism in terms of magnetic anisotropy of crystals has been proposed. Based on this mechanism, a crystal with an anisotropic magnetic susceptibility will rotate to an angle that minimizes the system energy when subjected to a magnetic field, B. The driving force for the magnetic orientation, i.e. the reduction of magnetic energy due to the rotation, can be described by E = −VB2 /20 , where is the difference between the susceptibility along each axis, V is the volume of the crystal and 0 is the vacuum permeability. From this equation, one can find that for a certain crystal at a magnetic field with a fixed magnetic flux density, the crystal size is crucial for the rotation of the crystal. In addition, Wang et al. [6] has argued that time is another key parameter which can influence the rotation.
In this work, the (h 0 l) and (h k 0) planes of both the primary and eutectic MnSb crystals in Mn–89.7 wt.%Sb alloy are oriented parallel and perpendicular to the imposed magnetic field direction, respectively, indicating an orientation of the MnSb phase with its c-plane parallel to the magnetic field direction. During the solidification process of the alloys under high magnetic field conditions, the combination of the increasing volume of the primary MnSb crystal and relatively larger temperature interval of the liquid–solid transitional region satisfied the conditions of the crystal size and the time needed for the rotation. It is, therefore, easily for the primary MnSb crystals to be oriented by the high magnetic field during the solidification process. However, the eutectic phase is created by the eutectic reaction which is corresponding to the final stage of the solidification process and characterized by the cooperative growth of two eutectic phases. This means an incapacity for the orientation of the eutectic crystals based on the above-mentioned rotation to orientation mechanism, because of the absence of the time and the liquid matrix needed by the crystals to be rotated freely. The phenomenon that the eutectic MnSb in a eutectic alloy solidified at 11.5 T still showed a randomly oriented structure may, therefore, be attributed to this incapacity. The changes in the intensity of some
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Fig. 3. Micrographs of Mn–89.7 wt.%Sb specimens solidified under various magnetic fields with different holding times indicated by “2a”, “2b”, “2c” and “2d” marked in Fig. 2(a)–(d), respectively.
diffraction peaks in the magnetic field-treated specimen (Fig. 6) may result from the variations of thermal gradient caused by the application of the high magnetic field by means of Lorentz force [18]. It is certain that the high magnetic field can induce the orientation of the primary MnSb crystals, but one cannot ensure whether
Fig. 4. X-ray diffraction patterns on the (a) longitudinal and (b) transverse sections of Mn–89.7 wt.%Sb alloy specimens solidified in various magnetic fields with different holding times.
the eutectic MnSb from the same specimen is also oriented or not, because it is difficult to quantitively analyze the XRD results. However, if the eutectic MnSb in a hypoeutectic MnSb alloy can also be oriented by a high magnetic field, it may arise an assumption that such orientation strongly depends on the crystallization behavior of the primary one, because some researchers have suggested that the nucleation and growth of eutectic can be strongly influenced by the previously nucleated and grown primary phases [19]. Generally, the growth direction of dendrites of an alloy is mainly controlled by heat flow direction and preferred crystallographic direction. That is, the dendrites will grow in preferred crystallographic direction and antiparallel or closely antiparallel to the heat flow direction under normal solidification condition, as illustrated in Fig. 7(a) [20–22]. In this work, the nondirectional solidification process exhibits a nonuniform distribution of the temperature gradient, this creates the microstructure with the primary MnSb dendrites randomly distributed on the longitudinal section of the alloy without the high magnetic field (Figs. 2(a) and 3(a)). On the assumption that the easy magnetization direction of a crystal is parallel to the magnetic field direction, when subjected to a magnetic field with an enough high magnetic flow density, the crystal will be oriented with its easy magnetization direction parallel to the field direction. In this case, the growth direction of the crystal will not be the preferred crystallographic direction and opposite to the heat flow direction, but affected by the interaction of both the heat flow and preferred crystallographic directions because they will not always be antiparallel to each other any longer. If the preferred crystallographic direction of the crystal is the same to the easy magnetization direction, the crystal will grow in the easy magnetization direction (also the preferred crystallographic direction) when the heat flow direction is antiparallel to the easy magnetization direction (Fig. 7(b)). While it will grow in the direction perpendicular to the easy magnetization direction when the heat flow direction is perpendicular to the preferred crystallographic direction (Fig. 7(c)) because the heat flow will control the growth direction of the crystal under this condition. Also, when the heat flow is neither parallel nor perpendicular to the easy magnetization direction, under the interaction of both the heat flow and preferred crystallographic
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Fig. 5. Micrographs of eutectic Mn–Sb alloy specimens solidified (a) without or (b) with an 11.5 T magnetic field present.
nor perpendicular to the easy magnetization direction, the crystal will grow in a direction other than those parallel or perpendicular to the easy magnetization direction, such as that as shown in Fig. 7(g). In the case of that these three directions are not parallel or perpendicular to each other, the growth direction of the crystal will not along the easy magnetization direction. From the above discussion, it can be found that the aligned dendritic structure along the magnetic field direction can only be obtained under conditions as shown in Fig. 7(b) and (d), of which the easy magnetization direction is parallel or perpendicular to the preferred crystallographic direction as well as antiparallel to the heat flow direction. It may thus be deduced that the easy magnetization direction of the primary MnSb is antiparallel to the heat flow direction in this work, resulting in the aligned dendritic structures of the primary MnSb in the 11.5 T magnetic field. But, it is not clear whether the preferred crystallographic direction is parallel or perpendicular to the easy magnetization direction. 4. Conclusions
Fig. 6. X-ray diffraction patterns on the (a) longitudinal and (b) transverse sections of eutectic Mn–Sb specimens solidified at 0 T and 11.5 T.
directions, the crystal will grow in a direction which is neither parallel nor perpendicular to the easy magnetization direction such as that as shown in (Fig. 7(d)). In the case of that the preferred crystallographic direction is perpendicular to the easy magnetization direction, the crystal will grow in the easy magnetization direction when the heat flow direction is antiparallel to the easy magnetization direction (Fig. 7(e)), while it will grow in the direction perpendicular to the easy magnetization direction when the heat flow direction is opposite to the preferred crystallographic direction (Fig. 7(f)). Also, if the heat flow direction is neither parallel
Mn–89.7 wt.%Sb alloy specimens were solidified in a high magnetic field up to 11.5 T with various holding times. The influence of the high magnetic field on the solidified microstructures of the specimens has been investigated. It was found that the primary MnSb crystals were oriented with their c-plane parallel to the imposed magnetic field direction, but the eutectic MnSb crystals from a eutectic Mn–Sb alloy could not be oriented by the high magnetic field. The primary MnSb dendrites were aligned along the magnetic field direction. This alignment may attributed to the combined effects of crystal orientation, crystal growth feature and heat flow. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant no. 50374027), the Program for New Century
Fig. 7. Schematic view of the relationships between easy magnetization, preferred crystallographic directions of crystals and heat flow direction.
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Excellent Talents in University, PR China (Grant no. NCET-06-0289) and the 111 project (Grant no. B07015). References [1] J.A. Kewus, J. Am. Ceram. Soc. 83 (2000) 2341–2359. [2] T.S. Suzuki, Y. Sakka, Scripta Mater. 52 (2005) 583–586. [3] D.E. Farrell, B.S. Chandrasekhar, K.W. DeGuire, M.M. Fang, V.G. Kogan, J.R. Klem, D.K. Finnemore, Phys. Rev. B 36 (1987) 4025–4027. [4] W.W. Chen, Y. Kinemuchi, T. Tamura, K. Miwa, K. Watari, Mater. Lett. 61 (2007) 6–9. [5] H. Yasuda, I. Ohnaka, Y. Yamamoto, A.S. Wismogroho, N. Takezawa, K. Kishio, Mater. Trans. 44 (2003) 2207–2212. [6] C.J. Wang, Q. Wang, Z.Y. Wang, H.T. Li, K. Nakajimaa, J.C. He, J. Cryst. Growth 310 (2008) 1256–1263. [7] A.E. Mikelson, Ya.Kh. Karklin, J. Cryst. Growth 52 (1981) 524–529. [8] P. de Rango, M. Lees, P. Lejay, A. Sulpice, R. Tournier, M. Ingold, P. Germi, M. Pernet, Nature 349 (1991) 770–772. [9] C.J. Wu, S.Q. Li, K. Sassa, Y. Chino, K. Hattori, S. Asai, Mater. Trans. JIM 46 (2005) 1311–1317.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Crystal orientation and grain alignment in a hypoeutectic Mn–Sb alloy under high magnetic field conditions Tie Liu, Qiang Wang ∗ , Ao Gao, Chao Zhang, Donggang Li, Jicheng He Key Laboratory of Electromagnetic Processing of Materials (Ministry of Education), Northeastern University, Shenyang 110004, China
a r t i c l e
i n f o
Article history: Received 30 August 2008 Received in revised form 9 March 2009 Accepted 14 March 2009 Available online 25 March 2009 Keywords: Metals and alloys Crystal growth Microstructure High magnetic fields
a b s t r a c t Mn–89.7 wt.%Sb alloy specimens were solidified in various magnetic fields with different holding times. The influence of the high magnetic field on the orientation of both the primary and eutectic MnSb crystals and on the alignment of the primary MnSb grains in the alloys has been investigated. It was found that the primary MnSb crystals were oriented with their c-plane parallel to the imposed magnetic field direction, but the eutectic MnSb from a eutectic MnSb alloy could not be oriented by the high magnetic field. The primary MnSb dendrites were aligned along the magnetic field direction. This alignment may be attributed to combined effects of crystal orientation, crystal growth feature and heat flow. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Crystal orientation and grain alignment, which are different in nature from each other with that the former is defined in terms of crystallization behavior and the later is characterized by macrostructural morphology, under high magnetic field conditions have been paid much attention recently. A great number of studies have been focused on the experimental and theoretical analyses of crystal oriented and grain aligned structures using colloidal [1,2], sintering [3,4] and solidification [5,6] processes with the aid of a high magnetic field. Especially, the solidification process, during which the crystalline materials with an anisotropic magnetic property can be driven to rotate freely by the imposed magnetic field for the reduction of the system energy, provides a promising method for the in situ fabrication of crystal oriented or grain aligned materials. In 1981, Mikelson and Karklin [7] firstly found that the oriented structures could be obtained when Al–Cu and Cd–Zn alloys solidified in 0.5–1.5 T magnetic fields. de Rango et al. [8] obtained crystal oriented YBa2 Cu3 O7 by solidification in a magnetic field and discussed the possibility of crystal orientation at a high temperature using a magnetic field. On the basis of magnetic anisotropy and magnetization energy, several researches have been carried out to theoretically analyze the crystal orientation [9,10]. On the other hand, Morikawa et al. [11] observed the alignment phenomena of
∗ Corresponding author. Tel.: +86 24 83681726; fax: +86 24 83681758. E-mail address: [email protected] (Q. Wang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.03.090
the precipitated primary MnBi and the AlFeSi intermetallic compound from a Bi–Mn alloy and an Al–Si–Fe alloy, respectively, by imposing a high magnetic field. Wang et al. [6] obtained primary Al3 Ni aligned structures in various hypoeutectic Al–Ni alloys during nondirectional solidification process in high magnetic fields. They attributed this alignment to the combined effects of magnetic orientation, crystal growth and the effects of the magnetic field on mass transport during the solidification. Li et al. [12] also obtained aligned structures of clustered primary Al3 Ni grains in hypereutectic Al–Ni alloys under high magnetic field conditions. They argued that this alignment of the primary Al3 Ni phase was the result of the orientation of the primary Al3 Ni crystals caused by the high magnetic field. Although those previous works have tried to discuss the forming mechanism of grain aligned structures, it is not clear yet due to the complexity of mechanism and multiplicity of phenomena for various materials. Especially, the grain alignment seems strongly dependent on the solidification condition as well as the application of the magnetic field. The purpose of this study is to investigate the mechanism for formation of the aligned structure from the view points of crystal orientation, crystal growth and heat flow. An Mn–89.7 wt.%Sb alloy was used in the experiments. Because MnSb is ferromagnetic below Tc = 587 K [13] and proposed to be a promising magnetic material, due to its strong magnetic–optical effect, high magnetic anisotropy and complicated magnetic phase transition [14]. Meanwhile, the primary phase of Mn–89.7wt.%Sb alloy, i.e. MnSb, has been observed to generally show a dendritic morphology after the soldification [15] and could be oriented by the high magnetic field during the semi-solid state process[16], it can serve as a good candi-
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T. Liu et al. / Journal of Alloys and Compounds 481 (2009) 755–760
Table 1 Experimental conditions for Mn–89.7wt.%Sb alloy. Magnetic flux density, B (T) Holding time (min)
0 30
4.4 30
11.5 30
11.5 60
date for characterizing the relationships among crystal orientation, crystal growth feature and heat flow. The alloy specimens were solidified in various magnetic fields. In the obtained structures of treated alloys, aligned primary MnSb dendrites were observed. With the aid of X-ray diffraction (XRD) analysis, the mechanism about this phenomenon was discussed. 2. Experimental An ingot of Mn–89.7 wt.%Sb was produced by induction-melting 99.99% Mn and 99.99% Sb under vacuum in a graphite crucible. From the obtained ingot, specimens were machined to cylinders 5 mm in diameter and 5 mm in length. A specimen was placed into an alumina crucible (inner diameter 6 mm; outer diameter 9 mm), heated under an argon atmosphere to 1123 K which is above the liquidus temperature at a heating rate of 5 K min−1 and held for 30 min to ensure its homogeneity. The specimen was cooled to 673 K at a cooling rate of approximately 1 K min−1 , and then cooled to room temperature by turning off the DC power source. The alloys were also solidified in the presence of various magnetic fields with different holding times. Details of the experimental apparatus have been reported previously [17]. The experimental conditions are summarized in Table 1. The magnetic fields were imposed and removed when the temperature of the charge was approximately 90 K higher than the Curie temperature of the MnSb phase in the alloys. The magnetic field-treated specimens were cut along longitudinal and transverse sections which were parallel and perpendicular to the magnetic field direction, respectively, and polished for metallographic analysis. No etchants were necessary for microstructures. The as-solidified structures of the specimens were examined by optical microscopy and XRD (CuK␣ radiation) analysis on these two sections. Energy dispersive X-ray (EDX) analysis was performed to characterize the chemical compositions of phases in the specimens.
3. Results and discussion Fig. 1. shows the typical microstructure of Mn–89.7 wt.%Sb master alloy. As checked by the EDX technique, there are two phases, i.e. MnSb (dark one) and Sb (bright one). The primary MnSb with a fine dendritic morphology is distributed homogeneously in MnSb/Sb eutectics which serve as a matrix. After remelting and solidifying under various magnetic field conditions, the phase pattern remains unaltered as confirmed by EDX analysis. But the primary MnSb dendrites become coarse obviously compared with that of the master alloy, because of the rather lower cooling rate of 1 K min−1 adopted in this work (Figs. 2 and 3). At 0 T, due to the difference in density between Mn and Sb elements, the primary MnSb dendrites segregate mainly in the upper region of the longitudinal section (Fig. 2(a), upside), while they are distributed uniformly on the transverse section (Fig. 2(a), downside).
Fig. 1. Microstructure of the Mn–89.7 wt.%Sb master alloy.
In the case of high magnetic field, the segregation of the primary MnSb dendrites is found to be inhibited throughout the longitudinal section (Fig. 2(b)–(d), upside). With increasing magnetic flux density and holding time, an increasing number of columnar-like dendrites with their main dendrite arm aligned highly parallel to the applied magnetic field direction are observed (Figs. 2 and 3). Especially for the case of a holding time of 60 min at 1.5 T, a columnar-like primary MnSb dendrite is found to grow almost throughout the longitudinal section of the specimen. For determination of the orientation of MnSb crystals, XRD measurement was performed on both the longitudinal and transverse sections which are parallel and perpendicular to the imposed magnetic field direction, respectively. The XRD patterns for the specimens after the solidification process in various magnetic fields with different holding times are shown in Fig. 4, together with that of the specimens without magnetic field for comparison. In the figures, the diffraction peaks are generally relevant to two phases, i.e. MnSb and Sb, but, in this work, only these diffraction peaks originating from MnSb are taken into account and indexed. The XRD patterns of the specimens solidified at 0 T reveal that there are only three reflections of (1 0 1), (1 0 3) and (3 0 2), corresponding to the longitudinal (Fig. 4(a)) and transverse (Fig. 4(a)) sections, strong enough to be detected. This is mainly due to the relatively lower cooling rate in this work. When imposed upon high magnetic fields, the intensities of the (h 0 l) i.e. (0 0 2) and (2 0 3) peaks obtained on the longitudinal sections are markedly enhanced (Fig. 4(a)). While on the transverse sections, both the (1 0 1) and (3 0 2) peaks are suppressed systematically and the (h k 0) i.e. (1 1 0) and (3 0 0) peaks are remarkably enhanced (Fig. 4(b)). Furthermore, a longer holding time gives no additional effect on the diffraction peaks. Considering the fact that MnSb has a NiAs-type crystal structure, i.e. a hexagonal structure, it can be concluded that the MnSb crystal is oriented with its c-plane parallel to the magnetic field direction. This result apparently indicates that the high magnetic field with a magnetic flux density above 4.4 T is high enough to induce the orientation of the MnSb phase in hypoeutectic Mn–Sb alloys, although the XRD pattern of the 4.4 T case on the transverse section exhibits a limited increase in (h k 0) peaks. However, due to the hypoeutectic composition of the alloy adopted in this work, the structures of all cases exhibit a coexistence of both the primary and eutectic MnSb phases as illustrated in Figs. 2 and 3. The diffraction peaks of the MnSb crystals mentioned above, thus, do not wholly originate from primary ones. One cannot distinguish the difference in crystallization behavior of each phase and detect the relationship between the crystallization processes of these two phases from the XRD measurement results. Therefore, additional solidification experiments using alloys with a eutectic composition to be the initial materials under similar conditions described above were carried out. It is because that the application of high magnetic fields on Mn–Sb alloy system could induce a slight shift of the eutectic point to lower Mn concentrations. That is, the alloy having a completely eutectic structure at 0 T, however, showed a hypereutectic solidified structure under high magnetic field conditions (this phenomenon will be discussed elsewhere). A hypereutectic Mn–Sb alloy was, therefore, adopted to do the solidification experiment under high magnetic field condition, of which there was no any primary phase to be detected. XRD measurement was also performed on the longitudinal and transverse sections of solidified specimens. The as-solidified eutectic microstructures of the alloys solidified at 0 T and 11.5 T with a holding time of 30 min are shown in Fig. 5(a) and (b), respectively. The XRD patterns corresponding to these two alloys are shown in Fig. 6. The figures indicate that although the intensities of some peaks are suppressed and some others are enhanced by the magnetic field, these changes are dissimilar to those obtained from the hypoeutectic case and corresponding to a randomly oriented structure (Fig. 4). This sug-
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Fig. 2. Longitudinal (upside) and transverse (downside) macrographs of Mn–89.7 wt.%Sb specimens solidified in various magnetic fields with different holding times. (a) 0 T, 30 min; (b) 4.4 T, 30 min; (c) 11.5 T, 30 min; and (d) 11.5 T, 60 min.
gests that high magnetic field cannot induce the orientation of the MnSb phase in a eutectic MnSb alloy. It can, thus, be concluded that the application of a high magnetic field can induce the orientation of the primary MnSb crystals of a hypoeutectic Mn–Sb alloy. The crystal orientation of materials under magnetic field conditions has been well investigated either experimentally or theoretically by researchers. A model known as rotation to orientation mechanism in terms of magnetic anisotropy of crystals has been proposed. Based on this mechanism, a crystal with an anisotropic magnetic susceptibility will rotate to an angle that minimizes the system energy when subjected to a magnetic field, B. The driving force for the magnetic orientation, i.e. the reduction of magnetic energy due to the rotation, can be described by E = −VB2 /20 , where is the difference between the susceptibility along each axis, V is the volume of the crystal and 0 is the vacuum permeability. From this equation, one can find that for a certain crystal at a magnetic field with a fixed magnetic flux density, the crystal size is crucial for the rotation of the crystal. In addition, Wang et al. [6] has argued that time is another key parameter which can influence the rotation.
In this work, the (h 0 l) and (h k 0) planes of both the primary and eutectic MnSb crystals in Mn–89.7 wt.%Sb alloy are oriented parallel and perpendicular to the imposed magnetic field direction, respectively, indicating an orientation of the MnSb phase with its c-plane parallel to the magnetic field direction. During the solidification process of the alloys under high magnetic field conditions, the combination of the increasing volume of the primary MnSb crystal and relatively larger temperature interval of the liquid–solid transitional region satisfied the conditions of the crystal size and the time needed for the rotation. It is, therefore, easily for the primary MnSb crystals to be oriented by the high magnetic field during the solidification process. However, the eutectic phase is created by the eutectic reaction which is corresponding to the final stage of the solidification process and characterized by the cooperative growth of two eutectic phases. This means an incapacity for the orientation of the eutectic crystals based on the above-mentioned rotation to orientation mechanism, because of the absence of the time and the liquid matrix needed by the crystals to be rotated freely. The phenomenon that the eutectic MnSb in a eutectic alloy solidified at 11.5 T still showed a randomly oriented structure may, therefore, be attributed to this incapacity. The changes in the intensity of some
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Fig. 3. Micrographs of Mn–89.7 wt.%Sb specimens solidified under various magnetic fields with different holding times indicated by “2a”, “2b”, “2c” and “2d” marked in Fig. 2(a)–(d), respectively.
diffraction peaks in the magnetic field-treated specimen (Fig. 6) may result from the variations of thermal gradient caused by the application of the high magnetic field by means of Lorentz force [18]. It is certain that the high magnetic field can induce the orientation of the primary MnSb crystals, but one cannot ensure whether
Fig. 4. X-ray diffraction patterns on the (a) longitudinal and (b) transverse sections of Mn–89.7 wt.%Sb alloy specimens solidified in various magnetic fields with different holding times.
the eutectic MnSb from the same specimen is also oriented or not, because it is difficult to quantitively analyze the XRD results. However, if the eutectic MnSb in a hypoeutectic MnSb alloy can also be oriented by a high magnetic field, it may arise an assumption that such orientation strongly depends on the crystallization behavior of the primary one, because some researchers have suggested that the nucleation and growth of eutectic can be strongly influenced by the previously nucleated and grown primary phases [19]. Generally, the growth direction of dendrites of an alloy is mainly controlled by heat flow direction and preferred crystallographic direction. That is, the dendrites will grow in preferred crystallographic direction and antiparallel or closely antiparallel to the heat flow direction under normal solidification condition, as illustrated in Fig. 7(a) [20–22]. In this work, the nondirectional solidification process exhibits a nonuniform distribution of the temperature gradient, this creates the microstructure with the primary MnSb dendrites randomly distributed on the longitudinal section of the alloy without the high magnetic field (Figs. 2(a) and 3(a)). On the assumption that the easy magnetization direction of a crystal is parallel to the magnetic field direction, when subjected to a magnetic field with an enough high magnetic flow density, the crystal will be oriented with its easy magnetization direction parallel to the field direction. In this case, the growth direction of the crystal will not be the preferred crystallographic direction and opposite to the heat flow direction, but affected by the interaction of both the heat flow and preferred crystallographic directions because they will not always be antiparallel to each other any longer. If the preferred crystallographic direction of the crystal is the same to the easy magnetization direction, the crystal will grow in the easy magnetization direction (also the preferred crystallographic direction) when the heat flow direction is antiparallel to the easy magnetization direction (Fig. 7(b)). While it will grow in the direction perpendicular to the easy magnetization direction when the heat flow direction is perpendicular to the preferred crystallographic direction (Fig. 7(c)) because the heat flow will control the growth direction of the crystal under this condition. Also, when the heat flow is neither parallel nor perpendicular to the easy magnetization direction, under the interaction of both the heat flow and preferred crystallographic
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Fig. 5. Micrographs of eutectic Mn–Sb alloy specimens solidified (a) without or (b) with an 11.5 T magnetic field present.
nor perpendicular to the easy magnetization direction, the crystal will grow in a direction other than those parallel or perpendicular to the easy magnetization direction, such as that as shown in Fig. 7(g). In the case of that these three directions are not parallel or perpendicular to each other, the growth direction of the crystal will not along the easy magnetization direction. From the above discussion, it can be found that the aligned dendritic structure along the magnetic field direction can only be obtained under conditions as shown in Fig. 7(b) and (d), of which the easy magnetization direction is parallel or perpendicular to the preferred crystallographic direction as well as antiparallel to the heat flow direction. It may thus be deduced that the easy magnetization direction of the primary MnSb is antiparallel to the heat flow direction in this work, resulting in the aligned dendritic structures of the primary MnSb in the 11.5 T magnetic field. But, it is not clear whether the preferred crystallographic direction is parallel or perpendicular to the easy magnetization direction. 4. Conclusions
Fig. 6. X-ray diffraction patterns on the (a) longitudinal and (b) transverse sections of eutectic Mn–Sb specimens solidified at 0 T and 11.5 T.
directions, the crystal will grow in a direction which is neither parallel nor perpendicular to the easy magnetization direction such as that as shown in (Fig. 7(d)). In the case of that the preferred crystallographic direction is perpendicular to the easy magnetization direction, the crystal will grow in the easy magnetization direction when the heat flow direction is antiparallel to the easy magnetization direction (Fig. 7(e)), while it will grow in the direction perpendicular to the easy magnetization direction when the heat flow direction is opposite to the preferred crystallographic direction (Fig. 7(f)). Also, if the heat flow direction is neither parallel
Mn–89.7 wt.%Sb alloy specimens were solidified in a high magnetic field up to 11.5 T with various holding times. The influence of the high magnetic field on the solidified microstructures of the specimens has been investigated. It was found that the primary MnSb crystals were oriented with their c-plane parallel to the imposed magnetic field direction, but the eutectic MnSb crystals from a eutectic Mn–Sb alloy could not be oriented by the high magnetic field. The primary MnSb dendrites were aligned along the magnetic field direction. This alignment may attributed to the combined effects of crystal orientation, crystal growth feature and heat flow. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant no. 50374027), the Program for New Century
Fig. 7. Schematic view of the relationships between easy magnetization, preferred crystallographic directions of crystals and heat flow direction.
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