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Crystal growth of iron-based compounds by the cold-crucible Czochralski technique
,. . . . . . . .
ELSEVIER
CRYSTAL OIROWTH
Journal of Crystal Growth 166 (1996) 298-302
Crystal growth of iron-based compounds by the cold-crucible Czochralski technique Y.J. Bi *, J.S. Abell, D. Fort School of Metallurgy and Materials', University of Birmingham, Birmingham B15 2TT, UK
Abstract
Bulk single crystals of NbFe 2, HoFe 2 and (Tb/Dy)Fe 2 have been grown from a melt with a cold-crucible Czochralski technique. The as-grown crystals were characterised by X-ray diffraction, optical and scanning electron microscopy. The microstructural defects of the pseudo-binary (Tb/Dy)Fe 2 crystals have been studied by transmission electron microscopy. The as-grown NbFe 2 crystals with C14 hexagonal structure contain many thermal cracks, but no secondary phases. Both X-ray lattice parameter measurement and EDX/SEM analysis show consistent stoichiometric composition. However, both HoFe 2 and (Tb/Dy)Fe 2 have C15 cubic Laves phase structure and form by peritectic reactions with a narrow separation between the liquidus and solidus lines. By adopting a slow growth rate, a planar growth front has been maintained and bulk single crystals have been achieved. The as-grown crystals of (Tb/Dy)Fe 2 contain many needle-shaped Widmanstatten precipitates formed by a solid state reaction. Microstructural study by transmission electron microscopy revealed the presence of many {l 11} plane stacking faults and a few twin boundaries. A large number of interracial dislocation networks have been observed on the Widmanstatten precipitates. The presence of thermal cracks, microstructural defects and Widmanstatten precipitates in the Laves phase compounds will strongly affect the measurement of the magnetic properties of the crystals.
1. I n t r o d u c t i o n
The magnetic properties of iron-based Laves phase compounds have been the subject of much study in recent years [1-3]. Among the compounds is the C14 Laves phase NbFe 2, which is a weak antiferromagnet with ferromagnetic spin fluctuations [4]. The compound solidifies congruently with a very broad homogeneity range [5], and the magnetic properties of the compound was found to be dependent on the shift of composition [6]. Rare-earth-iron Laves phase compounds have been more extensively investigated * Corresponding author.
[7]. The C15 Laves phase HoFe 2, as an ideal compound, has been used to study the magnetic Compton scattering to determine the spin-dependent Compton profile [1]. On the other hand, the pseudo-binary compound Tb0.z7Dy0.73Fex (1.8 _< x _< 2), known as Terfenol-D, has a very high magnetostrictive coefficient with a low magnetic anisotropy [7]. To realise the maximum strain, crystallographic alignment of the material is desirable. Therefore, high quality, well-characterised single crystals become more and more important not only for the interpretation of some fundamental physical properties, but for the optimum magnetic properties in the application as well.
0022-0248/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0022-0248(96)00045-0
Y.J. Bi et al. / Journal of Crystal Growth 166 (1996) 298-302 The commonly available refractory metal crucibles, such as W, Ta and Mo, are not suitable for the melting of iron compounds due to the strong reaction between the alloys and crucibles [8]. It has proved difficult to achieve any good quality crystals by the use of the Bridgman technique with BN crucibles due to crucible contamination [9]. In addition, the constraint induced by the crucibles during cooling strongly degrades the quality of crystals, particularly for very brittle Laves phase compounds. Since both cubic Laves phases of HoFe 2 and ( T b / D y ) F e 2 form by a peritectic reaction with a narrow separation between the liquidus and peritectic [8,10], a planar growth interface may be achieved by using a low growth rate [11], which inevitably increases the difficulty of the float zone technique [12]. Development in the cold-crucible Czochralski technique provides an alternative solution for the single crystal growth of iron-based Laves phase compounds. Earlier work indicated that large grains or even single crystals could be readily achieved using an induction heated cold-crucible at low growth rates [13-15].
2. Experimental procedure Master alloys with nominal compositions NbFe 2, HoF% and Tb0.27Dy0.73Fe x (x = 1.9, 1.95) were prepared from high quality starting materials by arc melting. Charges of about 50 g were used for each growth. The Czochralski growth was carried out in a water-cooled, double silica glass, tubular chamber with a vertical cold crucible (also known as a Hukin crucible), which consists of water-cooled copper segments with 1 mm gaps. Samples cut from both the cross section and the longitudinal section of the as-grown boules were examined by optical and scanning electron microscopy (SEM). Phase identification and lattice parameter measurements were carried out by X-ray diffraction. Compositional analysis has been conducted on SEM by X-ray energy dispersive analysis (EDX) and X-ray wavelength dispersive analysis (WDX) with a LiF crystal. Detailed microstructural defects of Tb0.27Dy0.73Fe x have been investigated by transmission electron microscopy (TEM).
299
3. Results and discussion 3.1. Crystal growth
Crystal growth was carried out by direct radio frequency (RF) (350 kHz) induction heating under purified argon at 2 bars in order to suppress the evaporation of any volatile constituents. The magnetic flux through the thin slits of the water-cooled copper crucible is strongly coupled with the charge and leads to over-heating and levitation simultaneously. The levitation force will not be sufficient to lift the whole charge so a solid shell of the master alloy is expected to form on the bottom, where the melt contacts the cold crucible. Subsequently, the temperature of the melt responds sluggishly to the change of power input, because any slight increase of the power leads to reduction of the solid shell; on the other hand, any slight decrease of power results in decrease of the volume of the liquid. For HoFe 2 and Terfenol-D, a 2 mm diameter tungsten rod, rotating at a typical rate of 5 rev/min, was dipped into the melts and a polycrystalline solid nucleated on the tungsten rod immediately. The tungsten rod was then pulled upwards at the rate of 28 m m / h as the polycrystalline mass was growing. However, in the case of NbFe 2, since the melt dissolves W rapidly, an arc-melted NbF% polycrystalline rod was used as a seed instead of W. A standard necking-and-widening process then followed. Since the charge was heated directly by the RF magnetic field, the strong magnetic stirring effect disturbed the solid-liquid interface continuously, and the solidified polycrystalline mass could only be necked down to about 2 mm in diameter for the crystal growth before the solid-liquid interface was disrupted. Typical crystal boules of about 10 mm in diameter and > 20 mm in length have been achieved for the iron-based Laves phase compounds (Fig. 1). 3.2. Microstructural examination
Both microstructural examination and Laue X-ray diffraction show that the Czochralski-grown NbFe 2 is a single crystal, though it contains many cracks. Since NbFe 2 solidifies congruently [5], a planar solidification interface with little or no solute segregation can be easily maintained during slow Czochral-
300
Y.J. Bi et al. / Journal of Crystal Growth 166 (1996) 298-302
ski growth. During the necking process, one of the competitive growing grains passes the thin neck and grows into a single crystal. Although the growing boule was pulled out of the RF heating region at the same rate as the growth, the temperature gradient might be large enough to induce many thermal cracks in the brittle compound. The SEM back-scattered electron image shows no secondary phases. EDX analysis indicates that the crystal has the exact stoichiometric composition of the starting alloy. Lattice parameter measurement by powder X-ray diffraction gives a = 4.841 A, and c = 7.894 A, which are in a good agreement with previous results [5]. The thermal cracks within the crystal increase the difficulty of sample fabrication and of some physical property measurements. By further reducing the pulling rate and using higher purity start materials, it may be possible to eliminate the thermal cracks. The peritectic formation of HoFe 2 and (Tb/Dy)Fe 2 makes it more difficult to prepare large and perfect single crystals [9]. The Czochralski-grown boules usually consist of a few large grains. Back-reflected Laue X-ray diffraction shows that the large grains in the as-grown boules are aligned to the growth axis within about 5 °. X-ray rocking curves obtained from a (220)-aligned crystal of HoFe 2 show a mosaic spread of less than 0.2 ° with a small shoulder, which indicates that the crystal is generally of good quality, probably containing a small subgrain (Fig. 2). Both optical and SEM back-scattered electron images of lightly etched HoFe 2 reveal no
Fig. 1. Tbo.27Dy0.73Fel.95 boule prepared by cold-crucible Czochralski growth at the rate of 28 m m / h . The boule consists of four large grains aligned to each other within 5 ° .
. . . .
100
I . . . .
I
....
I,
, I
o 80
. . . .
I . . . .
I . . . .
o o o
o
o
o
o
o 6O
o o o o
o
4O
o
o
o
o 20
o o
o
%
0
P 16.3
16,4
__.e
....
16.5
I .... 16.6
d .... 16.7
I,,, 16.8
, 16.9
17
0 Fig. 2. X-ray diffraction rocking curve obtained from a (220)aligned HoFe 2 single crystal showing the half-width of the peak about 0.15 ° . The small shoulder on the right side indicates the presence of small sub-grains.
secondary phases. On the other hand, back-scattered images reveal that both T b 0 . 2 7 D y 0 . 7 3 F e l . 9 and Tb0.27DY0.73Fel.95 contain many needle-shaped Widmanstatten phases homogeneously arranged in a crystallographic network. Twin boundaries have also been observed optically after a light etching. The Widmanstatten precipitates, which are believed to be (Tb/Dy)Fe 3, are believed to form by a solid precipitation of the excess iron during cooling [10]. Tb0.27Dy0.73Fel.95 contains a slightly higher density of Widmanstatten precipitates than that in Tbo.27 Dyo.73 F e 1.9, though no quantitative analysis has been done (see Fig. 3). S E M / E D X compositional analysis of HoFe 2 gives an average of 33.8 at% Ho, which is slightly off-stoichiometry towards the Ho-rich side. The result suggests that HoFe 2, like most rare-earth Laves phase compounds, may have a homogeneity range. There was no detectable compositional variation along the growth axis. S E M / W D X analysis of the as-grown boules from the nominal compositions of Tb0.27Dyo.73Fel.95 and Tb0.27Dy0.73Fel.9 gives the average composition of Tbo.26Dyo.74Fe2.0 and Tbo.26Dyo.74Fel.98, respectively. The results show the shifting tendency of the iron content towards stoichiometry, though the ratio between terbium and dysprosium has remained almost unchanged. Since the measured compositions include the contribution from the (Tb/Dy)Fe 3 phase, the actual iron contents of the matrix could be less. X-ray diffraction data
Y.J. Bi et aL / Journal of Crystal Growth 166 (1996) 298-302
give the lattice parameter of the C15 cubic phase as a = 7.331 A for Tb0.27Dy0.73Fel.95 and a = 7.330 ,~ for Tb0.27Dy0.73Fel.9, which are consistent with other published results [10]. Both microstructural examination and compositional analysis along the growth direction reveal no obvious change. Since the separation between the peritectic and liquidus is very small, the solidification of HoFe 2 and (Th/Dy)Fe 2 will be similar to congruent melting during a very slow Czochralski growth, i.e. a planar or pseudo-planar growth interface could be achieved. The initially nucleated material will be the Laves phase with composition close to the peritectic composition. Any of the rejected rare-earth constituents at the growth front would be removed by the constant magnetic stirring. Since the solute segregation at the growth interface is very small compared
Fig. 3. Optical micrographs from the cross sections of Czochralski-grown boules showing the existence of Widmanstatten precipitates. (a) Tbo 27Dyo.73Fel.9; (b) Tbo.27Dyo.73Fel.95.
301
Fig. 4. A TEM micrograph of as-grown Tbo.27Dyo.73Fe|. 9 showing a large Widmanstatten precipitate with interfacial dislocations and many {111} plane stacking faults.
with the volume of the remaining melt, any compositional change induced by the solute segregation is negligible. Therefore, no detectable compositional variation has been observed in the as-grown boules along the growth axis. However, if the growth rates change, the undercooling will be different, and subsequently, the composition of the growing alloy will be altered. The microstructural defects of Czochralski-grown Terfenol-D have been examined by TEM. It has been found that the Czochralski-grown compounds contain many {111} plane stacking faults in addition to the Widmanstatten precipitates [16] (Fig. 4). The large Widmanstatten precipitates are revealed to contain numerous interfacial dislocations, which have been observed to pin domain wall motion [17]. The formation of the stacking faults was believed to be due to the coalescence of vacancies during crystal growth, since the density of them does not change with iron content, but depends on growth rates [16]. In summary, bulk single crystals of NbFe 2, HoFe 2 and (Tb/Dy)Fe 2 can be readily prepared by the cold-crucible Czochralski technique. A planar or pseudo-planar growth interface can be maintained during the Czochralski growth of HoFe 2 and (Tb/Dy)Fe 2 by adopting a low growth rate. However, the formation of many thermal cracks in NbFe 2, and the presence of many Widmanstatten precipitates in Terfenol-D, can strongly affect the physical properties of the crystals.
302
Y.J. Bi et al. / Journal of Crystal Growth 166 (1996) 298-302
Acknowledgements The authors wish to acknowledge the technical support from Mr. A.R. Bradshaw and Mr. J. Sutton. Thanks are also due to Mr. S. Carpenter for his help on S E M / W D X analysis, and Mr. S. Sutton for his assistance on X-ray rocking curves. This work is financially supported by the UK EPSRC.
References [1] M.J. Cooper, Physica B 192 (1993) 191. [2] Y. Yamada, H. Nakamura, Y. Kitaota, K. Asayama, K. Koga, A. Sakata and T. Murakami, J. Magn. Magn. Mater. 90 (1990) 712. [3] D. Kendall and A.R. Piercy, J. Appl. Phys. 76 (1994) 7148. [4] Y. Yamada, J.G.M. Armitage, R.G. Graham and P.C. Riedi, J. Magn. Magn. Mater. 104 (1992) 1317. [5] R.P. Elliott, Constitution of Binary Alloys, 1st Suppl. (McGraw-Hill, New York, 1965) p. 254.
[6] J. Inoue and M. Shimizu, J. Magn. Magn. Mater. 79 (1989) 265. [7] A.E. Clark, Ferromagnetic Materials, Vol. 1 (North-Holland, Amsterdam, 1980) p. 533. [8] O. Kubaschewski, Iron-Binary Phase Diagram (Springer, Berlin, 1982) p. 111. [9] J.D. Verhoeven, E.D. Gibson, O.D. McMasters and H.H. Baker, Met. Trans. 18 A (1978) 223. [10] P. Westwood and J.S. Abell, J. Appl. Phys. 67 (1990) 4998. [11] Y.J. Bi and J.S. Abell, Scr. Met. Mater. 31 (1994) 751. [12] Y.J. Bi, A.M.H. Huang and J.S. Abell, J. Magn. Magn. Mater. 104-107 (1992) 1471. [13] J.B. Milstein, in: The Rare Earths in Modern Science and Technology, Eds. G.J. McCarthy and J.J. Rhyne (Plenum, New York, 1978) p. 315. [14] Q. Li, Y.L. Zhang, R.Z. Yuan, X.H. Huang and D.J. Jin, J. Crystal Growth 128 (1993) 1092. [15] Y.J. Bi, J.S. Abell and D. Fort, in: Proc. 2nd Int. Conf. Rare Earth Application and Development, China, Vol. 1 (Int. Acad. Publ., China, 1991) p. 100. [16] Y.J. Bi, J.S. Abell and A.M.H. Huang, J. Magn. Magn. Mater. 99 (1991) 159. [17] M. AI-Jiboory, D.G. Lord, Y.J. Bi, J.S. Abell and A.M.H. Huang, J. Appl. Phys. 73 (1993) 6168.
ELSEVIER
CRYSTAL OIROWTH
Journal of Crystal Growth 166 (1996) 298-302
Crystal growth of iron-based compounds by the cold-crucible Czochralski technique Y.J. Bi *, J.S. Abell, D. Fort School of Metallurgy and Materials', University of Birmingham, Birmingham B15 2TT, UK
Abstract
Bulk single crystals of NbFe 2, HoFe 2 and (Tb/Dy)Fe 2 have been grown from a melt with a cold-crucible Czochralski technique. The as-grown crystals were characterised by X-ray diffraction, optical and scanning electron microscopy. The microstructural defects of the pseudo-binary (Tb/Dy)Fe 2 crystals have been studied by transmission electron microscopy. The as-grown NbFe 2 crystals with C14 hexagonal structure contain many thermal cracks, but no secondary phases. Both X-ray lattice parameter measurement and EDX/SEM analysis show consistent stoichiometric composition. However, both HoFe 2 and (Tb/Dy)Fe 2 have C15 cubic Laves phase structure and form by peritectic reactions with a narrow separation between the liquidus and solidus lines. By adopting a slow growth rate, a planar growth front has been maintained and bulk single crystals have been achieved. The as-grown crystals of (Tb/Dy)Fe 2 contain many needle-shaped Widmanstatten precipitates formed by a solid state reaction. Microstructural study by transmission electron microscopy revealed the presence of many {l 11} plane stacking faults and a few twin boundaries. A large number of interracial dislocation networks have been observed on the Widmanstatten precipitates. The presence of thermal cracks, microstructural defects and Widmanstatten precipitates in the Laves phase compounds will strongly affect the measurement of the magnetic properties of the crystals.
1. I n t r o d u c t i o n
The magnetic properties of iron-based Laves phase compounds have been the subject of much study in recent years [1-3]. Among the compounds is the C14 Laves phase NbFe 2, which is a weak antiferromagnet with ferromagnetic spin fluctuations [4]. The compound solidifies congruently with a very broad homogeneity range [5], and the magnetic properties of the compound was found to be dependent on the shift of composition [6]. Rare-earth-iron Laves phase compounds have been more extensively investigated * Corresponding author.
[7]. The C15 Laves phase HoFe 2, as an ideal compound, has been used to study the magnetic Compton scattering to determine the spin-dependent Compton profile [1]. On the other hand, the pseudo-binary compound Tb0.z7Dy0.73Fex (1.8 _< x _< 2), known as Terfenol-D, has a very high magnetostrictive coefficient with a low magnetic anisotropy [7]. To realise the maximum strain, crystallographic alignment of the material is desirable. Therefore, high quality, well-characterised single crystals become more and more important not only for the interpretation of some fundamental physical properties, but for the optimum magnetic properties in the application as well.
0022-0248/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0022-0248(96)00045-0
Y.J. Bi et al. / Journal of Crystal Growth 166 (1996) 298-302 The commonly available refractory metal crucibles, such as W, Ta and Mo, are not suitable for the melting of iron compounds due to the strong reaction between the alloys and crucibles [8]. It has proved difficult to achieve any good quality crystals by the use of the Bridgman technique with BN crucibles due to crucible contamination [9]. In addition, the constraint induced by the crucibles during cooling strongly degrades the quality of crystals, particularly for very brittle Laves phase compounds. Since both cubic Laves phases of HoFe 2 and ( T b / D y ) F e 2 form by a peritectic reaction with a narrow separation between the liquidus and peritectic [8,10], a planar growth interface may be achieved by using a low growth rate [11], which inevitably increases the difficulty of the float zone technique [12]. Development in the cold-crucible Czochralski technique provides an alternative solution for the single crystal growth of iron-based Laves phase compounds. Earlier work indicated that large grains or even single crystals could be readily achieved using an induction heated cold-crucible at low growth rates [13-15].
2. Experimental procedure Master alloys with nominal compositions NbFe 2, HoF% and Tb0.27Dy0.73Fe x (x = 1.9, 1.95) were prepared from high quality starting materials by arc melting. Charges of about 50 g were used for each growth. The Czochralski growth was carried out in a water-cooled, double silica glass, tubular chamber with a vertical cold crucible (also known as a Hukin crucible), which consists of water-cooled copper segments with 1 mm gaps. Samples cut from both the cross section and the longitudinal section of the as-grown boules were examined by optical and scanning electron microscopy (SEM). Phase identification and lattice parameter measurements were carried out by X-ray diffraction. Compositional analysis has been conducted on SEM by X-ray energy dispersive analysis (EDX) and X-ray wavelength dispersive analysis (WDX) with a LiF crystal. Detailed microstructural defects of Tb0.27Dy0.73Fe x have been investigated by transmission electron microscopy (TEM).
299
3. Results and discussion 3.1. Crystal growth
Crystal growth was carried out by direct radio frequency (RF) (350 kHz) induction heating under purified argon at 2 bars in order to suppress the evaporation of any volatile constituents. The magnetic flux through the thin slits of the water-cooled copper crucible is strongly coupled with the charge and leads to over-heating and levitation simultaneously. The levitation force will not be sufficient to lift the whole charge so a solid shell of the master alloy is expected to form on the bottom, where the melt contacts the cold crucible. Subsequently, the temperature of the melt responds sluggishly to the change of power input, because any slight increase of the power leads to reduction of the solid shell; on the other hand, any slight decrease of power results in decrease of the volume of the liquid. For HoFe 2 and Terfenol-D, a 2 mm diameter tungsten rod, rotating at a typical rate of 5 rev/min, was dipped into the melts and a polycrystalline solid nucleated on the tungsten rod immediately. The tungsten rod was then pulled upwards at the rate of 28 m m / h as the polycrystalline mass was growing. However, in the case of NbFe 2, since the melt dissolves W rapidly, an arc-melted NbF% polycrystalline rod was used as a seed instead of W. A standard necking-and-widening process then followed. Since the charge was heated directly by the RF magnetic field, the strong magnetic stirring effect disturbed the solid-liquid interface continuously, and the solidified polycrystalline mass could only be necked down to about 2 mm in diameter for the crystal growth before the solid-liquid interface was disrupted. Typical crystal boules of about 10 mm in diameter and > 20 mm in length have been achieved for the iron-based Laves phase compounds (Fig. 1). 3.2. Microstructural examination
Both microstructural examination and Laue X-ray diffraction show that the Czochralski-grown NbFe 2 is a single crystal, though it contains many cracks. Since NbFe 2 solidifies congruently [5], a planar solidification interface with little or no solute segregation can be easily maintained during slow Czochral-
300
Y.J. Bi et al. / Journal of Crystal Growth 166 (1996) 298-302
ski growth. During the necking process, one of the competitive growing grains passes the thin neck and grows into a single crystal. Although the growing boule was pulled out of the RF heating region at the same rate as the growth, the temperature gradient might be large enough to induce many thermal cracks in the brittle compound. The SEM back-scattered electron image shows no secondary phases. EDX analysis indicates that the crystal has the exact stoichiometric composition of the starting alloy. Lattice parameter measurement by powder X-ray diffraction gives a = 4.841 A, and c = 7.894 A, which are in a good agreement with previous results [5]. The thermal cracks within the crystal increase the difficulty of sample fabrication and of some physical property measurements. By further reducing the pulling rate and using higher purity start materials, it may be possible to eliminate the thermal cracks. The peritectic formation of HoFe 2 and (Tb/Dy)Fe 2 makes it more difficult to prepare large and perfect single crystals [9]. The Czochralski-grown boules usually consist of a few large grains. Back-reflected Laue X-ray diffraction shows that the large grains in the as-grown boules are aligned to the growth axis within about 5 °. X-ray rocking curves obtained from a (220)-aligned crystal of HoFe 2 show a mosaic spread of less than 0.2 ° with a small shoulder, which indicates that the crystal is generally of good quality, probably containing a small subgrain (Fig. 2). Both optical and SEM back-scattered electron images of lightly etched HoFe 2 reveal no
Fig. 1. Tbo.27Dy0.73Fel.95 boule prepared by cold-crucible Czochralski growth at the rate of 28 m m / h . The boule consists of four large grains aligned to each other within 5 ° .
. . . .
100
I . . . .
I
....
I,
, I
o 80
. . . .
I . . . .
I . . . .
o o o
o
o
o
o
o 6O
o o o o
o
4O
o
o
o
o 20
o o
o
%
0
P 16.3
16,4
__.e
....
16.5
I .... 16.6
d .... 16.7
I,,, 16.8
, 16.9
17
0 Fig. 2. X-ray diffraction rocking curve obtained from a (220)aligned HoFe 2 single crystal showing the half-width of the peak about 0.15 ° . The small shoulder on the right side indicates the presence of small sub-grains.
secondary phases. On the other hand, back-scattered images reveal that both T b 0 . 2 7 D y 0 . 7 3 F e l . 9 and Tb0.27DY0.73Fel.95 contain many needle-shaped Widmanstatten phases homogeneously arranged in a crystallographic network. Twin boundaries have also been observed optically after a light etching. The Widmanstatten precipitates, which are believed to be (Tb/Dy)Fe 3, are believed to form by a solid precipitation of the excess iron during cooling [10]. Tb0.27Dy0.73Fel.95 contains a slightly higher density of Widmanstatten precipitates than that in Tbo.27 Dyo.73 F e 1.9, though no quantitative analysis has been done (see Fig. 3). S E M / E D X compositional analysis of HoFe 2 gives an average of 33.8 at% Ho, which is slightly off-stoichiometry towards the Ho-rich side. The result suggests that HoFe 2, like most rare-earth Laves phase compounds, may have a homogeneity range. There was no detectable compositional variation along the growth axis. S E M / W D X analysis of the as-grown boules from the nominal compositions of Tb0.27Dyo.73Fel.95 and Tb0.27Dy0.73Fel.9 gives the average composition of Tbo.26Dyo.74Fe2.0 and Tbo.26Dyo.74Fel.98, respectively. The results show the shifting tendency of the iron content towards stoichiometry, though the ratio between terbium and dysprosium has remained almost unchanged. Since the measured compositions include the contribution from the (Tb/Dy)Fe 3 phase, the actual iron contents of the matrix could be less. X-ray diffraction data
Y.J. Bi et aL / Journal of Crystal Growth 166 (1996) 298-302
give the lattice parameter of the C15 cubic phase as a = 7.331 A for Tb0.27Dy0.73Fel.95 and a = 7.330 ,~ for Tb0.27Dy0.73Fel.9, which are consistent with other published results [10]. Both microstructural examination and compositional analysis along the growth direction reveal no obvious change. Since the separation between the peritectic and liquidus is very small, the solidification of HoFe 2 and (Th/Dy)Fe 2 will be similar to congruent melting during a very slow Czochralski growth, i.e. a planar or pseudo-planar growth interface could be achieved. The initially nucleated material will be the Laves phase with composition close to the peritectic composition. Any of the rejected rare-earth constituents at the growth front would be removed by the constant magnetic stirring. Since the solute segregation at the growth interface is very small compared
Fig. 3. Optical micrographs from the cross sections of Czochralski-grown boules showing the existence of Widmanstatten precipitates. (a) Tbo 27Dyo.73Fel.9; (b) Tbo.27Dyo.73Fel.95.
301
Fig. 4. A TEM micrograph of as-grown Tbo.27Dyo.73Fe|. 9 showing a large Widmanstatten precipitate with interfacial dislocations and many {111} plane stacking faults.
with the volume of the remaining melt, any compositional change induced by the solute segregation is negligible. Therefore, no detectable compositional variation has been observed in the as-grown boules along the growth axis. However, if the growth rates change, the undercooling will be different, and subsequently, the composition of the growing alloy will be altered. The microstructural defects of Czochralski-grown Terfenol-D have been examined by TEM. It has been found that the Czochralski-grown compounds contain many {111} plane stacking faults in addition to the Widmanstatten precipitates [16] (Fig. 4). The large Widmanstatten precipitates are revealed to contain numerous interfacial dislocations, which have been observed to pin domain wall motion [17]. The formation of the stacking faults was believed to be due to the coalescence of vacancies during crystal growth, since the density of them does not change with iron content, but depends on growth rates [16]. In summary, bulk single crystals of NbFe 2, HoFe 2 and (Tb/Dy)Fe 2 can be readily prepared by the cold-crucible Czochralski technique. A planar or pseudo-planar growth interface can be maintained during the Czochralski growth of HoFe 2 and (Tb/Dy)Fe 2 by adopting a low growth rate. However, the formation of many thermal cracks in NbFe 2, and the presence of many Widmanstatten precipitates in Terfenol-D, can strongly affect the physical properties of the crystals.
302
Y.J. Bi et al. / Journal of Crystal Growth 166 (1996) 298-302
Acknowledgements The authors wish to acknowledge the technical support from Mr. A.R. Bradshaw and Mr. J. Sutton. Thanks are also due to Mr. S. Carpenter for his help on S E M / W D X analysis, and Mr. S. Sutton for his assistance on X-ray rocking curves. This work is financially supported by the UK EPSRC.
References [1] M.J. Cooper, Physica B 192 (1993) 191. [2] Y. Yamada, H. Nakamura, Y. Kitaota, K. Asayama, K. Koga, A. Sakata and T. Murakami, J. Magn. Magn. Mater. 90 (1990) 712. [3] D. Kendall and A.R. Piercy, J. Appl. Phys. 76 (1994) 7148. [4] Y. Yamada, J.G.M. Armitage, R.G. Graham and P.C. Riedi, J. Magn. Magn. Mater. 104 (1992) 1317. [5] R.P. Elliott, Constitution of Binary Alloys, 1st Suppl. (McGraw-Hill, New York, 1965) p. 254.
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