Crystal growth of giant magnetostrictive Tb-Dy-Fe alloy

Journal of

ALLOY~ A~D CONPOUND~ ELSEVIER

Journal of Alloys and Compounds 258 (I997) 34-38

Crystal growth of giant magnetostrictive Tb-Dy-Fe alloy Wu Mei*, Masateru Yoshizumi, Toshimitsu Okane, Takateru Umeda Department of Metallurgy, School of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113, Japan

Received 6 November 1996

Abstract Crystal growth of the giant magnetostrictive Tb-Dy-Fe alloy has been investigated. <112> and <110> twinned 'single' crystals were obtained utilizing the preferred growth of the Tb-Dy-Fe alloy. The twinning could be effectively suppressed by low growth rates such as 3 mm h-~. With seed crystals cut from the <112> twinned 'single' crystal, < 111 > oriented single crystals were successfully grown. High growth rates such as 60 mm h- ' induced the twinning and a growth tilted from axial heat flow direction, and thus destroyed the twinless growth along <111 > direction. In the initial growth regions of the < 111> crystals, a REF% banded structure was usually observed. © 1997 Elsevier Science S.A. Keywords: Crystal growth; Magnetostrictive; Twinning; Rare earth element; Intermetallic compound

1. Introduction

2. Experimental

In the last decade, Tbo,27_o,3oDYo.73_o,7oFe 2 compound drew much attention due to its excellent magnetostrictive properties [1]. Its preparation, microstructure and physical properties have been widely investigated [2-5]. One important characterization of its magnetostrictive properties is its magnetostriction in low external magnetic field. That is strongly application oriented. Considering the huge magnetostrictive anisotropy in the T b - D y - F e compound and the hindrance to magnetic domain process of inhomogeneity, boundaries, impurities etc., < 1 1 1 > oriented T b - D y - F e single crystal is desirable. Usually, crystal growth methods were adopted [6-10]. However, it was reported that the < 1 1 1 > crystal was not obtained even using seeded technique [8]. On the other hand, < 112> and < 1 1 0 > oriented twinned crystals were easily produced due to the preferred growth of the T b - D y - F e alloy [5]. The twinning in the crystals does harm to their magnetostrictive properties because of its hindrance to magnetization process. In this study, the crystal growth of the Tbo.3Dyo.7Fel.9o_l.gs was investigated. The rate dependence of the twinning was studied and the < 111 > oriented T b - D y - F e single crystals were successfully grown.

Starting materials were of 99.9 wt% purity. Crystal growth was carried out using a Bridgman furnace, an inductional zone melting furnace (IF) and an optical image zone melting furnace (OF) [5]. For the former two cases, master Tbo.3DY0.7Fet.90 alloy rods (diameter, 8 mm; length, 100 ram) were induction cast and then grown in A120 3 crucibles at 30-3600 mm h-~ pulling rate. For the image furnace, crucibleless growth is available. Two composition alloys, Tbo.3Dyo.TFel.go and Tbo.3DYo.7Fei.95, were initially alloyed by arc melting and then arc cast into rods (diameter, 6 ram; length, 100 ram). These rods were then grown at rates varying from 5 to 200 mm h-l, rotation rates of 10-60 rpm. The molten zone length is one important parameter in the crucibleless zone melting. In the OF furnace, a steady crucibleless growth could be achieved with a rod diameter of about 6 mm and molten zone length less than 6 ram. For the rod with 8 mm diameter, the molten zone was difficult to keep steady, unlike that in the induction crncibletess growth [7]. This is because for the image furnace, only surface tension of the molten zone contributes to its stability, while in the case of the induction furnace, an electromagnetic force also contributes. To detect the axial grain orientations of the as-grown crystal rods, thin sections were cut from the rods and analyzed using an X-ray diffractometer and a Laue apparatus. For the X-ray diffractometer, a Cu target was

*Corresponding author. 0925-8388/97/$17.00 © 1997 Elsevier Science S.A. AII rights reserved. PII S0925-8388(97)00064-9

W. Mel et al. I Journal o f Alloys and Compounds 258 (1997) 3 4 - 3 8

employed. Work conditions were 40-50 kV and 200-250 mA. The X-ray beam size in the Laue apparatus was about 3 mm in diameter. Work conditions were 40 kV, 30 mA and 35 mm film-sample distance. Microstructures were observed by an optical microscopy and a scanning electron microscopy (SEM). To reveal twin boundary and grain size, Vilella's reagent (1 g picric acid in 5 ml HC1 plus 95 ml methanol) was used for heavy etching. Axial magnetostriction was measured using a standard strain gauge method. Compressive prestress was supplied by a gas pressure cell. The directions of the external magnetic field and the applied compressive load were along the T b - D y - F e crystal rod axis. To reduce residual internal stresses in the crystals, a routine annealing of 950°C for 3 hours was usually employed.

35

one parallel array of the Laves phase originated from one grain. It should be noted that it was difficult to obtain the 'single' crystals in the IF furnace even using the seed crystal. Probably, it is because in this furnace, the solidliquid interface of the growing crystal was a little concave to the melt. This allowed the growth of the nuclei formed in the crucible walls. For the Bridgman method, although it was easy to obtain 'single' crystals, another problem occurred. Since the alloy was wholly melted during the growth, the contamination from AlzOz crucible material and atmosphere was severe. A significant RE loss along the axial direction was observed even at the growth rate of 600 mm h- ~. The use of BN or RE20 ~ crucibles can avoid this problem. In the present experiments, the 'single' crystals were mainly obtained in the OF furnace.

3.1.2. Twinning suppression 3. Results and discussions

3.1. Preparation of <112> and <110> co,stals 3.1.1. Gkowth of <112> and <110> 'single' crystal In our previous studies [5], it was found that, for the T b - D y - F e alloy, there are different preferred growth directions in different growth rate regimes. Higher rates that lead to cellular/dendritic growth usually forn~ < 1 1 2 > oriented crystals. Lower rates that obtain planar solidliquid interface often bring about < 110> oriented crystals. Taking advantage of such preferred growth, < 1 1 2 > oriented and < 1 1 0 > oriented twinned 'single' crystals were obtained without seed crystals. Fig. 1 shows a typical biphase morphology of the transverse section of one < 1 1 0 > oriented 'single' crystal. Although in this type of twinned crystals, there are two phases and strictly said, two grains, they were usually called 'single' crystals [8], since

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It was attempted to grow < 1 1 2 > and < 1 1 0 > oriented twinless crystal by (1) reducing the impurities as much as possible and (2) adopting low growth rates. Such methods were verified to be effective for III-V compounds [11,12]. It has been mentioned that twin formation in the T b - D y Fe could be eliminated at a growth rate of about 30 mm h-i in an induction floating zone melting furnace and a Czochralski furnace [9,13]. However, in the present experiments, even when the growth rate was decreased to 5 mm h -1, no significant decrease of the twin boundaries was observed. A further decrease to 3 m m h -1 led to obvious decrease. This indicated the twin formation can be suppressed effectively at a low enough growth rate that is dependent on the growth conditions of different furnaces. To reveal the twinning mechanism in the T b - D y - F e , another experiment was also done. During the growth of one < 112> oriented crystal at 10 mm h -1, the pulling rate was suddenly raised to 30 m m h -~. A planar-cellular transition growth occurred and an obvious increase of the number of the twin boundaries was observed. In previous studies [14-18], it was usually considered that the twin formation was mainly initiated by particular growth conditions, such as thermal instability, impurities, interface shape, on the basis of the internal atomic structure of the material. In the present experiments, the direct driving force for twin formation, twin propagation and twin suppression seems to be the growth rate. Thus, for the T b - D y - F e alloy, the twinning mainly reflects the kinetic requirements of its growth. The high growth rates requires twinned re-entrant growth mechanism and, higher rates prefer larger amount of twin boundaries, which provides more re-entrant corners as nucleation sites that can greatly improve the stepped growth of the T b - D y - F e intermetallic compound.

3.2. Preparation of < i i 1 > oriented single crystal Fig. l. Mierograph of the transverse section of one <110> oriented 'single' co, stal.

The preferred growth after a spontaneous nucleation usually leads to the formation of the < 1 1 2 > or < 1 1 0 >

36

W. Mei et a L I Journal of Alloys and Compounds 258 (1997) 34-38

oriented twinned crystals. To obtain < 1 t 1> single crystal with the present furnaces, the seed crystal technique was applied.

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3.2.1. Seed preparation

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An ideal seed is a < 111> oriented T b - D y - F e single crystal, which is usually obtained by cutting from a twinless crystal. Since < 1 1 2 > and < 1 1 0 > oriented twinless crystals were difficult to prepare in the present apparatus, this is not the preferred method. An alternative way is to use a seed crystal of another material to obtain the desired oriented crystals. In the present experiments, another simple method was found to grow the < 1 1 1 > twinless crystals directly from the twinned seed crystals. Fig. 2 schematically shows the preparation of such a < 1 1 1 > oriented twinned seed. One seed crystal was cut from the < 1 1 2 > oriented twinned 'single' crystal with the originally lateral (111) planes as its upper face. The perpendicular < 111 > direction was easily determined owing to the sheet-like morphology of the twinned crystals [5]. Along this direction, both of the twinned parts and parent parts exhibited the same {111} planes, and the obtained < 111 > twinned seed had its original twin planes perpendicular to the following growth direction. Since the twinning usually develops in the {111} planes of the T b - D y - F e alloy, it is reasonable to assume that the twinning is difficult to form from the obtained < 1 1 1 > oriented seed, even though it was heavily twinned.

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3.2.2. Growth of ~111> single crystal Grown from the aforementioned < 1 1 1 > oriented seed crystals, < 111 > twinless Tb0.3DymyFe~90_~.95 single crystals were successfully grown at the rates of 3-15 mm hin the image furnace. Fig. 3 shows one single crystal and the Laue pattern of its transverse section. In the microstructures of the < 111 > single crystals, apart from some oxidized particles, there was nearly single REF% (RE refers to Tb, Dy) Laves phase, resulting from

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the planar growth of the Tbo.3Dyo.7Fel.9o_l.95, which occurred below 20 mm h- ~ [5]. Besides this, in the initial solidified regions of the crystals (within 10 ram), a banded structure was usually observed, together with REFe 3 Wiedmanst'/itten precipitates (WSP). Fig. 4 shows one typical micrograph of the banded structure. In most cases there was only one band of REF% primary phase in the Laves phase matrix, where the cellular REFe 3 phase was formed. In some cases, a multi-band REFe 3 structure was observed. The banded structure under a planar growth was also observed in other alloys in peritectic systems [19,20]. It was regarded as a result of nucleation controlled growth [21,22]. In the present study, the length of the < 1 1 1 > single crystal was greatly limited by the atmospheric contamination in the optical image furnace. During growth, an oxidized shell was formed on the surface of the molten zone and the growing crystal, which created new nuclea-

37

W. Mei et aI, I Journal of Alloys and Compounds 258 (1997) 3 4 - 3 8

Fig. 4. Banded structure in the <111> single crystal.

tion sites. Although the solid-liquid interface was nearly flat, the large growth anisotropy of the T b - D y - F e alloy enabled those new random nuclei grow faster than the < 111 > grain. Hence this matched the < 111 > grain out. The oxidized shell also made the control of the growth difficult. Higher growth rates were explored to prepare the < 1 1 1 > single crystal. From a <111 > seed, one crystal was grown continuously at 10, 20 and 60 mm h -t. A large amount of twin boundaries and a tilted cellular biphase structure were observed in the part grown at 60 mm h-*, as shown in Fig. 5. The twin boundaries and the RE-rich phase deviated from the heat flow direction, i.e., the original axial direction, and were probably along the < 1 1 2 > direction on the {111} plane, 19.5 ° (from the < 1 1 1 > axial direction. The appearance of the large amount of twin boundaries reflected the kinetic requirement of the growth, which also resulted in the tilted cellular morphology. Thus, the twinless growth along

< 111> direction can only be realized at low growth rates. In the present image furnace, the rates lower than 15 m m h -~ were adopted to grow the Tb0.3Dyo.TFel.9o_~.95 crystals. The above results also confirmed the strong growth anisotropy in the T b - D y - F e alloy and that the preferred growth of the Tbo.3Dy0.7Fel.9o_>95 is contributed by the twinned re-entrant growth mechanism. From the present study, the difficulty of the twinless growth along < 1 1 1 > direction was also highlighted. Previous failures in preparing the < 1 1 1 > oriented TbDyFengo_~ 95 single crystal probably lie in (1) the bad quality of the seed crystal, e.g., the incorrect cut of the < 1 1 2 > twinned crystals; (2) the growth conditions that permits new nucleation and growth of new nuclei during the growth and (3) the growth conditions that cannot suppress the twin formation and preferred growth. In the present work, the < 1 1 1 > single crystals were grown only in the image furnace. Although their diameters were limited by the crucibleless zone melting method, < 111> single crystal with a large diameter can be grown in crucible as long as the twinless growth along the < 1 1 1 > direction is not destroyed. For this purpose, crucible made of BN or RE oxides should be selected and, besides low enough growth rates, an appropriate solidliquid interface convex to melt is also crucial.

3.2.3. Magnetosn'iction of the <111> single crystals Fig. 6 shows the A-H curves of two < 111 > oriented single crystals. In the crystal 1, even though the existing WSP precipitates and oxidized particles prevented magnetostrictive 'jump' effect [2], maximum magnetostriction Am nearly reached 2100 ppm under 20 MPa compressive prestress. In the crystal 2 with nearly single Laves phase, excellent low-field properties were obtained. A huge magnetostrictive jump was observed. The jump range is

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single Laves phase).

38

W. Meiet al. / Journal of Alloys and Compounds 258 (1997) 34-38

about 1700 ppm under 10 MPa. More importantly, after the jump, magnetostriction nearly saturated. That is different from the < 1 1 2 > crystals, in which there was still magnetostrictive increase after the jump usually lower than 1200 ppm [2]. In the figure, the crystal saturated in 500-1000 Oe under 3-10 MPa. To our knowledge, the saturation field of 500 Oe is the lowest for the T b - D y - F e alloy up to now. The jump range of 1700 ppm is also the highest. Since there were some oxidized particles in the present crystals, it can be predicted that a higher quality < 111 > single crystal will exhibit more excellent magnetostrictive properties. Probably, the theoretically maximum magnetostriction (3A~11/2 (2400 ppm) of the Tb0.3Dyo,TF%,o can be realized in such a high quality < 1 1 1 > crystal under prestress with the external magnetic field lower than 500 Oe.

4. Conclusions (1) Growth rate was crucial to the twin formation, propagation and suppression in the T b - D y - F e alloy, indicating that the twinning reflects the requirement of growth kinetics. Low enough growth rates, such as 3 mm h -~, could effectively suppress the twinning in the Tbo.3Dyo,vFel.9o_ 1.95 crystals. (2) < 1 1 1 > oriented twinless Tb03DYo,vFe~.90_l.95 single crystals were successfully grown from twinned seed crystals at 3-15 nmlh -~ in an optical image furnace. Excellent magnetostrictive properties were obtained in the <1 11> single crystals. Some of the reasons for failure in growing the < 1 1 1 > crystal were revealed such as, too high a growth rate, oxidized shell, etc. In the initial solidified regions of the < 1 1 1 > single crystals, a REF% banded structure was observed.

Acknowledgments We would like to thank Mr. T. Yamamoto (The University of Tokyo, Japan), Professor Zhenxing Shi and Dr.

JianGuo Li (Northwestern Polytechnical University, China) for their support in the crystal growth experiments, and Dr. T. Minowa (Shin-Estu Chemical Co., Ltd., Japan) for his materials support. This work was partly supported by Grant-in-aid for Scientific Research (B) under contract No. 07455275, and on Priority Areas under contract No. 07219205.

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