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Hydrogen permeable Ta–Ti–Ni duplex phase alloys with high resistance to hydrogen embrittlement
Journal of Alloys and Compounds 460 (2008) 353–356
Hydrogen permeable Ta–Ti–Ni duplex phase alloys with high resistance to hydrogen embrittlement W. Luo a , K. Ishikawa b , K. Aoki b,∗ a
b
Graduate School of Kitami Institute of Technology, 165 Koen-cho, Kitami, Hokkaido 090-8507, Japan Department of Materials Science, Kitami Institute of Technology, 165 Koen-cho, Kitami, Hokkaido 090-8507, Japan Received 27 April 2007; received in revised form 15 June 2007; accepted 15 June 2007 Available online 20 June 2007
Abstract Crystal structures, microstructures and hydrogen permeability Φ of as-cast Ta–TiNi alloys on the line connecting the compositions of the primary (Ta, Ti) and the ternary eutectic phases have been investigated to find out highly hydrogen permeable duplex phases alloys with high resistance to the hydrogen embrittlement. The alloys on this line show microstructures of (1) the eutectic {(Ta, Ti) + TiNi} phase, (2) the primary (Ta, Ti) phase + the eutectic {(Ta, Ti) + TiNi} phase, and (3) the (Ta, Ti) solid solution, although a little amount of unidentified (impurity) phases are included in these samples. The value of Φ increases with increasing Ta content and the volume fraction of the primary (Ta, Ti) phase, which indicates that the primary phase contributes mainly to the hydrogen permeation. The Ta56 Ti23 Ni21 alloy, containing the 61 vol.% primary phase, shows the highest Φ of 2.18 × 10−8 mol H2 m−1 s−1 Pa−0.5 at 673 K, which is 1.3 times higher than that of the previous most high Φ alloy (Ta53 Ti28 Ni19 ). The more Ta-rich alloys on this line, i.e., containing a small amount of the eutectic phase, are broken down by the hydrogen embrittlement, suggesting that the eutectic phase suppresses the hydrogen embrittlement. © 2007 Elsevier B.V. All rights reserved. Keywords: Hydrogen absorption; Hydrogen diffusion; Microstructures; Hydrogen embrittlement
1. Introduction The Pd–Ag alloy membranes are mainly used to purify hydrogen gas. However, since Pd is expensive and a rare natural resource, it is eagerly desired to develop low cost and high performance hydrogen permeation alloys other than Pd alloys [1–5]. Recently, Hashi et al. have reported that the Nb–TiNi [6,7], the V–TiNi [8] and the Ta–TiNi alloys [8] show high hydrogen permeability Φ comparable to that of pure Pd at 673 K. According to Hashi et al., the Ta53 Ti28 Ni19 alloy, which is the most Tarich alloys investigated by them, shows the highest Φ value of 1.66 × 10−8 mol H2 m−1 s−1 Pa−0.5 at 673 K [8]. On the other hand, it has recently been demonstrated that the Φ value of the Nb–TiNi alloys on the straight line connecting the eutectic and the primary phases increases with increasing volume fraction of the primary (Nb, Ti) phase [9], because only TiNi and the (Nb, Ti) phases appear on this line. On the analogy of the Nb–TiNi ∗
Corresponding author. Tel.: +81 157 26 9452; fax: +81 157 26 9452. E-mail address: [email protected] (K. Aoki).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.06.061
alloys, there is a possibility that the alloys showing higher Φ exist in the more Ta-rich Ta–TiNi alloys on the line connecting the primary and the eutectic phases. In the present work, crystal structures, microstructures and hydrogen permeability Φ in the Ta–TiNi alloys on the straight line connecting the primary (Ta, Ti) and the ternary eutectic phases are investigated in order to find out highly hydrogen permeable alloys. 2. Experimental The preliminary experiment has shown that the Ta44 Ti29 Ni27 (mol%) alloy consists mainly of the primary (Ta, Ti) phase (the white phase) and the ternary eutectic one (the fine lamellar microstructure) as shown in Fig. 1. Chemical compositions of the primary and the ternary eutectic phases are Ta87 Ti10 Ni3 and Ta20 Ti40 Ni40 , respectively. Then, the Ta–TiNi alloys on the line connecting these compositions were prepared by arc melting using high purity Ta (99.9 mass%), Ti (99.5 mass%) and Ni (99.9 mass%) in a purified argon atmosphere. The ascast alloys were cut into disks of 12 mm in diameter and 0.6–0.8 mm in thickness with a spark erosion wire-cutting machine. Both sides of the disks were polished with buff and ␣-alumina of 0.5 m particle, and coated with Pd in the thickness of 190 nm by the dc sputtering machine to avoid oxidation and to promote the hydrogen dissociation and the recombination. Crystal structures of the samples
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Fig. 2. The XRD patterns of the as-cast Ta–TiNi alloys on the straight line connecting the primary (Ta, Ti) and the ternary eutectic phases.
Fig. 1. SEM photographs (a) and (b) of the as-cast Ta44 Ti29 Ni27 alloy. The right side photograph (b) is the enlarged one of the left side photograph (a).
were identified with a powder X-ray diffractometer (XRD). Microstructures of the samples were observed with a scanning electron microscope (SEM). Chemical compositions of their constituting phases were analyzed by using an energy dispersive X-ray spectrometer (EDS). Hydrogen permeability Φ was measured by using an apparatus which has been described in the previous paper [6]. The absence of a potential short circuit for hydrogen permeation such as microcrackings and voids penetrating the disk sample was confirmed by the helium leak test before and after the permeation measurement.
3. Results and discussion 3.1. Crystal structures and microstructures of Ta–TiNi alloys Figs. 2 and 3 show XRD patterns and SEM photographs of the as-cast Ta–TiNi alloys, respectively, on the line connecting the primary (Ta, Ti) and the ternary eutectic phases. The XRD pattern of Ta20 Ti40 Ni40 alloy is indexed on the basis of the bcc-(Ta, Ti), the B2-TiNi and the unidentified phases. The microstructure of this alloy is heterogeneous and consists of the eutectic phase as well as black (unidentified) phases {Fig. 3(a)}. The XRD patterns of Ta34 Ti33 Ni33 and Ta44 Ti29 Ni27 alloys are identified on the basis of the bcc-(Ta, Ti) and the B2-TiNi phases. The white and the gray phases in the SEM photographs for them are the primary (Ta, Ti) and the eutectic {(Ta, Ti) + TiNi} phases, respectively {Fig. 3(b) and (c)}. The XRD patterns of Ta52 Ti25 Ni23 and Ta64 Ti19 Ni17 alloys are indexed on the basis
of the bcc-(Ta, Ti) and the B2-TiNi phase, although the unidentified phases are also observed. The SEM photographs of these alloys show the primary (white) phase and the gray (lamellar eutectic) ones {Fig. 3(d) and (e)}. The XRD pattern of the Ta87 Ti10 Ni3 alloy shows the Bragg peaks of bcc-(Ta, Ti). The SEM photograph of this alloy consists almost of the singlephase {Fig. 3(f)}. The lattice parameters of the bcc-(Ta, Ti) and the B2-TiNi in these alloys are 0.331 and 0.301 nm, respectively, and are almost independent of the alloy compositions. From Figs. 2 and 3, we can confirm that the alloys on the line connecting the primary (Ta, Ti) phase and the ternary eutectic phases consist of a combination of the primary bcc-(Ta, Ti) phase with the eutectic {bcc-(Ta, Ti) + B2-TiNi} one, although a small amount of impurity (unidentified) phases are included in these alloys. 3.2. Hydrogen permeability Φ of the Ta–TiNi alloys Fig. 4 shows Φ of the Ta–TiNi alloys on the line connecting the primary (Ta, Ti) and the ternary eutectic phases in the form of the Arrhenius plot. Those of Pd measured using the same apparatus and procedures as the present work are also plotted for reference in this figure. The Φ value increases with increasing temperature and the Ta content. Φ for the Ta–TiNi alloys having the Ta contents between 52 and 56 mol% is higher than that of Pd between 573 and 673 K. Fig. 5 shows the values of Φ for the above-mentioned alloys at 673 K plotted against the volume fraction of the primary phase (Ta, Ti). Φ increases with increasing the volume frac-
W. Luo et al. / Journal of Alloys and Compounds 460 (2008) 353–356
355
Fig. 3. SEM photographs of the as-cast Ta–TiNi alloys on the straight line connecting the primary (Ta, Ti) and the ternary eutectic phase. (a) The Ta20 Ti40 Ni40 alloy, (b) the Ta34 Ti33 Ni33 , (c) the Ta52 Ti25 Ni23 , (d) the Ta56 Ti23 Ni21 , (e) the Ta64 Ti19 Ni17 and (f) Ta87 Ti10 Ni3 alloys.
tion of the primary phase (Ta, Ti). The most Ta-rich hydrogen permeable Ta56 Ti23 Ni21 alloy consists of the 39 vol.% eutectic phase and the 61 vol.% primary phase. This alloy shows the highest Φ of 2.18 × 10−8 mol H2 m−1 s−1 Pa−0.5 at 673 K, which is 1.3 times higher than that of the previous most high Φ alloy(Ta53 Ti28 Ni19 alloy). That is, the hydrogen permeable Ta–TiNi alloys are extended to 56 mol% Ta by the present work. On the other hand, the more Ta-rich alloys, i.e., containing less amount of the eutectic phase, are broken down by the hydrogen embrittlement. This experimental result suggests that the eutectic phase suppresses the hydrogen embrittle-
ment of the Ta–TiNi alloys, although its mechanism is still uncertain. 4. Summary and conclusion Chemical compositions of the primary (Nb, Ti) and the ternary eutectic phases in the Ta44 Ti29 Ni27 (mol%) alloy have been analyzed by EDS. Crystal structures, microstructures and Φ of the as-cast Ta–TiNi alloys on the line connecting these two phases have been investigated. These alloys consist of the primary bcc-(Ta, Ti) phase and the eutectic
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{bcc-(Ta, Ti) + B2-TiNi} one, except that the Ta20 Ti40 Ni40 , the Ta56 Ti23 Ni21 and the Ta64 Ti19 Ni17 alloys which contain a little amount of unidentified phases. The hydrogen permeable Ta–TiNi alloys are extended to 56 mol% Ta. Hydrogen permeability Φ of the Ta–TiNi alloys increases with the volume fraction of the primary phase up to 61 vol.%. The most Ta-rich Ta56 Ti23 Ni21 alloy, consisting of the 39 vol.% eutectic phase and the 61 vol.% primary phase, shows the highest Φ of 2.18 × 10−8 mol H2 m−1 s−1 Pa−0.5 at 673 K. Acknowledgement Fig. 4. Hydrogen permeability Φ of the Ta–TiNi alloys on the straight line connecting the primary (Ta, Ti) and the ternary eutectic phases in the form of the Arrhenius plot.
This work was supported by New Energy and Industrial Technology Department Organization (NEDO) of Japan. References
Fig. 5. The values of Φ for the as-cast Ta–TiNi alloys on the straight line connecting the primary (Ta, Ti) and the ternary eutectic phases at 673 K plotted against the volume fraction of the primary (Ta, Ti) phase.
[1] M. Amano, M. Komaki, C. Nishimura, J. Less-Common Met. 172–174 (1991) 727–731. [2] C. Nishimura, M. Komaki, S. Hwang, M. Amano, J. Alloys Compd. 330–332 (2002) 902–906. [3] S. Hara, N. Hatakeyama, N. Itoh, H.-M. Kimura, A. Inoue, J. Membr. Sci. 211 (2003) 149–156. [4] S. Yamaura, Y. Shimpo, H. Okouchi, M. Nishida, O. Kajita, H. Kimura, A. Inoue, Mater. Trans. 44 (2003) 1885–1890. [5] Y. Shimpo, S. Yamaura, H. Okouchi, M. Nishida, O. Kajita, H. Kimura, A. Inoue, J. Alloys Compd. 372 (2004) 197–200. [6] K. Hashi, K. Ishikawa, T. Matsuda, K. Aoki, J. Alloys Compd. 368 (2004) 215–220. [7] K. Hashi, K. Ishikawa, T. Matsuda, K. Aoki, Mater. Trans. 46 (2005) 1026–1031. [8] K. Hashi, K. Ishikawa, T. Matsuda, K. Aoki, J. Alloys Compd. 404–406 (2005) 273–278. [9] W. Luo, K. Ishikawa, K. Aoki, J. Alloys Compd. 407 (2006) 115–117.
Hydrogen permeable Ta–Ti–Ni duplex phase alloys with high resistance to hydrogen embrittlement W. Luo a , K. Ishikawa b , K. Aoki b,∗ a
b
Graduate School of Kitami Institute of Technology, 165 Koen-cho, Kitami, Hokkaido 090-8507, Japan Department of Materials Science, Kitami Institute of Technology, 165 Koen-cho, Kitami, Hokkaido 090-8507, Japan Received 27 April 2007; received in revised form 15 June 2007; accepted 15 June 2007 Available online 20 June 2007
Abstract Crystal structures, microstructures and hydrogen permeability Φ of as-cast Ta–TiNi alloys on the line connecting the compositions of the primary (Ta, Ti) and the ternary eutectic phases have been investigated to find out highly hydrogen permeable duplex phases alloys with high resistance to the hydrogen embrittlement. The alloys on this line show microstructures of (1) the eutectic {(Ta, Ti) + TiNi} phase, (2) the primary (Ta, Ti) phase + the eutectic {(Ta, Ti) + TiNi} phase, and (3) the (Ta, Ti) solid solution, although a little amount of unidentified (impurity) phases are included in these samples. The value of Φ increases with increasing Ta content and the volume fraction of the primary (Ta, Ti) phase, which indicates that the primary phase contributes mainly to the hydrogen permeation. The Ta56 Ti23 Ni21 alloy, containing the 61 vol.% primary phase, shows the highest Φ of 2.18 × 10−8 mol H2 m−1 s−1 Pa−0.5 at 673 K, which is 1.3 times higher than that of the previous most high Φ alloy (Ta53 Ti28 Ni19 ). The more Ta-rich alloys on this line, i.e., containing a small amount of the eutectic phase, are broken down by the hydrogen embrittlement, suggesting that the eutectic phase suppresses the hydrogen embrittlement. © 2007 Elsevier B.V. All rights reserved. Keywords: Hydrogen absorption; Hydrogen diffusion; Microstructures; Hydrogen embrittlement
1. Introduction The Pd–Ag alloy membranes are mainly used to purify hydrogen gas. However, since Pd is expensive and a rare natural resource, it is eagerly desired to develop low cost and high performance hydrogen permeation alloys other than Pd alloys [1–5]. Recently, Hashi et al. have reported that the Nb–TiNi [6,7], the V–TiNi [8] and the Ta–TiNi alloys [8] show high hydrogen permeability Φ comparable to that of pure Pd at 673 K. According to Hashi et al., the Ta53 Ti28 Ni19 alloy, which is the most Tarich alloys investigated by them, shows the highest Φ value of 1.66 × 10−8 mol H2 m−1 s−1 Pa−0.5 at 673 K [8]. On the other hand, it has recently been demonstrated that the Φ value of the Nb–TiNi alloys on the straight line connecting the eutectic and the primary phases increases with increasing volume fraction of the primary (Nb, Ti) phase [9], because only TiNi and the (Nb, Ti) phases appear on this line. On the analogy of the Nb–TiNi ∗
Corresponding author. Tel.: +81 157 26 9452; fax: +81 157 26 9452. E-mail address: [email protected] (K. Aoki).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.06.061
alloys, there is a possibility that the alloys showing higher Φ exist in the more Ta-rich Ta–TiNi alloys on the line connecting the primary and the eutectic phases. In the present work, crystal structures, microstructures and hydrogen permeability Φ in the Ta–TiNi alloys on the straight line connecting the primary (Ta, Ti) and the ternary eutectic phases are investigated in order to find out highly hydrogen permeable alloys. 2. Experimental The preliminary experiment has shown that the Ta44 Ti29 Ni27 (mol%) alloy consists mainly of the primary (Ta, Ti) phase (the white phase) and the ternary eutectic one (the fine lamellar microstructure) as shown in Fig. 1. Chemical compositions of the primary and the ternary eutectic phases are Ta87 Ti10 Ni3 and Ta20 Ti40 Ni40 , respectively. Then, the Ta–TiNi alloys on the line connecting these compositions were prepared by arc melting using high purity Ta (99.9 mass%), Ti (99.5 mass%) and Ni (99.9 mass%) in a purified argon atmosphere. The ascast alloys were cut into disks of 12 mm in diameter and 0.6–0.8 mm in thickness with a spark erosion wire-cutting machine. Both sides of the disks were polished with buff and ␣-alumina of 0.5 m particle, and coated with Pd in the thickness of 190 nm by the dc sputtering machine to avoid oxidation and to promote the hydrogen dissociation and the recombination. Crystal structures of the samples
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Fig. 2. The XRD patterns of the as-cast Ta–TiNi alloys on the straight line connecting the primary (Ta, Ti) and the ternary eutectic phases.
Fig. 1. SEM photographs (a) and (b) of the as-cast Ta44 Ti29 Ni27 alloy. The right side photograph (b) is the enlarged one of the left side photograph (a).
were identified with a powder X-ray diffractometer (XRD). Microstructures of the samples were observed with a scanning electron microscope (SEM). Chemical compositions of their constituting phases were analyzed by using an energy dispersive X-ray spectrometer (EDS). Hydrogen permeability Φ was measured by using an apparatus which has been described in the previous paper [6]. The absence of a potential short circuit for hydrogen permeation such as microcrackings and voids penetrating the disk sample was confirmed by the helium leak test before and after the permeation measurement.
3. Results and discussion 3.1. Crystal structures and microstructures of Ta–TiNi alloys Figs. 2 and 3 show XRD patterns and SEM photographs of the as-cast Ta–TiNi alloys, respectively, on the line connecting the primary (Ta, Ti) and the ternary eutectic phases. The XRD pattern of Ta20 Ti40 Ni40 alloy is indexed on the basis of the bcc-(Ta, Ti), the B2-TiNi and the unidentified phases. The microstructure of this alloy is heterogeneous and consists of the eutectic phase as well as black (unidentified) phases {Fig. 3(a)}. The XRD patterns of Ta34 Ti33 Ni33 and Ta44 Ti29 Ni27 alloys are identified on the basis of the bcc-(Ta, Ti) and the B2-TiNi phases. The white and the gray phases in the SEM photographs for them are the primary (Ta, Ti) and the eutectic {(Ta, Ti) + TiNi} phases, respectively {Fig. 3(b) and (c)}. The XRD patterns of Ta52 Ti25 Ni23 and Ta64 Ti19 Ni17 alloys are indexed on the basis
of the bcc-(Ta, Ti) and the B2-TiNi phase, although the unidentified phases are also observed. The SEM photographs of these alloys show the primary (white) phase and the gray (lamellar eutectic) ones {Fig. 3(d) and (e)}. The XRD pattern of the Ta87 Ti10 Ni3 alloy shows the Bragg peaks of bcc-(Ta, Ti). The SEM photograph of this alloy consists almost of the singlephase {Fig. 3(f)}. The lattice parameters of the bcc-(Ta, Ti) and the B2-TiNi in these alloys are 0.331 and 0.301 nm, respectively, and are almost independent of the alloy compositions. From Figs. 2 and 3, we can confirm that the alloys on the line connecting the primary (Ta, Ti) phase and the ternary eutectic phases consist of a combination of the primary bcc-(Ta, Ti) phase with the eutectic {bcc-(Ta, Ti) + B2-TiNi} one, although a small amount of impurity (unidentified) phases are included in these alloys. 3.2. Hydrogen permeability Φ of the Ta–TiNi alloys Fig. 4 shows Φ of the Ta–TiNi alloys on the line connecting the primary (Ta, Ti) and the ternary eutectic phases in the form of the Arrhenius plot. Those of Pd measured using the same apparatus and procedures as the present work are also plotted for reference in this figure. The Φ value increases with increasing temperature and the Ta content. Φ for the Ta–TiNi alloys having the Ta contents between 52 and 56 mol% is higher than that of Pd between 573 and 673 K. Fig. 5 shows the values of Φ for the above-mentioned alloys at 673 K plotted against the volume fraction of the primary phase (Ta, Ti). Φ increases with increasing the volume frac-
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Fig. 3. SEM photographs of the as-cast Ta–TiNi alloys on the straight line connecting the primary (Ta, Ti) and the ternary eutectic phase. (a) The Ta20 Ti40 Ni40 alloy, (b) the Ta34 Ti33 Ni33 , (c) the Ta52 Ti25 Ni23 , (d) the Ta56 Ti23 Ni21 , (e) the Ta64 Ti19 Ni17 and (f) Ta87 Ti10 Ni3 alloys.
tion of the primary phase (Ta, Ti). The most Ta-rich hydrogen permeable Ta56 Ti23 Ni21 alloy consists of the 39 vol.% eutectic phase and the 61 vol.% primary phase. This alloy shows the highest Φ of 2.18 × 10−8 mol H2 m−1 s−1 Pa−0.5 at 673 K, which is 1.3 times higher than that of the previous most high Φ alloy(Ta53 Ti28 Ni19 alloy). That is, the hydrogen permeable Ta–TiNi alloys are extended to 56 mol% Ta by the present work. On the other hand, the more Ta-rich alloys, i.e., containing less amount of the eutectic phase, are broken down by the hydrogen embrittlement. This experimental result suggests that the eutectic phase suppresses the hydrogen embrittle-
ment of the Ta–TiNi alloys, although its mechanism is still uncertain. 4. Summary and conclusion Chemical compositions of the primary (Nb, Ti) and the ternary eutectic phases in the Ta44 Ti29 Ni27 (mol%) alloy have been analyzed by EDS. Crystal structures, microstructures and Φ of the as-cast Ta–TiNi alloys on the line connecting these two phases have been investigated. These alloys consist of the primary bcc-(Ta, Ti) phase and the eutectic
356
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{bcc-(Ta, Ti) + B2-TiNi} one, except that the Ta20 Ti40 Ni40 , the Ta56 Ti23 Ni21 and the Ta64 Ti19 Ni17 alloys which contain a little amount of unidentified phases. The hydrogen permeable Ta–TiNi alloys are extended to 56 mol% Ta. Hydrogen permeability Φ of the Ta–TiNi alloys increases with the volume fraction of the primary phase up to 61 vol.%. The most Ta-rich Ta56 Ti23 Ni21 alloy, consisting of the 39 vol.% eutectic phase and the 61 vol.% primary phase, shows the highest Φ of 2.18 × 10−8 mol H2 m−1 s−1 Pa−0.5 at 673 K. Acknowledgement Fig. 4. Hydrogen permeability Φ of the Ta–TiNi alloys on the straight line connecting the primary (Ta, Ti) and the ternary eutectic phases in the form of the Arrhenius plot.
This work was supported by New Energy and Industrial Technology Department Organization (NEDO) of Japan. References
Fig. 5. The values of Φ for the as-cast Ta–TiNi alloys on the straight line connecting the primary (Ta, Ti) and the ternary eutectic phases at 673 K plotted against the volume fraction of the primary (Ta, Ti) phase.
[1] M. Amano, M. Komaki, C. Nishimura, J. Less-Common Met. 172–174 (1991) 727–731. [2] C. Nishimura, M. Komaki, S. Hwang, M. Amano, J. Alloys Compd. 330–332 (2002) 902–906. [3] S. Hara, N. Hatakeyama, N. Itoh, H.-M. Kimura, A. Inoue, J. Membr. Sci. 211 (2003) 149–156. [4] S. Yamaura, Y. Shimpo, H. Okouchi, M. Nishida, O. Kajita, H. Kimura, A. Inoue, Mater. Trans. 44 (2003) 1885–1890. [5] Y. Shimpo, S. Yamaura, H. Okouchi, M. Nishida, O. Kajita, H. Kimura, A. Inoue, J. Alloys Compd. 372 (2004) 197–200. [6] K. Hashi, K. Ishikawa, T. Matsuda, K. Aoki, J. Alloys Compd. 368 (2004) 215–220. [7] K. Hashi, K. Ishikawa, T. Matsuda, K. Aoki, Mater. Trans. 46 (2005) 1026–1031. [8] K. Hashi, K. Ishikawa, T. Matsuda, K. Aoki, J. Alloys Compd. 404–406 (2005) 273–278. [9] W. Luo, K. Ishikawa, K. Aoki, J. Alloys Compd. 407 (2006) 115–117.