Hydrogen permeation characteristics of multi-phase NiTiNb alloys

Journal of Alloys and Compounds 368 (2004) 215–220

Hydrogen permeation characteristics of multi-phase Ni–Ti–Nb alloys K. Hashi, K. Ishikawa, T. Matsuda, K. Aoki∗ Department of Materials Science, Kitami Institute of Technology, Koen-cho 165, Kitami, Hokkaido 090-8507, Japan Received 28 July 2003; received in revised form 20 August 2003; accepted 20 August 2003

Abstract Hydrogen permeation characteristics of ternary Ni–Ti–Nb alloys were investigated using a mass flow meter in the pressure range of 0.2–0.97 MPa on the upstream side and the temperature range of 523–673 K. Hydrogen permeation through the as-cast alloys with Ti and Ni of 20 mol% and less could not be measured by brittleness and by hydrogen brittleness, respectively. Hydrogen permeability of the ductile B2-NiTi phase was 10−10 mol H2 (m−1 s−1 Pa−0.5 ), and increased when alloying with Nb. The Ni30 Ti31 Nb39 alloy provided a hydrogen permeability of 1.93 × 10−8 mol H2 (m−1 s−1 Pa−0.5 ) at 673 K, which was equivalent to that of pure Pd measured under the same conditions. The Ni30 Ti31 Nb39 alloy consisted of the primary phase, bcc-NbTi, and the eutectic phases, NiTi + NbTi. The eutectic phases played a major role in prevention of hydrogen brittleness of the NbTi phase, while the NbTi phase contributed mainly to the hydrogen permeation of this duplex alloy. This paper represents the observation of hydrogen permeation in the multiphase alloys for the first time. © 2003 Elsevier B.V. All rights reserved. Keywords: Hydrogen permeation; NiTi intermetallic compound; Scanning electron microscopy; Alloy design

1. Introduction In recent years, non-palladium based hydrogen permeation membranes for hydrogen purification have actively been investigated by Nishimura et al. [1,2] and Nishimura and co-workers [3,4]. Group 5A metals, such as V, Nb and Ta showing large hydrogen solubility and high hydrogen diffusivity [5] are promising for hydrogen permeation membranes, because hydrogen permeability (Φ) is the product of hydrogen solubility (K) and hydrogen diffusivity (D). However, these metals suffer from severe hydrogen brittleness and are pulverized spontaneously during hydrogenation, so that these metals are unusable as a hydrogen permeation membrane. However, their alloys substituted with the metals which do not form metal hydride are promising candidates for hydrogen permeation materials. Then, hydrogen permeation characteristics of V-based alloys have mainly been investigated so far [1–4]. In addition to high hydrogen solubility and diffusivity, mechanical properties are also one of the important characteristics for hydrogen permeation membranes, because membranes have to endure the large pressure difference between upstream and downstream sides. In general, a metal having large hydrogen ∗

Corresponding author. E-mail address: [email protected] (K. Aoki).

0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.08.064

permeation ability is susceptible to hydrogen brittleness. Thus, it is difficult to improve the resistance of V, Nb and Ta-based alloys to hydrogen brittleness without much loss of hydrogen permeability. We have paid attention to the NiTi intermetallic compound, which is well known as a shape-memory alloy and has good mechanical properties. NiTi absorbs hydrogen forming a solid solution, but its hydrogen diffusivity is considerably lower than those of V, Nb and Ta [6]. Hence, we try to improve the hydrogen diffusivity of NiTi by alloying with Nb. In this paper, we present new hydrogen permeation alloys consisting of duplex phases, NiTi and NbTi, in the Ni–Ti–Nb system.

2. Experimental 2.1. Sample preparation About 20 g ingots of Nix Tiy Nb(100−x−y) alloys were prepared by arc melting using Ni (99.9% purity), Ti (99.5% purity) and Nb (99.9% purity) in an argon atmosphere. Disks of 12 mm in diameter and 0.55–0.75 mm in thickness were cut out from the ingots by the spark erosion method. Microstructural and structural examinations of the Ni–Ti–Nb alloys were carried out with a scanning electron microscope (SEM) and an X-ray diffractometer (XRD),

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K. Hashi et al. / Journal of Alloys and Compounds 368 (2004) 215–220 pressure gause

sample disk furnace

H2 Inlet

V1

V5

mass flow meter

V4 V2 back pressure valve

V3 R.P.+D.P.

V1~V5 = valve

Fig. 1. A schematic illustration of the hydrogen permeation measuring apparatus used in this work.

respectively. Chemical compositions of the samples were determined by the SEM–EDS method. Determination of volume percentage of a phase was carried out on a Macintosh model computer using the public domain NIH image program. Both sides of the disks were polished with an abrasive paper, buff and ␣-alumina of 0.5 ␮m particle size, followed by coating with palladium of 190 nm in thickness using a RF magnetron sputtering machine. 2.2. Measurement of hydrogen permeability Fig. 1 shows a schematic illustration of the hydrogen permeation measuring apparatus used in the present work. The Pd-coated disk was sealed by copper gaskets. At first, both sides of the disk were evacuated using a diffusion pump up to below 3 × 10−3 Pa, and then the disk was heated to 673 K

and kept at this temperature. Hydrogen gas (99.99999% purity) of 0.1 and 0.2 MPa was introduced into the downstream and the upstream sides, respectively, followed by hydrogen permeation experiments. Hydrogen pressure on the upstream side was raised from 0.2 to 0.97 MPa and temperature was stepped down from 673 to 523 K. Hydrogen flux J (mol H2 (m−2 s−1 )) was measured using a mass flow meter. In general, hydrogen permeation materials are used in the region forming the hydrogen solid solution, because they have to resist hydrogen brittleness. In the case of the ideal solid solution state, the hydrogen concentration in a material is proportional to the square root of the H2 pressure. In such a case, the relation between the hydrogen flux J and the hydrogen permeability Φ (mol H2 (m−1 s−1 Pa−0.5 )) is given by the following equation: J × L = Φ(Pu0.5 − Pd0.5 )

(1)

where Pu and Pd (Pa) are hydrogen pressures at the upstream and the downstream sides, respectively, and L is the thickness of the disk (m). If Eq. (1) is applicable to the present Ni–Ti–Nb alloys, hydrogen permeability Φ can be calculated from the slope of a (J × L) versus (Pu0.5 − Pd0.5 ) plot. This assumption was checked by examination of the linearity of the slope analyzed by a linear regression analysis.

3. Results and discussion The chemical compositions (mol%) of alloys investigated in the present work are marked in Fig. 2. The alloys, which

Fig. 2. Chemical compositions (mol%) of the alloys investigated in this work. The solid squares and the solid triangle denote the brittle alloys in the as-cast state and embrittled alloys by hydrogenation, respectively. The open circles indicate the alloys, which are possible to permeate hydrogen. The figures under the open circles indicate the hydrogen permeability.

K. Hashi et al. / Journal of Alloys and Compounds 368 (2004) 215–220

Fig. 3. SEM photograph of the as-cast Ni21 Ti51 Nb28 alloy which is embrittled by hydrogenation.

1.4x10

-5

1.2x10

-5

in the as-cast states. NbTi absorbs much hydrogen and becomes brittle, resulting in poor resistance to hydrogen brittleness. The Ni–Ti–Nb alloys with a Ni content of about 20 mol% provide the same property. Therefore, the quantities of NiTi2 and NbTi must be reduced to suppress brittleness and hydrogen brittleness, respectively. The numbers under the open circles in Fig. 2 shows the hydrogen permeability. The hydrogen permeability of the alloys containing 50 mol% Ni is 10−10 mol H2 (m−1 s−1 Pa−0.5 ), although this value is inaccurate because of the very small hydrogen flux. As the content of Ni decreases, the hydrogen permeability gradually increased. In particular, the Ni30 Ti31 Nb39 alloy has two–three times higher hydrogen permeability than the others. The hydrogen permeability of this alloy at 673 K shows the highest value in the Ni–Ti–Nb system investigated in the present work and is comparable to that of pure Pd. Fig. 4 shows plots of (J × L) versus (Pu0.5 − Pd0.5 ) for the Ni30 Ti31 Nb39 alloy at several temperatures. The linear correlation coefficient, R2 values were obtained by a linear regression analysis. The R2 values are over 0.99 and approach 1 with increasing temperature in the temperature range from 573 to 673 K. It can, therefore, be said that hydrogen permeation through this alloy almost follows the mechanism represented by Eq. (1) in this temperature range, but not at 523 K. Fig. 5 demonstrates the temperature dependence of the hydrogen permeability Φ calculated from the slope of (J × L) versus (Pu0.5 − Pd0.5 ) plots for the Ni30 Ti31 Nb39 alloy in the form of an Arrhenius plot. For comparison, that of pure Pd is also shown. The data of the Ni30 Ti31 Nb39 alloy at 523 K is treated as reference data, because the Ni30 Ti31 Nb39 alloy does not give a good linearity in Eq. (1).

Ni30Ti31Nb39

673 K R 2 = 0.997

1.0x10-5

-1

-1

(J x L) [mol H2 m s ]

are brittle in the as-cast state and broke down during hydrogen permeation experiments, are denoted by the solid squares and triangles, respectively. The open circles indicate the alloys which are capable of permeating hydrogen. The Ni–Ti–Nb alloys have a wide testable area between the NiTi intermetallic compound toward pure Nb. The alloys with compositions near the NiTi2 compound are brittle in the as-cast state. Brittleness also appears in the Ni–Ti–Nb alloys with 20 mol% Ti. As an example of hydrogen embrittled alloys, the SEM photograph of the as-cast Ni21 Ti51 Nb28 alloy is shown in Fig. 3. White, dark and black regions are NbTi, NiTi and NiTi2 phases, respectively. This alloy has a NbTi content of about 50 vol.%, and the amount of NiTi is very limited. NiTi2 is brittle, but NiTi and NbTi are ductile

217

623 K 2 R = 0.996

8.0x10-6

6.0x10

-6

573 K 2 R = 0.993 4.0x10

-6

2.0x10

-6

523 K R 2 = 0.962

0 0

100

200

300

400

500

600

700

800

(Pu0.5 - Pd0.5) [Pa0.5] Fig. 4. Plots of (J × L) vs. (Pu0.5 − Pd0.5 ) for Ni30 Ti31 Nb39 alloy at each temperature and R2 value obtained by linear regression analysis.

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Φ (mol H2 m-1 s-1 Pa-0.5)

10-7

400˚C

350˚C

300˚C

250˚C

Pd

10-8

Ni30Ti31Nb39

10-9

10-10 1.4

1.5

1.6

1.7

1.8

1.9

2.0

-1

1000 / T ( K ) Fig. 5. Temperature dependence of the hydrogen permeability Φ calculated from the slope of (J × L) vs. (Pu0.5 − Pd0.5 ) plots for Ni30 Ti31 Nb39 alloy and for pure Pd.

The hydrogen permeability falls with decreasing temperature in both the materials. The effect of temperature on the hydrogen permeability of Ni30 Ti31 Nb39 alloy is larger than that of pure Pd, indicating that the activation energy for hydrogen permeation through the Ni30 Ti31 Nb39 alloy is larger than that of pure Pd. The activation energy of hydrogen permeation through pure Pd is determined to be 15.9 kJ mol−1 , and is consistent with the reported value of 13.9 kJ mol−1 [7]. The Ni30 Ti31 Nb39 alloy gives an activation energy of 31.1 kJ mol−1 for hydrogen permeation. Table 1 summarizes the R2 values obtained for the Ni–Ti–Nb alloys and pure Pd at several temperatures. At 673 K, almost all alloys give rise to R2 values of about 1, but the values decrease with decreasing temperature. Fig. 6 shows the SEM photograph of the as-cast Ni30 Ti31 Nb39 alloy. This alloy consists of the primary phase, NbTi, and the eutectic phases (NiTi + NbTi). The NbTi phase is surrounded by the eutectic phases and its volume fraction is about 40 vol.%. That is, this alloy contains a large amount of NbTi, although its amount is small compared with that of Ni21 Ti51 Nb28 . The chemical com-

position of the NbTi phase is Ni4 Ti13 Nb83 and that of the eutectic phases is Ni41.1 Ti38.4 Nb20.5 . Fig. 7 shows the SEM photograph of the as-cast Ni41 Ti42 Nb17 alloy. This alloy almost consists of the eutectic structures of NiTi and NbTi. The composition of this alloy is approximately equal to the composition of the eutectic phase, Ni41.1 Ti38.4 Nb20.5 , in the Ni30 Ti31 Nb39 alloy. The hydrogen permeability of this alloy at 673 K is 6.42 × 10−9 mol H2 (m−1 s−1 Pa−0.5 ), which is one-third of that (1.93 × 10−8 ) of the Ni30 Ti31 Nb39 alloy. Consequently, the hydrogen permeation passing through the Ni30 Ti31 Nb39 alloy is considered to be realized not by the eutectic phases, but by the primary phase NbTi (Ni4 Ti13 Nb83 ). In other words, the NbTi phase contributes mainly to the hydrogen permeation of this duplex alloy. The single-phase alloy of

Table 1 R2 values of Ni–Ti–Nb alloys and pure palladium at several temperatures Composition (at.%)

673 K

623 K

573 K

523 K

Ni39 Ti50 Nb11 Ni41 Ti42 Nb17 Ni40 Ti31 Nb29 Ni29 Ti50 Nb21 Ni30 Ti42 Nb28 Ni33 Ti31 Nb39 Pure palladium

0.987 0.995 0.992 0.940 0.992 0.997 0.988

0.982 0.988 0.988 0.934 0.969 0.996 0.983

0.952 0.934 0.975 0.896 0.972 0.993 0.980

0.897 0.824 0.883 0.702 0.934 0.962 0.982

Fig. 6. SEM photograph of the as-cast Ni30 Ti31 Nb39 alloy which consists of the primary phase, NbTi, and eutectic phases (NiTi + NbTi).

K. Hashi et al. / Journal of Alloys and Compounds 368 (2004) 215–220

219

Fig. 7. SEM photograph of the as-cast Ni41 Ti42 Nb17 alloy which almost consists of the ternary eutectic phases (NiTi + NbTi).

NbTi (Ni4 Ti13 Nb83 ) becomes brittle and breaks down by hydrogenation, but in the Ni30 Ti31 Nb39 alloy it keeps its original shape. Consequently, the eutectic structure (NiTi+NbTi) is considered to suppresses the volume expansion of NbTi, resulting in the prevention of hydrogen brittleness of NbTi. That is, the eutectic structure (NiTi + NbTi) plays a major role in the suppression of hydrogen brittleness in the Ni30 Ti31 Nb39 alloy. We conclude from the above results that NbTi plays a major role in hydrogen permeation through the ternary Ni–Ti–Nb alloy, and its hydrogen brittleness is reduced in the presence of the fine eutectic structure. Fig. 8 shows the SEM photograph of the as-cast Ni29 Ti50 Nb21 alloy which presents the worst linearity in the (J × L) versus (Pu0.5 − Pd0.5 ) plots among the alloys tested (Table 1). This alloy consists of NiTi, the eutectic structure (NiTi + NbTi), NbTi and NiTi2 . The volume fraction of NiTi2 is 28%. The composition of NbTi, in the Ni29 Ti50 Nb21 alloy is determined to be Ni3 Ti31 Nb66 , and is

different from that in the Ni30 Ti31 Nb39 alloy. The NbTi in the Ni29 Ti50 Nb21 seems to form a hydride easily, resulting in the low linearity of the (J × L) versus (Pu0.5 − Pd0.5 ) plot.

4. Conclusion Hydrogen permeation passing through the ternary Ni–Ti–Nb alloys in the pressure range of 0.2–0.97 MPa on the upstream side and in the temperature range of 523–673 K were investigated. The hydrogen permeability of the NiTi intermetallic compound was 10−10 mol H2 (m−1 s−1 Pa−0.5 ). The hydrogen permeability of Ni–Ti–Nb alloys is improved when increasing the Nb content. Unless the Ni and Ti content is over 30 mol%, hydrogen permeation could not be measured due to severe hydrogen brittleness. The Ni30 Ti31 Nb39 alloy provided the highest hydrogen permeability, 1.93 × 10−8 mol H2 (m−1 s−1 Pa−0.5 ) at 673 K, which is almost identical with that of Pd. From hydrogen permeation experiments and structural observations, the following conclusions are drawn. 1. The eutectic structure (NiTi + NbTi) plays a major role in preventing the hydrogen brittleness of the NbTi phase. 2. Hydrogen permeability is enlarged by an increase in the amount of the NbTi phase, indicating that the NbTi phase is responsible for hydrogen permeation in the Ni–Ti–Nb alloys.

Acknowledgements

Fig. 8. SEM photograph of the as-cast Ni29 Ti50 Nb21 alloy which shows the worst linearity in the alloys shown in Table 1.

This work has been supported in part by a Grant-in-Aid for Scientific Research (B) of Japan Society for the Promotion of Science.

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