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Microstructures and hydrogen permeability of directionally solidified Nb–Ni–Ti alloys with the Nb–NiTi eutectic microstructure
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Intermetallics 16 (2008) 88e95 www.elsevier.com/locate/intermet
Microstructures and hydrogen permeability of directionally solidified NbeNieTi alloys with the NbeNiTi eutectic microstructure Kyosuke Kishida a,*, Yuji Yamaguchi a, Katsushi Tanaka a, Haruyuki Inui a, Sho Tokui b, Kazuhiro Ishikawa b, Kiyoshi Aoki b a b
Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan Department of Materials Science, Kitami Institute of Technology, 165 Koen-cho, Kitami, 090-8507, Japan Received 25 March 2007; received in revised form 2 August 2007; accepted 8 August 2007 Available online 24 September 2007
Abstract The microstructures and hydrogen permeability properties were investigated for NbeNiTi eutectic alloys directionally solidified in an optical floating zone furnace. Rod-type eutectic microstructures with Nb rods aligned parallel to the growth direction are obtained with an alloy having a composition of Nbe41Nie40Ti grown at relatively slow growth rates below 2.5 mm/h. The hydrogen permeability depends on the relative direction of aligned Nb rods within the membrane. The values for the specimens with Nb rods aligned perpendicular to the membrane surface are about an order of magnitude larger than those for the specimens with Nb rods parallel to the surface. The largest value for the former specimen is as high as 2.60 108 mol H2 m1 s1 Pa1/2 (673 K), which is more than twice that reported for the as-cast NbeNiTi eutectic alloy. No hydrogen embrittlement is observed between 573 and 673 K, indicating that the NbeNiTi eutectic structure suppresses hydrogen embrittlement of Nb during hydrogen permeation. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: A. Composites; B. Anisotropy; B. Hydrogen embrittlement; C. Crystal growth; D. Microstructure
1. Introduction Pd-based alloys, in particular PdeAg alloys have been used as hydrogen permeation membranes for separation and purification of hydrogen gas produced by the steam reforming of natural gas containing methane and so on [1,2], because of the high hydrogen permeability and high resistance to hydrogen embrittlement of Pd. Palladium is, however, too expensive for large-scale industrial applications as hydrogen permeation membranes for separation and purification of hydrogen gas. The development of alternative membrane materials with lower cost and higher hydrogen purification efficiency is therefore strongly desired. From the viewpoint of the hydrogen
* Corresponding author. Tel.: þ81 75 753 5481; fax: þ81 75 753 5461. E-mail address: [email protected] (K. Kishida). 0966-9795/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2007.08.001
permeability (V), which is defined as the product of hydrogen diffusion coefficient (D) and the hydrogen solubility coefficient (K ), some BCC (body-centered cubic) metals of group 5 such as V, Nb and Ta are promising candidates since their predicted hydrogen permeability is larger than that of Pd [3e8]. Of these BCC metals, niobium has been considered as the best candidate due to the highest predicted permeability. Although the predicted hydrogen permeability of pure Nb is much higher than that of PdeAg alloys [3e5], hydrogen embrittlement of this material, which occurs as a result of hydrogen permeability, hinders its practical applications as hydrogen permeation membranes [9e11]. Recently, however, one of the author groups has developed a new class of hydrogen permeation membrane materials based on NbeNieTi alloy system; alloys containing NbeNiTi eutectic microstructures exhibit a good combination of the resistance to hydrogen embrittlement and high hydrogen permeability [9e11]. The NbeNiTi eutectic microstructure is believed to play a key
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role in achieving both high hydrogen permeability and high resistance to hydrogen embrittlement simultaneously. The former property is realized with Nb phase, while the latter with NiTi phase in the eutectic microstructure. Then, the distribution of Nb phase in the membrane is expected to play a decisive role in determining the hydrogen permeability. If all eutectic grains penetrate the thickness of the membrane so that no Nb grains in the eutectic microstructure are suspended in the membrane, the highest hydrogen permeability is expected to be achieved. We have tried many different ways to realize such a distribution of Nb phase in the membrane. One way to achieve this is hot forging and rolling [12] and we believe that directional solidification is an alternative way. However, there is no detailed study on directional solidification of NbeNieTi alloys with the NbeNiTi eutectic microstructure and it is not known yet whether or not the NbeNiTi eutectic microstructure can be aligned so that all Nb grains in the eutectic microstructure penetrate the thickness of the membrane. In the present study, we investigate the feasibility of directional solidification techniques in aligning the NbeNiTi eutectic microstructure. For this purpose, we first investigate the alloy composition which is the most favorable for aligning the eutectic microstructure. We then investigate the hydrogen permeability properties of membranes with the aligned Nbe NiTi eutectic microstructure produced by directional solidification.
2. Experimental procedure Rod ingots of NbeNieTi alloys with various alloy compositions near Nbe40Nie40Ti, which has been reported to be on the NbeNiTi monovariant eutectic line [13], were produced by arc-melting elemental Nb (99.9%), Ni (99.97%) and Ti (99.9%) in an Ar atmosphere. These ingots were directionally solidified with an optical floating zone (FZ) furnace at various growth rates ranging from 1.0 to 20 mm/h. These DS ingots were first sliced perpendicular to and/or parallel to the growth direction and then cut into disk-shaped samples (12 mm in diameter and 0.6 mm in thickness) by electrical discharge machining. Surfaces of the disks were polished mechanically and then finished with alumina suspension (0.3 mm). Both sides of the disks were coated with palladium with a thickness of 0.1 mm prior to hydrogen permeability measurements, in order to catalyze hydrogen dissociation and combination reactions and to protect the surfaces from oxidation. Measurements of hydrogen permeability were made at 573, 623 and 673 K with the apparatus described previously [9]. Microstructures of the DS ingots were examined before hydrogen permeability measurements by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). TEM samples were mechanically thinned and finally polished by a standard double-jet electropolishing method in a solution of sulfuric acid and methanol (15:85) at 243 K. Chemical compositions for selected regions of interest in the DS ingots were evaluated by energy dispersive X-ray spectroscopy (EDS) in SEM.
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3. Results 3.1. Microstructures SEM backscattered electron images of transverse sections (sections cut perpendicular to the growth direction) are shown in Fig. 1(a)e(c) for DS ingots with nominal compositions of NbexNiexTi (x ¼ 40, 40.5, 41; all alloy compositions will be given in at.% throughout the present paper) solidified at 20 mm/h. Bright and dark regions in the SEM images correspond to Nb and NiTi phases. Two-phase structures composed of Nb and NiTi phases are observed for all samples. Although the alloy composition of Nbe40Nie40Ti has been believed to be on NbeNiTi monovariant eutectic line from the inspection of arc-melted ingots [13], round-shaped grains of the primary Nb phase are observed in addition to the NiTi eutectic microstructure in the corresponding DS ingot (Fig. 1(a)). Roundshaped grains of the primary NiTi phase are observed in addition to the NiTi eutectic microstructure in the DS ingot with a composition of Nbe41Nie41Ti (Fig. 1(b)). In contrast, no grains that can be assigned as the primary solidification phase are observed and only the NbeNiTi eutectic microstructure is observed in the DS ingot with a composition of Nbe40.5Nie40.5Ti (Fig. 1(c)), indicating that of the three alloy compositions, Nbe40.5Nie40.5Ti is the closest composition to the NbeNiTi monovariant eutectic line. In contrast to the lamellar-type eutectic microstructure observed previously for arc-melted ingots with similar compositions [14], the eutectic microstructure observed in these DS ingots is of the rod-type. Since the volume fraction of the Nb phase in the NbeNiTi eutectic is estimated to be 25%, which is below the critical volume fraction (about 30%) for the lamellar-type eutectic growth [15,16], the rod-type eutectic growth should be preferred, as observed in the present study. The formation of lamellar-type eutectic microstructures in arc-melted ingots is mainly due to the high solidification rate [15,17]. Although no grains of the primary phase are observed in the DS ingot with the composition of Nbe40.5Nie40.5Ti, the eutectic microstructure in the vicinity of grain boundaries is rather coarse and irregular, in contrast to the eutectic microstructure within the grain, where Nb rods in the eutectic well align parallel to the growth direction. The coarse and irregular eutectic microstructure observed in the vicinity of grain boundaries is obviously due to the cellular growth during solidification. Since Nb rods in the eutectic microstructure in the vicinity of grain boundaries are not aligned at all, the reduction of such cellular boundary area is clearly important to achieve high hydrogen permeability. In order to investigate how the cellular boundary area varies with alloy composition, directional solidification was made for three different compositions (Nbe40Nie41Ti, Nbe40.5Nie 40.5Ti, and Nbe41Nie40Ti) at a slower growth rate of 5.0 mm/h, as shown in Fig. 1(d)e(f). No grains of the primary phase are observed for all three alloys, indicating that these compositions are all approximately on the NbeNiTi monovariant eutectic line. The thickness of the cellular boundary region decreases with the increase in the Ni content so that
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Fig. 1. SEM backscattered electron images of directionally solidified NbeNiTi alloys: (a) Nbe40Nie40Ti grown at 20 mm/h, (b) Nbe40.5Nie40.5Ti at 20 mm/h, (c) Nbe41Nie41Ti at 20 mm/h, (d) Nbe40Nie41Ti at 5.0 mm/h, (e) Nbe40.5Nie40.5Ti at 5.0 mm/h, and (f) Nbe41Nie40Ti at 5.0 mm/h.
a relatively homogeneous distribution of the Nb phase is achieved with the cell diameter of 20e50 mm for the DS ingot with a composition of Nbe41Nie40Ti (Fig. 1(f)). The chemical composition of Nbe41Nie40Ti is thus confirmed to be suitable for obtaining a homogeneous and well-aligned NbeNiTi eutectic microstructure. Since the cellular-type solidification has been known to be converted to the planar-type one by increasing the G/R ratio (G and R denote growth rate and temperature gradient, respectively) [18e21], directional solidification was made at different growth rates (1.0 and 2.5 mm/h) for the alloy with the composition of Nbe41Nie40Ti, in order to investigate how the cellular boundary area varies with growth rate. Fig. 2 shows typical microstructures of the Nbe41Nie40Ti DS alloys solidified at 2.5 and 1.0 mm/h. As seen from Fig. 2(a) and (d), the average cell size increases with the decrease in the growth rate approximately from 100 mm at 2.5 mm/h to 500 mm at 1.0 mm/h. Nb rods align
parallel to the growth directions well within the cellular grain (Fig. 2(c) and (f)), whereas those rods in the cellular boundary region tend to incline toward the cellular boundary (Fig. 2(c)) so as to exhibit the so-called fan-like arrangement, which is observed in the cellular solidification microstructure of many eutectic systems [17,18,21e23]. The thickness of the cellular boundary region does not depend much on the growth rate and is about 10e40 mm with an average of 20 mm in thickness for both samples estimated from SEM images (for example, Fig. 2(c) for sample solidified at 2.5 mm/h). The area fraction of the region containing well-aligned Nb rods consequently increases with the increase in the average cell size, i.e., with the decrease in the growth rate, it is increased from 35% at 2.5 mm/h to 85% at 1.0 mm/h. Although the alignment of Nb rods is not perfect because of the cellular boundary region, Nbe41Nie40Ti DS alloys solidified at 1.0 and 2.5 mm/h have a sufficiently high fraction of well-aligned Nb rods parallel to the growth direction. Hydrogen
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Fig. 2. Microstructures of directionally solidified Nbe41Nie40Ti alloys grown at (aec) 2.5 mm/h and (def) 1.0 mm/h, (a,b,d,e) transverse and (c,f) longitudinal sections.
permeability measurements were thus made for these DS ingots, as described later in Section 3.2. Chemical compositions of the constituent phases in the eutectic microstructure of Nbe41Nie40Ti DS ingot solidified at 1.0 mm/h are examined by EDS in SEM. SEM backscattered electron image and the corresponding composition maps taken from a cellular grain are depicted in Fig. 3(a) and (b)e(d),
respectively. As tabulated in Table 1, Nb phase contains nonnegligible amount of Ni and Ti, and the NiTi phase also contains non-negligible amount of Nb. Typical examples of TEM microstructures of Nbe41Nie40Ti DS ingot solidified at 1.0 mm/h are shown in Fig. 4 for the section cut perpendicular to the growth direction. Diffraction analysis reveals that the growth direction is nearly parallel to the [001] direction of the Nb rods (the inset
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Fig. 3. (a) SEM backscattered electron image and (bed) composition map of a transverse section of directionally solidified Nbe41Nie40Ti alloys grown at 1.0 mm/h.
of Fig. 4(a)). Nb rods are mostly faceted parallel to {100} and, to a lesser extent, to {110}. Surprisingly, all Nb rods are so heavily deformed as to contain a high density of dislocations. This is due obviously to the martensitic transformation of the surrounding NiTi phase that occurs during cooling following solidification, as is evident from the herringbone-type microstructure in the NiTi phase. As indicated in the experimental selected area electron diffraction (SAED) pattern (the inset of Fig. 4(a)), some of the diffraction spots are indexed with those of the B2 NiTi and BCC Nb phases with the cube-on-cube orientation relationships of (100)B2//(100)BCC and h001iB2//h001iBCC. Furthermore, the experimental SAED patterns contain many additional spots, most of which are confirmed to be indexed with that of B190 transformed variants of NiTi phase with the orientation relationships that (101)B2 is nearly parallel to (001)B190 and ½111B2 ==½110B190 [24]. Thus, Nb and B2 NiTi composite structures are considered to developed originally with the cube-on-cube orientation relationships of (100)B2//(100)BCC and h001iB2//h001iBCC. 3.2. Hydrogen permeability Hydrogen permeability measurements were made for the directionally solidified Nbe41Nie40Ti alloy with the wellaligned NbeNiTi eutectic microstructure grown at the rates of 2.5 and 1.0 mm/h, as seen in Fig. 5. Measurements were made for disks cut perpendicular to and parallel to the growth direction so that Nb rods in the eutectic microstructure penetrate the thickness of the disks in the former case and Nb rods in the eutectic microstructure lay parallel to the surface
of the disks in the latter case. These two cases are designated the ‘perpendicular’ and ‘parallel’ configurations, respectively. The value of hydrogen permeability generally increases with the increase in temperature and it strongly depends on the configuration of specimen. For the alloy grown at 2.5 mm/h, the values for the perpendicular configuration are about an order of magnitude larger than those for the parallel configuration, clearly indicating the importance of aligning Nb rods so as to penetrate the thickness of the membrane. For the perpendicular configuration, the value of hydrogen permeability for the specimen grown at 1.0 mm/h is almost twice that of the specimen grown at 2.5 mm/h. This is due obviously to the difference in the fraction of the area where Nb rods in the eutectic microstructure align parallel to the growth direction during cellular solidification (Fig. 2(a) and (d)), again indicating the importance of aligning Nb rods so as to penetrate the thickness of the membrane. The largest value of hydrogen permeability obtained at 673 K for the perpendicular configuration specimen grown at 1.0 mm/h is as high as 2.60 108 mol H2 m1 s1 Pa1/2, which is more than twice that reported for the as-cast Nbe40Nie40Ti eutectic alloy and is comparable to that reported for the NbeNiTi alloys consisting of grains of the primary Nb phase and those of the eutectic Table 1 Chemical compositions of Nb and NiTi phases in the directionally solidified Nbe41Nie40Ti alloy grown at 1.0 mm/h Phase
Nb (at.%)
Ni (at.%)
Ti (at.%)
Nb NiTi
82.85 6.43
7.31 47.70
9.84 45.87
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Fig. 4. TEM bright field images of directionally solidified Nbe41Nie40Ti alloys grown at 1.0 mm/h.
microstructure in 1:1 by volume [11]. Of importance to note is that no cracks are observed after hydrogen permeability measurements for all the specimens investigated, indicating that the directionally solidified alloys with the aligned NbeNiTi eutectic microstructure is not susceptible to hydrogen embrittlement at all during hydrogen permeation. These results thus indicate that the rod-type NbeNiTi eutectic structure possesses very good hydrogen permeability as well as a good resistance to hydrogen embrittlement. 4. Discussion The present experimental results indicate that the value of hydrogen permeability is closely related to the amount of Nb rods that align parallel to the growth direction during cellular solidification. The cellular grain size for the specimen grown at 2.5 mm/h is about 100 mm and only about 35% of Nb rods in the NbeNiTi eutectic microstructure align parallel to the growth
Fig. 5. Hydrogen permeability of Nbe41Nie40Ti DS ingots grown at 1.0 and 2.5 mm/h. Measurements were made for disks cut perpendicular and parallel to the growth direction, which are designated the ‘perpendicular’ and ‘parallel’ configurations, respectively.
direction (Fig. 2(a)). In contrast, about 85% of Nb rods in the eutectic microstructure align parallel to the growth direction for the specimen grown at 1.0 mm/h having the large cellular grain size of 500 mm (Fig. 2(a)). If hydrogen permeability of the alloys with the NbeNiTi eutectic microstructure is assumed to be due solely to the Nb phase, the value of hydrogen permeability at 673 K expected for the eutectic alloy with 100% of Nb rods being aligned perpendicular to the membrane surface is about 3.0 108 mol H2 m1 s1 Pa1/2, as shown in Fig. 6. Since the volume fraction of the Nb phase in the NbeNiTi eutectic is about 24%, the value of hydrogen permeability of pure Nb phase expected from the value (3.0 108 mol H2 m1 s1 Pa1/2) for the Nb phase in the NbeNiTi eutectic at 673 K is 1.3 107 mol H2 m1 s1 Pa1/2, which is by far smaller than the theoretical value (3.6 106 mol H2 m1 s1 Pa1/2) and is only about one third of the experimental value (3.6 107 mol H2 m1 s1 Pa1/2) [3,5]. There are some factors to be considered to understand the lowering mechanisms for the hydrogen permeability of the Nb phase in the eutectic structure. As revealed by the EDS analysis (Fig. 3 and Table 1), the Nb phase contains non-negligible amount of Ni and Ti. Similarly to the case for some ternary alloyed Nb reported in literature [25,26], these alloying elements are expected to act as trapping site for the interstitial hydrogen, which may result in the decrease of the hydrogen diffusion coefficient (D) and therefore the hydrogen permeability. We believe that the existence of a high density of dislocations introduced in the Nb rods (Fig. 4) presumably due to the B2eB190 martensitic transformation of the NiTi phase during cooling is also one of the factors lowering the value of hydrogen permeability of the NbeNiTi eutectic alloys. The high density of defects such as dislocations may also act as trapping site for the hydrogen and therefore reduce diffusivity of hydrogen in the Nb phase [27]. Eliminating the effects of defects in the Nb phase by annealing at a temperature below the B2eB190 martensitic transformation of NiTi is currently under way in our group. Another factor for lowering the value of hydrogen permeability in the eutectic alloys, we believe, is constraint force exerted on the Nb phase from the surrounding NiTi phase upon
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Fig. 6. Hydrogen permeability of Nbe41Nie40Ti DS ingots at 673 K plotted as a function of the volume fraction of the aligned Nb rods. Filled circles and error bars indicate the volume fractions estimated using the average thickness of 20 mm of the boundary regions in the cellular structure and the ranges of estimation errors relating to the thickness variation of the cellular boundary region, respectively.
hydrogen permeation. Once hydrogen permeates, Nb rods dilate with the extent depending on the solubility of hydrogen. However, these Nb rods cannot dilate freely because of the surrounding NiTi phase. Then, constraint force is expected to be exerted on the Nb phase, which reduces the hydrogen solubility in the Nb phase at a given hydrogen pressure (Fig. 7), thereby, the hydrogen permeability. Although the existence of the NiTi phase is thus expected to reduce the value of hydrogen permeability of the Nb phase in the NbeNiTi eutectic alloys, it greatly reduces the susceptibility to hydrogen embrittlement by reducing the extent of dilatation of the Nb phase upon hydrogen permeation. On top of that, high deformability of NiTi plays a role in reducing the susceptibility to hydrogen embrittlement. Although the volume fraction of the Nb phase in the Nbe NiTi eutectic cannot be changed, we may be able to further increase the value of hydrogen permeability of the NbeNiTi
eutectic by increasing the fraction of Nb rods aligned parallel to the growth direction. Judging from the microstructures of Figs. 1 and 2, unaligned Nb rods in the eutectic are resulted from cellular-type solidification. These unaligned Nb rods are expected to change to aligned rods by changing the solidification front from cellular to planar. In order to achieve the planar solidification, the growth rate and thermal gradient should be sufficiently small and large, respectively. In the present experiment, the growth rate is already sufficiently small but the thermal gradient achieved with the optical floating zone furnace is rather low because of the low melting temperature (about 1423 K) of the NbeNiTi eutectic alloy. If we employ a furnace that can achieve higher thermal gradient, the planar solidification front may be achieved, resulting in the increased fraction of Nb rods aligned parallel to the growth direction. Experiments with a Bridgeman-type furnace having a higher thermal gradient are undertaken in our group. 5. Conclusions (1) Rod-type eutectic microstructures with Nb rods aligned parallel to the growth direction are obtained with an alloy having a composition of Nbe41Nie40Ti grown at relatively slow growth rates below 2.5 mm/h. (2) Since the solidification occurs with the cellular-type front, Nb rods in regions between cellular grains are not aligned parallel to the growth direction. The fractional area of unaligned Nb rods can be reduced by reducing the growth rate. (3) The value of hydrogen permeability depends on the relative direction of aligned Nb rods against the surface of the membrane. The values for the specimen with Nb rods aligned perpendicular to the membrane surface are about an order of magnitude larger than those for the specimen with Nb rods aligned parallel to the membrane surface. (4) The largest value of hydrogen permeability obtained at 673 K for the former specimen is as high as 2.60 108 mol H2 m1 s1 Pa1/2, which is more than twice that reported for the as-cast polycrystalline Nbe NiTi eutectic alloy, indicating the importance of aligning the Nb phase in the NbeNiTi eutectic microstructure. (5) No hydrogen embrittlement is observed between 573 and 673 K, indicating that the NbeNiTi eutectic structure suppresses hydrogen embrittlement of Nb during hydrogen permeation.
Acknowledgements This work was partly supported by the 21st Century COE (Center of Excellence) Program on United Approach for New Materials Science from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. References Fig. 7. Schematic illustration of the hydrogen concentration in NbeNiTi twophase hydrogen permeation membranes with aligned Nb rods.
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K. Kishida et al. / Intermetallics 16 (2008) 88e95 [2] Kikuchi E. Membrane reactor application to hydrogen production. Catal Today 2000;56:97e101. [3] Steward SA. Review of hydrogen isotope permeability through materials, Lawrence Livermore National Laboratory report UCRL-53441; 1983. [4] Buxbaum RE, Marker TL. Hydrogen transport through non-porous membranes of palladium-coated niobium, tantalum and vanadium. J Membr Sci 1993;85:29e38. [5] Buxbaum RE, Kinney AB. Hydrogen transport through tubular membranes of palladium-coated tantalum and niobium. Ind Eng Chem Res 1996;35:530e7. [6] Nishimura C, Komaki M, Hwang S, Amano M. VeNi alloy membranes for hydrogen purification. J Alloy Compd 2002;330e332:902e6. [7] Zhang Y, Ozaki T, Komaki M, Nishimura C. Hydrogen permeation characteristics of vanadiumealuminium alloys. Scripta Mater 2002;47: 601e6. [8] Ozaki T, Zhang Y, Komaki M, Nishimura C. Preparation of palladiumcoated V and Ve15Ni membranes for hydrogen purification by electroless plating technique. Int J Hydrogen Energy 2003;28:297e302. [9] Hashi K, Ishikawa K, Matsuda T, Aoki K. Hydrogen permeation characteristics of multi-phase NieTieNb alloys. J Alloys Compd 2004;368: 215e20. [10] Hashi K, Ishikawa K, Matsuda T, Aoki K. Microstructures and hydrogen permeabililty of NbeTieNi alloys with high resistance to hydrogen embrittlement. Mater Trans 2005;46:1026e31. [11] Luo W, Ishikawa K, Aoki K. Hydrogen permeability in NbeTieNi alloys containing much primary (Nb,Ti) phase. Mater Trans 2005;46:2253e9. [12] Tokui S, Ishikawa K, Aoki K. Anisotropic microstructure and hydrogen permeability of NbeTieNi duplex phase alloy prepared by hot forgingrolling technique. In: Fu CL, Clemens H, Wiezorek J, Takeyama M, Morris D, editors. Advanced intermetallic-based alloys. Warrendale: Materials Research Society; 2007. p. 461e6 [Mater Res Soc Symp Proc 980].
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Intermetallics 16 (2008) 88e95 www.elsevier.com/locate/intermet
Microstructures and hydrogen permeability of directionally solidified NbeNieTi alloys with the NbeNiTi eutectic microstructure Kyosuke Kishida a,*, Yuji Yamaguchi a, Katsushi Tanaka a, Haruyuki Inui a, Sho Tokui b, Kazuhiro Ishikawa b, Kiyoshi Aoki b a b
Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan Department of Materials Science, Kitami Institute of Technology, 165 Koen-cho, Kitami, 090-8507, Japan Received 25 March 2007; received in revised form 2 August 2007; accepted 8 August 2007 Available online 24 September 2007
Abstract The microstructures and hydrogen permeability properties were investigated for NbeNiTi eutectic alloys directionally solidified in an optical floating zone furnace. Rod-type eutectic microstructures with Nb rods aligned parallel to the growth direction are obtained with an alloy having a composition of Nbe41Nie40Ti grown at relatively slow growth rates below 2.5 mm/h. The hydrogen permeability depends on the relative direction of aligned Nb rods within the membrane. The values for the specimens with Nb rods aligned perpendicular to the membrane surface are about an order of magnitude larger than those for the specimens with Nb rods parallel to the surface. The largest value for the former specimen is as high as 2.60 108 mol H2 m1 s1 Pa1/2 (673 K), which is more than twice that reported for the as-cast NbeNiTi eutectic alloy. No hydrogen embrittlement is observed between 573 and 673 K, indicating that the NbeNiTi eutectic structure suppresses hydrogen embrittlement of Nb during hydrogen permeation. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: A. Composites; B. Anisotropy; B. Hydrogen embrittlement; C. Crystal growth; D. Microstructure
1. Introduction Pd-based alloys, in particular PdeAg alloys have been used as hydrogen permeation membranes for separation and purification of hydrogen gas produced by the steam reforming of natural gas containing methane and so on [1,2], because of the high hydrogen permeability and high resistance to hydrogen embrittlement of Pd. Palladium is, however, too expensive for large-scale industrial applications as hydrogen permeation membranes for separation and purification of hydrogen gas. The development of alternative membrane materials with lower cost and higher hydrogen purification efficiency is therefore strongly desired. From the viewpoint of the hydrogen
* Corresponding author. Tel.: þ81 75 753 5481; fax: þ81 75 753 5461. E-mail address: [email protected] (K. Kishida). 0966-9795/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2007.08.001
permeability (V), which is defined as the product of hydrogen diffusion coefficient (D) and the hydrogen solubility coefficient (K ), some BCC (body-centered cubic) metals of group 5 such as V, Nb and Ta are promising candidates since their predicted hydrogen permeability is larger than that of Pd [3e8]. Of these BCC metals, niobium has been considered as the best candidate due to the highest predicted permeability. Although the predicted hydrogen permeability of pure Nb is much higher than that of PdeAg alloys [3e5], hydrogen embrittlement of this material, which occurs as a result of hydrogen permeability, hinders its practical applications as hydrogen permeation membranes [9e11]. Recently, however, one of the author groups has developed a new class of hydrogen permeation membrane materials based on NbeNieTi alloy system; alloys containing NbeNiTi eutectic microstructures exhibit a good combination of the resistance to hydrogen embrittlement and high hydrogen permeability [9e11]. The NbeNiTi eutectic microstructure is believed to play a key
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role in achieving both high hydrogen permeability and high resistance to hydrogen embrittlement simultaneously. The former property is realized with Nb phase, while the latter with NiTi phase in the eutectic microstructure. Then, the distribution of Nb phase in the membrane is expected to play a decisive role in determining the hydrogen permeability. If all eutectic grains penetrate the thickness of the membrane so that no Nb grains in the eutectic microstructure are suspended in the membrane, the highest hydrogen permeability is expected to be achieved. We have tried many different ways to realize such a distribution of Nb phase in the membrane. One way to achieve this is hot forging and rolling [12] and we believe that directional solidification is an alternative way. However, there is no detailed study on directional solidification of NbeNieTi alloys with the NbeNiTi eutectic microstructure and it is not known yet whether or not the NbeNiTi eutectic microstructure can be aligned so that all Nb grains in the eutectic microstructure penetrate the thickness of the membrane. In the present study, we investigate the feasibility of directional solidification techniques in aligning the NbeNiTi eutectic microstructure. For this purpose, we first investigate the alloy composition which is the most favorable for aligning the eutectic microstructure. We then investigate the hydrogen permeability properties of membranes with the aligned Nbe NiTi eutectic microstructure produced by directional solidification.
2. Experimental procedure Rod ingots of NbeNieTi alloys with various alloy compositions near Nbe40Nie40Ti, which has been reported to be on the NbeNiTi monovariant eutectic line [13], were produced by arc-melting elemental Nb (99.9%), Ni (99.97%) and Ti (99.9%) in an Ar atmosphere. These ingots were directionally solidified with an optical floating zone (FZ) furnace at various growth rates ranging from 1.0 to 20 mm/h. These DS ingots were first sliced perpendicular to and/or parallel to the growth direction and then cut into disk-shaped samples (12 mm in diameter and 0.6 mm in thickness) by electrical discharge machining. Surfaces of the disks were polished mechanically and then finished with alumina suspension (0.3 mm). Both sides of the disks were coated with palladium with a thickness of 0.1 mm prior to hydrogen permeability measurements, in order to catalyze hydrogen dissociation and combination reactions and to protect the surfaces from oxidation. Measurements of hydrogen permeability were made at 573, 623 and 673 K with the apparatus described previously [9]. Microstructures of the DS ingots were examined before hydrogen permeability measurements by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). TEM samples were mechanically thinned and finally polished by a standard double-jet electropolishing method in a solution of sulfuric acid and methanol (15:85) at 243 K. Chemical compositions for selected regions of interest in the DS ingots were evaluated by energy dispersive X-ray spectroscopy (EDS) in SEM.
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3. Results 3.1. Microstructures SEM backscattered electron images of transverse sections (sections cut perpendicular to the growth direction) are shown in Fig. 1(a)e(c) for DS ingots with nominal compositions of NbexNiexTi (x ¼ 40, 40.5, 41; all alloy compositions will be given in at.% throughout the present paper) solidified at 20 mm/h. Bright and dark regions in the SEM images correspond to Nb and NiTi phases. Two-phase structures composed of Nb and NiTi phases are observed for all samples. Although the alloy composition of Nbe40Nie40Ti has been believed to be on NbeNiTi monovariant eutectic line from the inspection of arc-melted ingots [13], round-shaped grains of the primary Nb phase are observed in addition to the NiTi eutectic microstructure in the corresponding DS ingot (Fig. 1(a)). Roundshaped grains of the primary NiTi phase are observed in addition to the NiTi eutectic microstructure in the DS ingot with a composition of Nbe41Nie41Ti (Fig. 1(b)). In contrast, no grains that can be assigned as the primary solidification phase are observed and only the NbeNiTi eutectic microstructure is observed in the DS ingot with a composition of Nbe40.5Nie40.5Ti (Fig. 1(c)), indicating that of the three alloy compositions, Nbe40.5Nie40.5Ti is the closest composition to the NbeNiTi monovariant eutectic line. In contrast to the lamellar-type eutectic microstructure observed previously for arc-melted ingots with similar compositions [14], the eutectic microstructure observed in these DS ingots is of the rod-type. Since the volume fraction of the Nb phase in the NbeNiTi eutectic is estimated to be 25%, which is below the critical volume fraction (about 30%) for the lamellar-type eutectic growth [15,16], the rod-type eutectic growth should be preferred, as observed in the present study. The formation of lamellar-type eutectic microstructures in arc-melted ingots is mainly due to the high solidification rate [15,17]. Although no grains of the primary phase are observed in the DS ingot with the composition of Nbe40.5Nie40.5Ti, the eutectic microstructure in the vicinity of grain boundaries is rather coarse and irregular, in contrast to the eutectic microstructure within the grain, where Nb rods in the eutectic well align parallel to the growth direction. The coarse and irregular eutectic microstructure observed in the vicinity of grain boundaries is obviously due to the cellular growth during solidification. Since Nb rods in the eutectic microstructure in the vicinity of grain boundaries are not aligned at all, the reduction of such cellular boundary area is clearly important to achieve high hydrogen permeability. In order to investigate how the cellular boundary area varies with alloy composition, directional solidification was made for three different compositions (Nbe40Nie41Ti, Nbe40.5Nie 40.5Ti, and Nbe41Nie40Ti) at a slower growth rate of 5.0 mm/h, as shown in Fig. 1(d)e(f). No grains of the primary phase are observed for all three alloys, indicating that these compositions are all approximately on the NbeNiTi monovariant eutectic line. The thickness of the cellular boundary region decreases with the increase in the Ni content so that
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Fig. 1. SEM backscattered electron images of directionally solidified NbeNiTi alloys: (a) Nbe40Nie40Ti grown at 20 mm/h, (b) Nbe40.5Nie40.5Ti at 20 mm/h, (c) Nbe41Nie41Ti at 20 mm/h, (d) Nbe40Nie41Ti at 5.0 mm/h, (e) Nbe40.5Nie40.5Ti at 5.0 mm/h, and (f) Nbe41Nie40Ti at 5.0 mm/h.
a relatively homogeneous distribution of the Nb phase is achieved with the cell diameter of 20e50 mm for the DS ingot with a composition of Nbe41Nie40Ti (Fig. 1(f)). The chemical composition of Nbe41Nie40Ti is thus confirmed to be suitable for obtaining a homogeneous and well-aligned NbeNiTi eutectic microstructure. Since the cellular-type solidification has been known to be converted to the planar-type one by increasing the G/R ratio (G and R denote growth rate and temperature gradient, respectively) [18e21], directional solidification was made at different growth rates (1.0 and 2.5 mm/h) for the alloy with the composition of Nbe41Nie40Ti, in order to investigate how the cellular boundary area varies with growth rate. Fig. 2 shows typical microstructures of the Nbe41Nie40Ti DS alloys solidified at 2.5 and 1.0 mm/h. As seen from Fig. 2(a) and (d), the average cell size increases with the decrease in the growth rate approximately from 100 mm at 2.5 mm/h to 500 mm at 1.0 mm/h. Nb rods align
parallel to the growth directions well within the cellular grain (Fig. 2(c) and (f)), whereas those rods in the cellular boundary region tend to incline toward the cellular boundary (Fig. 2(c)) so as to exhibit the so-called fan-like arrangement, which is observed in the cellular solidification microstructure of many eutectic systems [17,18,21e23]. The thickness of the cellular boundary region does not depend much on the growth rate and is about 10e40 mm with an average of 20 mm in thickness for both samples estimated from SEM images (for example, Fig. 2(c) for sample solidified at 2.5 mm/h). The area fraction of the region containing well-aligned Nb rods consequently increases with the increase in the average cell size, i.e., with the decrease in the growth rate, it is increased from 35% at 2.5 mm/h to 85% at 1.0 mm/h. Although the alignment of Nb rods is not perfect because of the cellular boundary region, Nbe41Nie40Ti DS alloys solidified at 1.0 and 2.5 mm/h have a sufficiently high fraction of well-aligned Nb rods parallel to the growth direction. Hydrogen
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Fig. 2. Microstructures of directionally solidified Nbe41Nie40Ti alloys grown at (aec) 2.5 mm/h and (def) 1.0 mm/h, (a,b,d,e) transverse and (c,f) longitudinal sections.
permeability measurements were thus made for these DS ingots, as described later in Section 3.2. Chemical compositions of the constituent phases in the eutectic microstructure of Nbe41Nie40Ti DS ingot solidified at 1.0 mm/h are examined by EDS in SEM. SEM backscattered electron image and the corresponding composition maps taken from a cellular grain are depicted in Fig. 3(a) and (b)e(d),
respectively. As tabulated in Table 1, Nb phase contains nonnegligible amount of Ni and Ti, and the NiTi phase also contains non-negligible amount of Nb. Typical examples of TEM microstructures of Nbe41Nie40Ti DS ingot solidified at 1.0 mm/h are shown in Fig. 4 for the section cut perpendicular to the growth direction. Diffraction analysis reveals that the growth direction is nearly parallel to the [001] direction of the Nb rods (the inset
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Fig. 3. (a) SEM backscattered electron image and (bed) composition map of a transverse section of directionally solidified Nbe41Nie40Ti alloys grown at 1.0 mm/h.
of Fig. 4(a)). Nb rods are mostly faceted parallel to {100} and, to a lesser extent, to {110}. Surprisingly, all Nb rods are so heavily deformed as to contain a high density of dislocations. This is due obviously to the martensitic transformation of the surrounding NiTi phase that occurs during cooling following solidification, as is evident from the herringbone-type microstructure in the NiTi phase. As indicated in the experimental selected area electron diffraction (SAED) pattern (the inset of Fig. 4(a)), some of the diffraction spots are indexed with those of the B2 NiTi and BCC Nb phases with the cube-on-cube orientation relationships of (100)B2//(100)BCC and h001iB2//h001iBCC. Furthermore, the experimental SAED patterns contain many additional spots, most of which are confirmed to be indexed with that of B190 transformed variants of NiTi phase with the orientation relationships that (101)B2 is nearly parallel to (001)B190 and ½111B2 ==½110B190 [24]. Thus, Nb and B2 NiTi composite structures are considered to developed originally with the cube-on-cube orientation relationships of (100)B2//(100)BCC and h001iB2//h001iBCC. 3.2. Hydrogen permeability Hydrogen permeability measurements were made for the directionally solidified Nbe41Nie40Ti alloy with the wellaligned NbeNiTi eutectic microstructure grown at the rates of 2.5 and 1.0 mm/h, as seen in Fig. 5. Measurements were made for disks cut perpendicular to and parallel to the growth direction so that Nb rods in the eutectic microstructure penetrate the thickness of the disks in the former case and Nb rods in the eutectic microstructure lay parallel to the surface
of the disks in the latter case. These two cases are designated the ‘perpendicular’ and ‘parallel’ configurations, respectively. The value of hydrogen permeability generally increases with the increase in temperature and it strongly depends on the configuration of specimen. For the alloy grown at 2.5 mm/h, the values for the perpendicular configuration are about an order of magnitude larger than those for the parallel configuration, clearly indicating the importance of aligning Nb rods so as to penetrate the thickness of the membrane. For the perpendicular configuration, the value of hydrogen permeability for the specimen grown at 1.0 mm/h is almost twice that of the specimen grown at 2.5 mm/h. This is due obviously to the difference in the fraction of the area where Nb rods in the eutectic microstructure align parallel to the growth direction during cellular solidification (Fig. 2(a) and (d)), again indicating the importance of aligning Nb rods so as to penetrate the thickness of the membrane. The largest value of hydrogen permeability obtained at 673 K for the perpendicular configuration specimen grown at 1.0 mm/h is as high as 2.60 108 mol H2 m1 s1 Pa1/2, which is more than twice that reported for the as-cast Nbe40Nie40Ti eutectic alloy and is comparable to that reported for the NbeNiTi alloys consisting of grains of the primary Nb phase and those of the eutectic Table 1 Chemical compositions of Nb and NiTi phases in the directionally solidified Nbe41Nie40Ti alloy grown at 1.0 mm/h Phase
Nb (at.%)
Ni (at.%)
Ti (at.%)
Nb NiTi
82.85 6.43
7.31 47.70
9.84 45.87
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Fig. 4. TEM bright field images of directionally solidified Nbe41Nie40Ti alloys grown at 1.0 mm/h.
microstructure in 1:1 by volume [11]. Of importance to note is that no cracks are observed after hydrogen permeability measurements for all the specimens investigated, indicating that the directionally solidified alloys with the aligned NbeNiTi eutectic microstructure is not susceptible to hydrogen embrittlement at all during hydrogen permeation. These results thus indicate that the rod-type NbeNiTi eutectic structure possesses very good hydrogen permeability as well as a good resistance to hydrogen embrittlement. 4. Discussion The present experimental results indicate that the value of hydrogen permeability is closely related to the amount of Nb rods that align parallel to the growth direction during cellular solidification. The cellular grain size for the specimen grown at 2.5 mm/h is about 100 mm and only about 35% of Nb rods in the NbeNiTi eutectic microstructure align parallel to the growth
Fig. 5. Hydrogen permeability of Nbe41Nie40Ti DS ingots grown at 1.0 and 2.5 mm/h. Measurements were made for disks cut perpendicular and parallel to the growth direction, which are designated the ‘perpendicular’ and ‘parallel’ configurations, respectively.
direction (Fig. 2(a)). In contrast, about 85% of Nb rods in the eutectic microstructure align parallel to the growth direction for the specimen grown at 1.0 mm/h having the large cellular grain size of 500 mm (Fig. 2(a)). If hydrogen permeability of the alloys with the NbeNiTi eutectic microstructure is assumed to be due solely to the Nb phase, the value of hydrogen permeability at 673 K expected for the eutectic alloy with 100% of Nb rods being aligned perpendicular to the membrane surface is about 3.0 108 mol H2 m1 s1 Pa1/2, as shown in Fig. 6. Since the volume fraction of the Nb phase in the NbeNiTi eutectic is about 24%, the value of hydrogen permeability of pure Nb phase expected from the value (3.0 108 mol H2 m1 s1 Pa1/2) for the Nb phase in the NbeNiTi eutectic at 673 K is 1.3 107 mol H2 m1 s1 Pa1/2, which is by far smaller than the theoretical value (3.6 106 mol H2 m1 s1 Pa1/2) and is only about one third of the experimental value (3.6 107 mol H2 m1 s1 Pa1/2) [3,5]. There are some factors to be considered to understand the lowering mechanisms for the hydrogen permeability of the Nb phase in the eutectic structure. As revealed by the EDS analysis (Fig. 3 and Table 1), the Nb phase contains non-negligible amount of Ni and Ti. Similarly to the case for some ternary alloyed Nb reported in literature [25,26], these alloying elements are expected to act as trapping site for the interstitial hydrogen, which may result in the decrease of the hydrogen diffusion coefficient (D) and therefore the hydrogen permeability. We believe that the existence of a high density of dislocations introduced in the Nb rods (Fig. 4) presumably due to the B2eB190 martensitic transformation of the NiTi phase during cooling is also one of the factors lowering the value of hydrogen permeability of the NbeNiTi eutectic alloys. The high density of defects such as dislocations may also act as trapping site for the hydrogen and therefore reduce diffusivity of hydrogen in the Nb phase [27]. Eliminating the effects of defects in the Nb phase by annealing at a temperature below the B2eB190 martensitic transformation of NiTi is currently under way in our group. Another factor for lowering the value of hydrogen permeability in the eutectic alloys, we believe, is constraint force exerted on the Nb phase from the surrounding NiTi phase upon
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Fig. 6. Hydrogen permeability of Nbe41Nie40Ti DS ingots at 673 K plotted as a function of the volume fraction of the aligned Nb rods. Filled circles and error bars indicate the volume fractions estimated using the average thickness of 20 mm of the boundary regions in the cellular structure and the ranges of estimation errors relating to the thickness variation of the cellular boundary region, respectively.
hydrogen permeation. Once hydrogen permeates, Nb rods dilate with the extent depending on the solubility of hydrogen. However, these Nb rods cannot dilate freely because of the surrounding NiTi phase. Then, constraint force is expected to be exerted on the Nb phase, which reduces the hydrogen solubility in the Nb phase at a given hydrogen pressure (Fig. 7), thereby, the hydrogen permeability. Although the existence of the NiTi phase is thus expected to reduce the value of hydrogen permeability of the Nb phase in the NbeNiTi eutectic alloys, it greatly reduces the susceptibility to hydrogen embrittlement by reducing the extent of dilatation of the Nb phase upon hydrogen permeation. On top of that, high deformability of NiTi plays a role in reducing the susceptibility to hydrogen embrittlement. Although the volume fraction of the Nb phase in the Nbe NiTi eutectic cannot be changed, we may be able to further increase the value of hydrogen permeability of the NbeNiTi
eutectic by increasing the fraction of Nb rods aligned parallel to the growth direction. Judging from the microstructures of Figs. 1 and 2, unaligned Nb rods in the eutectic are resulted from cellular-type solidification. These unaligned Nb rods are expected to change to aligned rods by changing the solidification front from cellular to planar. In order to achieve the planar solidification, the growth rate and thermal gradient should be sufficiently small and large, respectively. In the present experiment, the growth rate is already sufficiently small but the thermal gradient achieved with the optical floating zone furnace is rather low because of the low melting temperature (about 1423 K) of the NbeNiTi eutectic alloy. If we employ a furnace that can achieve higher thermal gradient, the planar solidification front may be achieved, resulting in the increased fraction of Nb rods aligned parallel to the growth direction. Experiments with a Bridgeman-type furnace having a higher thermal gradient are undertaken in our group. 5. Conclusions (1) Rod-type eutectic microstructures with Nb rods aligned parallel to the growth direction are obtained with an alloy having a composition of Nbe41Nie40Ti grown at relatively slow growth rates below 2.5 mm/h. (2) Since the solidification occurs with the cellular-type front, Nb rods in regions between cellular grains are not aligned parallel to the growth direction. The fractional area of unaligned Nb rods can be reduced by reducing the growth rate. (3) The value of hydrogen permeability depends on the relative direction of aligned Nb rods against the surface of the membrane. The values for the specimen with Nb rods aligned perpendicular to the membrane surface are about an order of magnitude larger than those for the specimen with Nb rods aligned parallel to the membrane surface. (4) The largest value of hydrogen permeability obtained at 673 K for the former specimen is as high as 2.60 108 mol H2 m1 s1 Pa1/2, which is more than twice that reported for the as-cast polycrystalline Nbe NiTi eutectic alloy, indicating the importance of aligning the Nb phase in the NbeNiTi eutectic microstructure. (5) No hydrogen embrittlement is observed between 573 and 673 K, indicating that the NbeNiTi eutectic structure suppresses hydrogen embrittlement of Nb during hydrogen permeation.
Acknowledgements This work was partly supported by the 21st Century COE (Center of Excellence) Program on United Approach for New Materials Science from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. References Fig. 7. Schematic illustration of the hydrogen concentration in NbeNiTi twophase hydrogen permeation membranes with aligned Nb rods.
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