Microstructures and Hydrogen Permeability of Nb-NiTi Eutectic Alloys Prepared by Directional Solidification
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0980-II05-52
Microstructures and Hydrogen Permeability of Nb-NiTi Eutectic Alloys Prepared by Directional Solidification Yuji Yamaguchi1, Kyosuke Kishida1, Sho Tokui2, Kazuhiro Ishikawa2, Katsushi Tanaka1, Kiyoshi Aoki2, and Haruyuki Inui1 1 Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto, 6068501, Japan 2 Department of Materials Science, Kitami Institute of Technology, 165 Koen-cho, Kitami, 0908507, Japan
ABSTRACT The Nb-NiTi eutectic alloys were directionally solidified (DS) in an optical floating zone furnace. Columnar grains of eutectic structures containing rod-shaped Nb in the NiTi matrix is observed in the DS ingots irrespective of the alloy composition. Well-aligned rod-type eutectic microstructures can be obtained for samples with a composition of Nb-41Ni-40Ti grown at relatively slow growth rates below 2.5mm/h. The hydrogen permeability along the direction parallel to the Nb rods of Nb-41Ni-40Ti DS alloy grown at 1mm/h is 2.60×10-8mol H2 m-1 Pa-1/2 at 673K, which is over 20 times higher than that measured perpendicular to the Nb-rod direction. No hydrogen embrittlement is observed between 573 and 673K indicating that the eutectic structure itself is favorable in suppressing the hydrogen embrittlement of Nb. INTRODUCTION Pd-Ag alloys have been used as hydrogen permeation membranes for obtaining very pure hydrogen at laboratory scale. Palladium is, however, too expensive for the large-scale hydrogen production. Alternative membrane materials with lower cost and higher hydrogen purification efficiency are therefore strongly required to be developed. As shown in figure 1, some BCC metals are predicted to have greater hydrogen permeability than palladium because of their high hydrogen solubility and diffusivity [1,2]. Niobium has been considered as the best candidate with the highest predicted permeability among them. Although the predicted hydrogen permeability of pure Nb is much higher than that of Pd-Ag alloys [2], hydrogen embrittlement of this material hinders its practical applications [1]. Recent studies on the development of new class of hydrogen permeation membranes have revealed that multi-phase materials such as Nb-Ni-Ti ternary alloys composed of Nb-NiTi pseudo-binary eutectic structure and proeutectic Nb phase exhibit a good combination of the resistance to hydrogen embrittlement and high hydrogen permeability [3,4]. The Nb-NiTi eutectic structure has been considered to play an important role in suppressing the hydrogen embrittlement in the Nb-Ni-Ti alloys [3,4]. However, the mechanism of suppression of the hydrogen embrittlement and the inherent hydrogen permeability of the eutectic structure has not been clarified yet.
Figure 1. Predicted hydrogen permeability of some metals [1,2]. In order to clarify the inherent properties of the eutectic structure, we prepared directionally solidified (DS) ingots of the Nb-NiTi alloys with various alloy compositions near the Nb-NiTi pseudo-binary eutectic point and investigated the relationship between the microstructures
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