Anisotropic Microstructure and Hydrogen Permeability of Nb-Ti-Ni Duplex Phase Alloy Prepared by Hot Forging-Rolling Tech
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0980-II05-54
Anisotropic Microstructure and Hydrogen Permeability of Nb-Ti-Ni Duplex Phase Alloy Prepared by Hot Forging-Rolling Technique Sho Tokui1, Kazuhiro Ishikawa2, and Kiyoshi Aoki2 1
Materials Science, Graduate student of Kitami Institute of Technology, Koen-cho 165, Kitami, 090-8507, Japan 2
Materials Science, Kitami Institute of Technology, Koen-cho 165, Kitami, 090-8507, Japan
ABSTRACT Microstructures produced by the forging and the rolling of the Nb40Ti30Ni30 alloy at 1173K, and hydrogen permeability (Φ) measured along the different directions of this worked alloy were investigated using a scanning electron microscope (SEM), and a gas flow meter, respectively. The primary (Nb, Ti) phase was elongated along the working direction, and anisotropic microstructures were formed in this alloy. Hydrogen permeability (Φ) measured along the working direction (RD) was 4.4 x 10-8 [molH2m-1s-1Pa-1/2] at 673 K, which was 2.5 times higher than that of the as-cast one. On the other hand, the values of Φ measured along the other directions were lower than this. Thus, Φ of the Nb40Ti30Ni30 alloy had a great dependence on the direction of the (Nb, Ti) phase.
INTRODUCTION Hydrogen produced by a steam reforming of hydrocarbons such as CH4 contains CO gas, which damages the catalytic effect of Pt electrodes in fuel cells. Then, such hydrogen must be purified using a hydrogen permeation membrane and so on. The Pd-Ag alloy membranes are commonly used for hydrogen purification [1]. However, Pd is an expensive and rare metal. Consequently, it is strongly desired to develop low cost and high performance hydrogen permeation alloys other than Pd alloys [2-4]. Hydrogen flux (J) through the alloy membrane is described on the basis of the Fick’s first law and the Sieverts’ law as follows; J = Φ ∆P1/2/L
(1)
where Φ, ∆P1/2 and L are the hydrogen permeability, difference of the square root of the hydrogen pressures at both sides of the alloy and a thickness of the alloy membrane, respectively. Large J is obtainable by large pressure difference and by the thin membrane, which imply that the high yield strength and large resistance to the hydrogen embrittlement are essential for hydrogen permeation alloys. However, high hydrogen permeability is generally incomparable with large resistance to the hydrogen embrittlement in single-phase alloys. Recently, we have demonstrated that the Nb40Ti30Ni30 alloy consisting of the primary bcc-(Nb, Ti) and the eutectic
{B2-TiNi + bcc-(Nb, Ti)} phases show higher hydrogen permeability (Φ) without the hydrogen embrittlement [5-8]. Since Φ of the (Nb, Ti) phase is much higher than that of the eutectic {B2TiNi + bcc- (Nb, Ti)} phase, the arrangement of these phases is important for high Φ. According to the composite law, Φ of series arrangement (Φs) and parallel arrangement (Φp) are expressed as next equations; 1/Φs = Vα/Φα+Vβ/Φβ
Φp=ΦαVα+ΦβVβ
(2) (3)
where Vα and Vβ are volume fraction of α and β phases, Φα and Φβ (Φα < Φβ) are hydrogen permeability of each phase, respectively. From these formulae
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