Reversible Hydrogen Absorption/Desorption and Related Lattice Deformation of Ti 3 Al Based Alloys in the Ti-Al-Nb System
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Reversible Hydrogen Absorption/Desorption and Related Lattice Deformation of Ti3Al Based Alloys in the Ti-Al-Nb System Kazuhiro Ito1, Lanting T. Zhang1, Yoshiharu Okabe1, Vijay K. Vasudevan2 and Masaharu Yamaguchi1 1 Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan 2 Department of Materials Science and Engineering, University of Cincinnati, Cincinnati, OH 45221-0012, U.S.A.
ABSTRACT Hydrogen absorption/desorption of Ti3Al-based alloys with α2 single-phase, β 0 single-phase and β/O two-phase microstructures was investigated to obtain systematic understanding microstructure effects. On exposing all the alloys to hydrogen, β (βH) and γ (γH) hydrides are formed. The βH and γH have bcc and bct (close to fcc) based structures, respectively. The hydrogen/metal ratio of the γH is larger than that of βH. Thus, solute hydrogen in Ti3Al based alloys stabilizes the bcc-based phase just like Nb, regardless of their original structure before hydrogenation. On the other hand, reversible hydrogen absorption/desorption around 100°C and related reversible β H-γH transformation was observed in β/O two-phase (Ti,Nb)3Al and α2-Ti3Al alloys, but not obviously observed in a β 0-(Ti,Nb)3Al alloy. In forming the βH, the expansion of the β0 and β phases occurred isotropically keeping their structures B2 and bcc, respectively. In contrast, the O and α2 phases expanded in anisotropic manner. The lattice deformation for the βH γH transformation can be described similarly to those for the O βH and α2 βH transformations. Such lattice deformation in anisotropic manner most likely agrees with that observed in a martensitic displacive transformation, because of the surface relief observed on the γH after the βH γH transformation in the β0-(Ti,Nb)3Al alloy and many twins formed during the α2 β H transformation in the α2-Ti3Al alloy. Mechanisms of the reversible βH-γH transformation in the β/O two-phase (Ti,Nb)3Al and α2-Ti3Al alloys was proposed on the basis of similarities in the lattice deformation and ordering of hydrogen atoms required for the βH-γH, O-βH and α2-βH transformations.
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INTRODUCTION It has been known that the hydrogen absorption and desorption of hydrogen absorbing alloys depend on their microstructure. In this study, the hydrogen absorption/desorption of Ti3Al-based alloys was investigated to obtain systematic understanding such microstructure effects [1-5]. The microstructure of Ti-Al-Nb alloys consisting of α2 (D019), β (bcc), β0(B2), and/or O (bcc-based orthorhombic, based on Ti2AlNb) phases can be systematically controlled on the basis of the well established knowledge of the nature of phase equilibria and transformations in the Ti-Al-Nb system [6-11]. Thus, Ti-Al-Nb alloys are not only candidates of alloys with a large hydrogen absorbing capacity but also they can be used as a good vehicle for studying the microstructure effect on hydrogen absorption/desorption. Figure 1 shows the crystal structures and structural relationship of the β0, O and α2 phases
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