Reversible Hydrogen Absorption/Desorption and Related Lattice Deformation of Ti₃Al Based Alloys in the Ti-Al-Nb System

<jats:title>ABSTRACT</jats:title><jats:p>Hydrogen absorption/desorption of Ti<jats:sub>3</jats:sub>Al-based alloys with α<jats:sub>2</jats:sub> single-phase, β<jats:sub>0</jats:sub> single-phase and β/O two-phase microstructures was investigated to obtain systematic understanding microstructure effects. On exposing all the alloys to hydrogen, β (β<jats:sub>H</jats:sub>) and γ (γ<jats:sub>H</jats:sub>) hydrides are formed. The β<jats:sub>H</jats:sub> and γ<jats:sub>H</jats:sub> have bcc and bct (close to fcc) based structures, respectively. The hydrogen/metal ratio of the γ<jats:sub>H</jats:sub> is larger than that of β<jats:sub>H</jats:sub>. Thus, solute hydrogen in Ti<jats:sub>3</jats:sub>Al 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<jats:sub>H</jats:sub>-γ<jats:sub>H</jats:sub> transformation was observed in β/O two-phase (Ti,Nb)<jats:sub>3</jats:sub>Al and α<jats:sub>2</jats:sub>-Ti<jats:sub>3</jats:sub>Al alloys, but not obviously observed in a β<jats:sub>0</jats:sub>-(Ti,Nb)<jats:sub>3</jats:sub>Al alloy. In forming the β<jats:sub>H</jats:sub>, the expansion of the β<jats:sub>0</jats:sub> and β phases occurred isotropically keeping their structures B2 and bcc, respectively. In contrast, the O and α<jats:sub>2</jats:sub> phases expanded in anisotropic manner. The lattice deformation for the β<jats:sub>H</jats:sub>→γ<jats:sub>H</jats:sub> transformation can be described similarly to those for the O→β<jats:sub>H</jats:sub> and α<jats:sub>2</jats:sub>→β<jats:sub>H</jats:sub> 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 γ<jats:sub>H</jats:sub> after the β<jats:sub>H</jats:sub>→γ<jats:sub>H</jats:sub> transformation in the β<jats:sub>0</jats:sub>-(Ti,Nb)<jats:sub>3</jats:sub>Al alloy and many twins formed during the α<jats:sub>2</jats:sub>→β<jats:sub>H</jats:sub> transformation in the α<jats:sub>2</jats:sub>-Ti<jats:sub>3</jats:sub>Al alloy. Mechanisms of the reversible β<jats:sub>H</jats:sub>-γ<jats:sub>H</jats:sub> transformation in the β/O two-phase (Ti,Nb)<jats:sub>3</jats:sub>Al and α<jats:sub>2</jats:sub>-Ti<jats:sub>3</jats:sub>Al alloys was proposed on the basis of similarities in the lattice deformation and ordering of hydrogen atoms required for the β<jats:sub>H</jats:sub>-γ<jats:sub>H</jats:sub>, O-β<jats:sub>H</jats:sub> and α<jats:sub>2</jats:sub>-β<jats:sub>H</jats:sub> transformations.</jats:p>