Hydride formation in two-phase (Ti₃Al + TiAl) titanium aluminides during cathodic charging and its dissociation

A (TiAl)H_x hydride which has a tetragonal crystal structure with lattice parameters a = 0.452 nm and c = 0.326 nm (c/a = 0.721) has been observed in Ti42Al, Ti45Al and Ti50Al (at.%) two-phase (Ti₃Al (α₂) + TiAl (γ)) titanium aluminides by cathodic charging in a 5% H₂SO₄ solution. Cracks or pits are also observed within the γ phase regions in the two-phase (α₂ + γ) coexisting grains (such as lamellar grains) but not within the α₂ phase or single γ grains. Weights of the samples decrease with increasing charging time owing to the crack or pit formation, and this is more drastic in the Ti-50Al alloy than in the Ti-42Al and Ti-45Al alloys. The hydride is thermally stable at temperatures up to about 550 K (277 °C) and dissociates completely at temperatures between 673 K (400 °C) and 723 K (450 °C).