Hydrogen evolution from cathodically charged two-phase (Ti₃Al + TiAl) titanium aluminides

Ti-45Al and Ti-50Al (at.%) titanium aluminides, whose microstructures consisted of Ti₃Al (α₂) and TiAl (γ), were cathodically hydrogen-charged in a 5% H₂SO₄ solution for charging times up to 14.4 ks (4 h), and the dissociation process of a hydride and the hydrogen evolution process during heating were investigated by thermal analyses (differential thermal analysis and thermal desorption spectroscopy). The hydride formed during cathodic charging dissociated at the temperature of about 700 K (427 °C), and corresponding to the hydride dissociation, hydrogen gas was evolved from the alloys at the dissociation temperature. In both alloys, accelerated hydrogen evolutions were observed at the lower temperatures than that for hydride dissociation. The evolution of hydrogen in the Ti-50Al alloy was extremely accelerated at about 523 K (250 °C) and the Ti-45Al at about 600 K (323 °C). The difference in the accelerated evolution temperatures was strongly dependent on the microstructures, in which structural imperfections, such as microvoids or internal cracks, could be formed during cathodic charging. The Ti-45Al alloy picked up about 1.5-times as much hydrogen as the Ti-50Al alloy, and more than 80% of the hydrogen was concentrated at the surface layer up to 20 μm in depth from the surface of the sample.