Hydrogen evolution from chemically etched titanium aluminides

Ti-45Al (Ti₃Al (α₂) and TiAl (γ)) and Ti-53Al (single γ) titanium aluminides were chemically etched in a solution of 10 ml HF + 5 ml HNO₃ + 85 ml H₂O for etching times between 10 and 120 s at room temperature, and hydrogen evolution behavior during heating and microstructural change were investigated by means of thermal desorption spectroscopy (TDS), X-ray diffractometry and scanning electron microscopy. The TDS spectra indicated that hydrogen was dissolved in the alloy matrix at an early stage of etching for both alloys and an additional etching enhanced the formation of a hydride phase in the Ti-45Al alloy and the formation of pit in the Ti-53Al alloy. No hydride formation was observed in the Ti-53Al alloy. The solubility limit of hydrogen during chemical etching was estimated to be about 8 wppm in the TI-45Al alloy and about 3 wppm in the Ti-53Al alloy. The activation energy of hydrogen evolution, which could be determined from TDS peak temperatures at different heating rates, resulting from hydrogen dissolution in the α₂ phase and the y phase, hydride dissociation in the T1-45Al alloy and hydrogen trapping at pit sites were determined to be 1029, 144, 37 and 30 kJ mol^-1 respectively.