Hydrogen uptake in titanium aluminides covered with oxide layers

Abstract

Two-phase (Ti3Al and TiAl) Ti-45Al and Ti-50Al (at. pct) alloys, which were preoxidized in static air at 1098 K (825 °C) for times between 900 seconds (0.25 hours) and 14.4 ks (4 hours), were charged thermally at 773 K (500 °C) with flowing hydrogen gas at a pressure of 0.1 MPa for 72 ks (20 hours), and the effect of oxide layers on hydrogen penetration (or occlusion) in the alloys was investigated by thermal desorption spectroscopy (TDS). The TDS main peak (accelerated hydrogen evolution) temperature increased with an increasing thickness of oxide layers for both alloys, which results in a diffusion of hydrogen through oxide layers. The onset temperature of hydrogen evolution showed the highest values for the alloys with thinner oxide layers and then decreased with increasing thickness of oxide layers, due to hydrogen trapping at the oxide surface. Total hydrogen uptake was the lowest for both of the alloys with the thinnest oxide layers and then increased with an increasing thickness of oxide layers. The thinnest oxide layer on the Ti-45Al alloy (about 600 nm) avoided about 97 pct of the hydrogen occluded in the alloy without an oxide layer, whereas that on the Ti-50Al alloy (about 300 nm) avoided about 83 pct. Titanium oxide (TiO2) was unstable and might be reduced to α-titanium during heating (TDS analysis) at a vacuum level of 10−6 Pa, whereas aluminum oxide (Al2O3) did not change its chemical form.