HYDROGEN UPTAKE IN TITANIUM ALUMINIDES COVERED WITH OXIDE LAYERS

Two-phase (Ti3Al and TiAl) Ti-45Al and Ti-50Al (at. pet) alloys, which were preoxidized in static air at 1098 K (825 degrees C) for times be tween 900 seconds (0.25 hours) and 14.4 ks (4 hours), were charged the rmally at 773 K (500 degrees C) with flowing hydrogen gas at a pressur e of 0.1 MPa for 72 ks (20 hours), and the effect of oxide layers on h ydrogen penetration (or occlusion) in the alloys was investigated by t hermal desorption spectroscopy (TDS). The TDS main peak (accelerated h ydrogen evolution) temperature increased with an increasing thickness of oxide layers for both alloys, which results in a diffusion of hydro gen 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 hydr ogen trapping at the oxide surface. Total hydrogen uptake was the lowe st for both of the alloys with the thinnest oxide layers and then incr eased 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 o n the Ti-50Al alloy (about 300 nm) avoided about 83 pct. Titanium oxid e (TiO2) was unstable and might be reduced to a-titanium during heatin g (TDS analysis) at a vacuum level of 10(-6) Pa, whereas aluminum oxid e (Al2O3) did not change its chemical form.