On the effect of low and high pressure hydrogen charging on the oxidation of a Ti–48Al–2Cr–2Nb alloy

The phases present in Ti–48Al–2Cr–2Nb γ titanium aluminide samples charged in hydrogen atmospheres containing very low partial pressures of oxygen have been analyzed in detail, particularly in near-surface regions of post-charged samples. Gaseous hydrogen exposure was conducted with gas pressure of 0.1 MPa at 740°C and 13.8 MPa at 800°C. A body-centered tetragonal hydride, designated θ, has been observed to be preferentially present in the surface regions of samples following charging. At a low hydrogen pressure of 0.1 MPa, hydrides are observed only in near-surface regions. At higher pressure of 13.8 Mpa, the hydrides form throughout the bulk of the material, but are present in much higher volume fractions in near-surface regions due to the availability of oxygen. The surface structure consists of a complex mixture of oxides and hydrides. The oxides exist in two layers: a TiO2 outer layer and an Al2O3 inner layer, with θ hydrides associated with the discontinuous inner alumina layer. The presence of hydrogen significantly increased the oxidation rate, compared to that observed in pure oxygen.