The diffusivity of D and H in γ-based alloys was determined at room temperature by using an electrochemical technique. Cast Ti-48Al-2Cr and Ti-46.5Al-4(Cr,Nb,Ta,B) sheet, with primary and fully lamellar microstructures, were subjected to cathodic H-charging at room temperature in the galvanostatic mode. The variation in potential with time was monitored, and this data was used to deduce the H diffusion coefficient from error-function solutions to Fick’s second law. Very good agreement was obtained with regard to theoretical calculations. The diffusion coefficients appeared to be in close agreement with those for the cast alloy, as deduced from microhardness measurements. The results showed that neither the microstructure (lamellar colony size) nor the charging-current density appeared to have any significant effect upon the diffusivity. The Ti-48Al-2Cr alloy, with its duplex microstructure, appeared to have a slightly higher H diffusivity than that of Ti-46.5Al-4(Cr,Nb,Ta,B) sheet with either primary or fully lamellar microstructures. It appeared that the use of thicker samples resulted in an over-estimation of the diffusion coefficient. For thin samples, the diffusivity was equal to 9.73 x 10-11 cm2/s. For thick samples, it was equal to 8.45 x 10-9cm2/s. These values were compared with published values of the room-temperature diffusion coefficients for commercially pure polycrystalline Ti (2.6 x 10-10cm2/s for lattice diffusion, 9.1 x 10-5cm2/s for grain boundary diffusion). The diffusion of H in γ-based Ti alloys was therefore expected to occur through the lattice. The charging current density was found to have no effect upon the diffusivity values, and lattice diffusion was concluded to be the rate-controlling factor in the diffusion process.

Determination of the Diffusion Coefficient of Hydrogen in Gamma Titanium Aluminides during Electrolytic Charging. P.A.Sundaram, E.Wessel, H.Clemens, H.Kestler, P.J.Ennis, W.J.Quadakkers, L.Singheiser: Acta Materialia, 2000, 48[5], 1005-19