Effect of pressure on the fast diffusion in beta-titanium

The impurity diffusion coefficients of Co and Ni in beta-Ti have been determined at a constant temperature of 1223 K under pressures of 0.1 MPa to 3.2 GPa using Ti/Ti-2.25 at%Co and Ti/Ti-2.07 at%Ni diffusion couples. The activation volumes, Delta V, evaluated from the isothermal pressure dependence of the diffusion coefficients are 0.22 Omega for Co and 0.21 Omega for Ni, where Omega is the atomic volume of the host metal. Their signs are the opposite of those for the fast diffusion of Fe in beta-Ti previously reported by Peart, Gertsriken and Pryanishnikov, and their magnitudes are one-third of that normally expected for diffusion in bce metals via monovacancy mechanism. However, the positive and small values of Delta V/Omega are close to those for the fast diffusion in Pb, and can be interpreted in terms of the interstitial-substitutional dissociative model.