Pressure dependence of anomalous diffusion of zirconium in beta-titanium

The impurity diffusion coefficients of Zr in beta-Ti have been determined at temperatures from 1173 to 1773 K under pressures of 0.1 MPa and 1.0, 2.1, and 3.0 GPa using Ti/Ti-3.06 at, pct Zr diffusion couples. The Arrhenius plots of the diffusion coefficients exhibit strong curvature, and their degree becomes larger with increasing pressure. The activation volumes, Delta V evaluated from the isothermal pressure dependence of the diffusion coefficient are 0.22 to 0.36 Omega, where Omega is the atomic volume of the host metal. The values of Delta V/Omega are very close to the ratio of 0.33 for self-diffusion in beta-Ti reported by Jeffery and the ratios of 0.28 to 0.41 for W and Sn diffusion in beta-Ti reported by Araki et at, whereas they are considerably smaller than that normally expected for diffusion in bcc metals via ''simple'' monovacancy mechanism. The increase in degree of curvature in the Arrhenius plot by pressure and the small values of Delta V/Omega can be interpreted in terms of the model of phonon-assisted diffusion jumps via monovacancies.