Effect of Pressure on the Anomalous Diffusion of Tin in β-Titanium

The diffusion coefficients of Sn in β-Ti have been determined at temperatures from 1173 to 1773 K under pressures from 0.1 MPa to 2.8 GPa using Ti/Ti-4.9 at%Sn diffusion couples. The diffusion coefficients at each of the pressures exhibit upward curvature in the Arrhenius plot, and its degree becomes larger with increasing pressure. The activation volume, ΔV⁄V0, has been evaluated from their isothermal pressure dependence to be (0.27–0.41), where V0 is the molar volume of Ti at room temperature and atmospheric pressure. The magnitude of ΔV⁄V0 is approximately one-half of ΔV⁄V0 measured for bcc systems characterized by the “simple” monovacancy diffusion mechanism. However, it is very close to the value of 0.33 for self-diffusion in β-Ti reported by Jeffery and the values of (0.28–0.41) for W diffusion in β-Ti reported by Araki et al. The increase of degree of curvature in the Arrhenius plot by pressure and the small values of ΔV⁄V0 can be interpreted in terms of the softening of phonons by pressure.