ACTIVATION VOLUME FOR TUNGSTEN DIFFUSION IN BETA-TITANIUM

The diffusion coefficient of W in beta-Ti has been determined at temperatures from 1323 to 1673 K under pressures from 90 kPa to 2.8 GPa using the diffusion couple method. The values for the activation volume, DELTAV, calculated from the pressure dependence of the diffusion coefficients at various temperatures, are (0.28-0.41) V0, where V0 is the molar volume of Ti at room temperature and the atmospheric pressure. These values are considerably smaller than those measured for the bcc systems characterized by the ''simple'' monovacancy diffusion mechanism. We applied the model for phonon controlled diffusion to diffusion in beta-Ti under high pressure and evaluated DELTAV for self- and W diffusion in beta-Ti as functions of T and partial derivative T0/partial derivative P, where T is the temperature, P the pressure, and T0 the hypothetical temperature at which the bcc lattice becomes instable for atomic displacements in the [111] direction. The DELTAV estimated for partial derivative T0/partial derivative P = 11 K/GPa is in agreement with both experimental results for W diffusion in beta-Ti in the present work and for self-diffusion in beta-Ti reported by Jeffery. This good agreement suggests that diffusion in beta-Ti occurs with the mechanism of phonon-assisted diffusion jumps via monovacancies, which is characterized by a positive temperature-dependent migration enthalpy and a positive temperature-dependent migration volume.