ACTIVATION VOLUME FOR TUNGSTEN DIFFUSION IN BETA-TITANIUM

The diffusion coefficient of W in beta-Ti has been determined at tempe ratures from 1323 to 1673 K under pressures from 90 kPa to 2.8 GPa usi ng the diffusion couple method. The values for the activation volume, DELTAV, calculated from the pressure dependence of the diffusion coeff icients at various temperatures, are (0.28-0.41) V0, where V0 is the m olar volume of Ti at room temperature and the atmospheric pressure. Th ese values are considerably smaller than those measured for the bcc sy stems 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 diffu sion in beta-Ti as functions of T and partial derivative T0/partial de rivative P, where T is the temperature, P the pressure, and T0 the hyp othetical temperature at which the bcc lattice becomes instable for at omic displacements in the [111] direction. The DELTAV estimated for pa rtial derivative T0/partial derivative P = 11 K/GPa is in agreement wi th 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 mechanis m of phonon-assisted diffusion jumps via monovacancies, which is chara cterized by a positive temperature-dependent migration enthalpy and a positive temperature-dependent migration volume.