Self-diffusion along stationary and moving grain boundaries in α-Hf

Grain boundary (GB) self-diffusion of Hf has been studied in the α(HCP) phase of Hf-3 wt.% Zr in the temperature range 910 to 1493 K using the radiotracer serial sectioning technique. We have developed a comprehensive procedure for processing such profiles based on a simple model of tracer diffusion in a polycrystal containing both stationary and moving GBs. By applying this procedure we could determine the product P=DGB δ (DGB is the GB self-diffusion coefficient, δ the GB width) for stationary GBs and estimate the GB migration velocities assuming that the P values for stationary and migrating GBs were the same. For the activation enthalpy and the preexponential factor of self-diffusion along stationary GBs the values of HGB=(212±9) kJ mol-1 and P0=3.5.10-13 m3 s-1, respectively, have been obtained. Inspite of the 3%-addition of Zr, the obtained HGB and P0 values are supposed to closely represent self-diffusion in pure α-Hf. They give evidence for a vacancy mechanism of GB self-diffusion in α-Hf and are well consistent with previous results for other HCP transition metals. A roughly estimated activation enthalpy for GB migration, Hm=(195±18) kJ mol-1, demonstrates a good agreement with HGB and indicates that the activation barriers for atomic transport across and along the GBs are nearly identical. (orig./MM)