Self-Diffusion of Ti, Zr, and Hf in their HCP Phases, and Diffusion of Nb-95 in HCP Zr

Preliminary values for the self-diffusion constants of alpha-(hexagonal cp) Hf and new values for the self-diffusion constants of alpha-(hcp) Zr are given. A new determination of the latter was considered necessary since existing values disagree widely and the experimental methods employed in their determination are not considered to be sound. Values obtained by the authors for the self-diffusion constants of alpha-Ti are reviewed. The activation energies obtained are much smaller than those predicted by relationships based on physical properties of the elements and, within the experimental error, they have similar values to those corresponding to the beta-(body-centred cubic) phases. Frequency factors are too small to satisfy Zener's theory; when interpreted according to a vacancy model they give negative activation entropies, and the relationship D-o(alpha)/D-o(beta) is approximately the same for the three elements. It is suggested that the same diffusion mechanism operates in both the alpha- and beta-phases. If two mechanisms operate in the beta-phase, the low temperature one is the same as operates in the a-phase.