Point defects and sink strength in h.c.p. metals

We present in this paper, a comprehensive study of the static and dynamic properties of point defects and of their interaction with dislocations and grain boundaries. First, by using short-range potentials fitted to physical parameters of Mg and Zr, we calculate defect configurations and associated energies. Two different sets of potentials are used: one weakly repulsive below first-neighbour interaction range and the second strongly repulsive. Both predict the basal octahedral interstitial to be the most stable, and either the axial dumbbell or the trigonal respectively as the second equilibrium configuration. Based on these results and those for the vacancy, we predict the corresponding anisotropic diffusion coefficients. We find that for all cases the interstitial moves faster along basal plane directions than along the c axis. For the vacancy, the ratio between diffusivities depends on the potential being used. Sink strengths are calculated for grain boundaries and dislocations. Those strengths in an anisotropic hexagonal medium appear to depend mainly on the Dc/Da ratio. Under this premise, the anisotropic deformation under irradiation of Zr (radiation growth) is consistent with the anisotropy in the defect diffusivities predicted by the strongly repulsive potentials. (author)