ON THE ANISOTROPIC MIGRATION OF POINT-DEFECTS IN HCP ZIRCONIUM

Point-defect calculations in h.c.p. zirconium have been carried out wi th a pair potential constructed in terms of the generalized pseudopote ntial theory. Two models of the h.c.p. lattice with either the ideal o r the experimental c/a ratio were considered. It has been found with b oth models that the monovacancy has the same migration energy in and o ut of the basal plane, whereas the divacancy migration is anisotropic and favours out-of-plane mobility. A significant anisotropy for inters titial migration is observed, with greater in-plane mobility at the ex perimental c/a ratio. The di-interstitial migrates in the basal plane by the crowdion mechanism. The results of the calculations permit inte rpretation of some annealing stages of irradiated zirconium and allow conclusions to be drawn on the possibility of the diffusional anisotro py difference in h.c.p. zirconium.