INVESTIGATION OF DEFECT STRUCTURES IN DEFORMED AND HELIUM-IRRADIATED REFRACTORY-METALS BY POSITRON-ANNIHILATION

The defect structures in deformed and helium-irradiated refractory met als, vanadium, niobium and tantalum have been investigated by means of positron lifetime and Doppler broadening techniques. The evolution of the defects was studied by performing isochronal annealing programmes . In all three metals after deformation a single, well defined lifetim e was observed which was 30 ps to 40 ps lower than that reported for v acancies. The recovery of the deformation-induced defects however, is remarkably different for niobium and tantalum as compared to vanadium. In helium-irradiated vanadium and niobium two distinctly different de fect types were observed with characteristic lifetimes of about 190 ps and 375 ps, respectively. The longer lifetime is attributed to large helium bubbles. Both metals showed a similar behaviour in the annealin g studies. All measured lifetime data are consistent with the simple t rapping model. The influence of oxygen impurities and oxygen resorptio n during the isochronal annealing procedure is discussed.