MICROSTRUCTURAL EVOLUTION OF DEFECTS AND THE NUCLEATION OF HELIUM BUBBLES IN NIOBIUM STUDIED BY POSITRON-ANNIHILATION

The nucleation of helium bubbles during the isochronal annealing of he lium-implanted niobium has been studied by positron annihilation measu rements. The evolution of implantation-induced radiation damage before the nucleation stage consists of processes such as the dissociation o f helium from dislocation loops, coarsening of vacancy clusters, heliu m-vacancy interaction and loop annealing. The helium bubble nucleation stage is characterised by a remarkable decrease in the positron lifet ime and is observed in the temperature interval from 750 degrees C to 850 degrees C. The stabilization of the helium bubbles in the post-nuc leation stage has been attributed to the effects of segregation of imp urities on the bubble surfaces. The positron lifetime data in the heli um bubble regime has been analysed in terms of the positron surface-st ate model, which revealed the presence of overpressurized bubbles. The properties of the helium bubbles observed in the present work have be en compared with those of athermal bubbles reported in earlier studies . A lower limit of 4.7 eV for helium-vacancy binding energy in niobium has been obtained. The helium-vacancy binding appears to be stronger than hydrogen-vacancy binding in niobium.