Microstructural evolution of defects and the nucleation of helium bubbles in niobium studied by positron annihilation

Abstract

The nucleation of helium bubbles during the isochronal annealing of helium-implanted niobium has been studied by positron annihilation measurements. The evolution of implantation-induced radiation damage before the nucleation stage consists of processes such as the dissociation of helium from dislocation loops, coarsening of vacancy clusters, helium-vacancy interaction and loop annealing. The helium bubble nucleation stage is characterised by a remarkable decrease in the positron lifetime and is observed in the temperature interval from 750°C to 850°C. The stabilization of the helium bubbles in the post-nucleation stage has been attributed to the effects of segregation of impurities on the bubble surfaces. The positron lifetime data in the helium bubble regime has been analysed in terms of the positron surface-state model, which revealed the presence of overpressurized bubbles. The properties of the helium bubbles observed in the present work have been 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.

Key Words:

• microstructural
• positron annihilation
• He bubble
• niobium
• vacancy clusters
• isochronal annealing
• helium-vacancy interaction.