Helium bubbles in bcc Fe and their interactions with irradiation

The properties of helium bubbles in a body-centred cubic (bcc) Fe lattice have been examined. The atomic configurations and formation energies of different He-vacancy complexes were determined. The 0 K results show that the most energetically favourable He to Fe vacancy ratio increases from about 1:1 for approximately 5 vacancies up to about 4:1 for 36 vacancies. The formation mechanisms for small He clusters have also been considered. Isolated interstitials and small clusters can diffuse quickly through the lattice. MD simulations of randomly placed interstitial He atoms at 500 K showed clustering over the time scale of nanoseconds with He clusters containing up to 4 atoms being mobile over this time scale. He clusters containing 4 or 5 atoms were shown to eject an Fe dumbbell interstitial which could then detach from the He cluster and diffuse with the remaining He-vacancy complex being effectively immobile. Collision cascades initiated near larger bubbles showed that Fe vacancies produced by the cascades readily become part of the He-vacancy complexes. Energy barriers for He to join an existing bubble as a function of the He-vacancy ratio are also calculated. These can be larger than the diffusion barrier in the pristine lattice, but are lower when the bubbles contain excess vacancies, thus indicating that bubble growth may be kinetically constrained.