Abstract
First principles calculations have been performed to energetically investigate the helium
cluster nucleation, formation and growth behavior in the nano-structured ferritic alloy 14YWT.
The helium displays strong affinity to the oxygen:vacancy (O:Vac) pair. By investigating various
local environments of the vacancy, we find that the energy cost for He cluster growth increases with
the appearance of solutes in the reference unit. He atom tends to join the He cluster in the directions
away from the solute atoms. Meanwhile, the He cluster tends to expand in the directions away from
the solute atoms. A growth criterion is proposed based on the elastic instability strain of the perfect
iron lattice in order to determine the maximum number of He atoms at the vacancy site. We find
that up to seven He atoms can be trapped at a single vacancy. However, it is reduced to five if the
vacancy is pre-occupied by an oxygen atom. Furthermore, the solute atoms within nanoclusters,
such as Ti and Y, will greatly limit the growth of the He cluster. A migration energy barrier study is
performed to discuss the reduced mobility of the He atom/He cluster in 14YWT.
