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The properties of the cluster He(n)V, an iron vacancy with an increasing number of He atoms, is studied with molecular statics and molecular dynamics simulations. A study of the binding energy of the self-interstitial atom (SIA) and the He, shows that from n = 6 the He(n)V cluster is stable and cannot shrink anymore, and from n = 16 the He(n)V(2) cluster is stabilized by the emission of SIA in the form of a (110) dumbbell. Calculation of the pressure exercised by the He(n)V cluster shows local peak normal stress and shear stress values up to 9 GPa and 4 GPa, respectively. The local configurations of He(n)V suggest that with increasing helium content, a high symmetry configuration close to a face centered cubic lattice is formed.

The clustering of helium in bcc (body centered cubic) iron and the growth of a helium bubble are simulated at the atomistic level for the helium-rich vacancy-poor condition. It is shown that a ½ 111 dislocation loop is formed as a sequential collection of 111 crowdions, the latter being the most stable self-interstitial atom configuration in the presence of a He cluster.

Complex states of nanoscale interstitial dislocation loop can be described by its habit plane and Burgers vector. Using atomistic simulations, we provide direct evidences on the change of the habit plane of a 1/2〈1 1 1〉 loop from {1 1 1} to {1 1 0} and {2 1 1}, in agreement with TEM observations. A new {1 0 0} habit plane of this loop is also predicted by simulations. The non-conservation of the Burgers vector is approved theoretically for: (1) dislocation reactions between loops with different Burgers vectors and (2) the transition between 〈1 0 0〉 loops and 1/2〈1 1 1〉 loops. The rotation from a 1/2〈1 1 1〉 to a 〈1 0 0〉 loop has also been explored, which occurs at 570 K for time on the order of 10 s. The dislocation-precipitate phase duality and change of habit plane are then proposed as new features for nano-scale dislocation loops.