Interaction between helium and self-defects in α-iron from first principles

The interaction between helium and self-defects in α-iron has been studied by means of density functional theory calculations. We show that the fast migrating interstitial helium atoms, which are known to strongly interact with vacancies, can also be weakly trapped by self-interstitial atoms, with a binding energy of about 0.3 eV. The small He n clusters, which can form as a result of the attractive interaction between interstitial helium atoms, induce strong lattice distortions and we predict that the emission of a self-interstitial atom is energetically favorable for n > 4. We also report on the structural and energetic properties of small helium-vacancy clusters (He nV m) with n/m ratio around the optimal value with respect to the emission of vacancy or helium (n/m ≅ 1.3). We suggest that helium dissociation from these clusters can be responsible for stages above 700 K (II and IV) observed in thermal helium desorption spectra