Structure and stability of He and He-vacancy clusters at a Σ5(310)/[001] grain boundary in bcc Fe from first-principles.

We have studied the atomic structure and energetic stability of helium (He) and He-vacancy clusters in an iron (Fe) Σ5(310)/[001] grain boundary (GB) using a first-principles method. The He and He-vacancy clusters in the Fe GB are shown to exhibit high-symmetry structures. The equilibrium He-He distance in the clusters is ~1.70 Å, much smaller than 2.80 Å in the vacuum or 2.94 Å in a face centred cubic (fcc) crystal, indicating the attractive interaction between the He atoms due to the presence of Fe. The charge density surrounding He is demonstrated to decrease with an increasing number of He atoms in the clusters, leading to a positive binding energy of a He atom to the clusters. This suggests He and He-vacancy clusters can energetically trap more He atoms, which is responsible for the growth of the He-related clusters (He and He-vacancy clusters) and thus the He bubbles in the GB. The binding energy of an interstitial He atom to the He-related clusters is found generally lower in the GB than in a bcc crystal. Besides, the binding strengths of small He clusters to the GB and to a vacancy in a bcc matrix are compared, and the latter shows greater trapping strength to an interstitial He and a He(2) cluster. The magnetism of the Fe atoms near the GB as well as its variation caused by the He-related clusters is also investigated. The local magnetic moment variation of the Fe atoms in the system is enhanced to a different extent, depending on the size of the He-related clusters.