Helium atoms in chromium-rich Fe–Cr alloys – Ab initio results

Helium generation in Fe–Cr alloys degrades the material by causing swelling and eventually the nucleation of bubbles. When the alloys are used as construction materials the resulting embrittlement is especially undesirable. The effects of helium atoms in pure iron have been recently elucidated by ab initio calculations. The different magnetic behavior – ferromagnetic for iron and anti-ferromagnetic for chromium – makes simple extrapolation to alloys and pure chromium uncertain. In the present study helium in pure chromium as well as chromium alloys with a small concentration of iron is investigated by ab initio density-functional theory calculations. The results indicate that interstitial helium atoms occupy the tetrahedral site in pure chromium. A pair of interstitial helium atoms have binding energies of about 0.3–0.6 eV when they are first or second nearest neighbors. When a substitutional iron atom is involved, the binding energy goes up to 1 eV. The formation energy of a substitutional helium atom is slightly higher (about 0.2 eV) when a substitutional iron atom is close by. The same holds for two helium atoms sitting in a vacancy, but then the energy difference is roughly twice as large. A substitutional helium atom is relatively strongly bound to a substitutional iron atom, with a binding energy of about 0.3 eV.