First-principles study of helium trapping in cementite Fe3C

We report a first-principles density functional theory study of helium distribution in cementite Fe3C. The solution energy of interstitial He is similar to that in bcc Fe; by contrast, the substitutional He (replacing Fe) is remarkably (0.74 eV) more stable than in the latter, due to the easiness of Fe vacancy formation in Fe3C. Therefore, He is predicted to be significantly more soluble in cementite than in Fe matrix. We find the binding potencies of both a substitutional-interstitial He pair (0.21 eV) and a substitutional-substitutional He pair (0.22 eV) are noticeably weaker in cementite than in bcc Fe, indicating a less powerful self-trapping. As a consequence, small size cementite in ferritic steels might serve as scattered trapping centers for He, mitigate helium bubble growth, and make the steel more swelling resistant while under neutron irradiation, just as dispersed oxide particles do.