  The formation/binding energetics and length scales associated with the
interaction between He atoms and grain boundaries in BCC alpha-Fe was explored.
Ten different low Sigma grain boundaries from the <100> and <110> symmetric
tilt grain boundary systems were used. In this work, we then calculated
formation/binding energies for 1-2 He atoms in the substitutional and
interstitial sites (HeV, He2V, HeInt, He2Int) at all potential grain boundary
sites within 15 Angstroms of the boundary (52826 simulations total). The
present results provide detailed information about the interaction energies and
length scales of 1-2 He atoms with grain boundaries for the structures
examined. A number of interesting new findings emerge from the present study.
For instance, we find that the Sigma3(112) twin boundary in BCC Fe possesses a
much smaller binding energy than other boundaries, which corresponds in long
time dynamics simulations to the ability of an interstitial He defect to break
away from the boundary in simulations on the order of nanoseconds, in contrast
to other boundaries. Additionally, we find that the calculated
formation/binding energies for substitutional He (i.e., He in a monovacancy)
correlates well (R>0.9) with interstitial He, substitutional He2, and
interstitial He2 energies for the same atomic site. This correlation asserts
that the local environment surrounding each site strongly influences the He
defect energies and that highly accurate quantum mechanics calculations of
lower order defects may be an adequate predictor of higher order defects. The
present work shows that the binding and formation energies for He defects are
important for understanding the physics of He diffusion and trapping by grain
boundaries, which can be important for modeling He interactions in
polycrystalline steels.
