He-V cluster nucleation and growth in α-Fe grain boundaries

The nucleation of helium (He) clusters and their growth in a monovacancy (V) eventually leads to the formation of He bubbles on grain boundaries and within the lattice for alpha-Fe, which can cause the macroscopic deterioration of material properties. The research objective herein is to model the nucleation and growth of small He clusters by calculating the formation/binding energetics of a single He atom to a HenV cluster (n = {1-7}) and by capturing how the grain boundary affects this behavior in bcc alpha-Fe. The formation energies for 1-8 He atoms in a monovacancy are calculated at all potential grain boundary sites within 15 A of ten select high angle grain boundaries. These results are combined with previously calculated vacancy formation energies and interstitial He formation energies to quantify how the local grain boundary structure impacts the binding of an interstitial He atom to a HenV cluster. We find that, despite the large range of different local environments within the grain boundaries, it is nearly always energetically favorable for a nearby interstitial He atom to combine with either a monovacancy or a HenV cluster to form a larger HenV cluster, with a binding energy that can be much greater (as much as 100% greater) than in the bulk crystal. Furthermore, a model is presented that captures the formation and binding energies of the various He V clusters while capturing the subsequent binding energies of different clusters/defects in the presence of grain boundaries both of which are important when accounting for the total energetics pertaining to He V cluster growth in the presence of the high angle grain boundaries. Published by Elsevier Ltd on behalf of Acta Materialia Inc.