Atomistic studies of helium defect properties in bcc iron: Comparison of He-Fe potentials

In the fusion irradiation environment, helium created by transmutation will play an important role in the response of structural materials to neutron radiation damage. Atomistic simulations have been carried out using a new three-body He-Fe interatomic potential and the results have been compared to those obtained using two He-Fe pair potentials. In simulations with the three-body potential, helium interstitials are very mobile and multiple He interstitials can coalesce to form interstitial clusters which are also mobile. The He interstitial cluster binding energy is in good agreement with DFT calculations. If the He cluster is sufficiently large, it can create additional free volume by ejecting an Fe interstitial atom, creating a Frenkel pair. The corresponding vacancy is incorporated into the existing He cluster, and the resulting helium-vacancy cluster is not mobile. The ejected self-interstitial atom is mobile, but is trapped by the He-vacancy cluster. If additional helium atoms join a He-vacancy cluster, more Fe interstitials can be ejected and they are observed to form small interstitial clusters (nascent dislocation loop). Although multiple helium atoms can be trapped in a single vacancy, a vacancy containing only a small number of helium atoms can recombine with an Fe interstitial to recreate a helium interstitial cluster. The He binding energy with one of the He-Fe pair potentials (Wilson's) is much higher, leading to more rapid He clustering and Frenkel pair formation. Very little He clustering occurs with the second He-Fe pair potential.