Calculation of the substitutional fraction of ion-implanted He in an Fe target

TL;DR

This study uses atomistic simulations to estimate the fraction of helium atoms occupying substitutional sites shortly after ion implantation in iron, providing insights into initial helium behavior relevant for nuclear fusion materials.

Contribution

It offers the first detailed atomistic calculation of the short-term substitutional fraction of implanted helium in iron and explores helium migration near vacancies.

Findings

At most 3% of He atoms occupy substitutional sites shortly after implantation.

Interstitial He migration barriers vary from 20 to 60 meV near vacancies.

Helium behavior in early implantation stages can be quantitatively estimated.

Abstract

Ion-implantation is a useful technique to study irradiation damage in nuclear materials. To study He effects in nuclear fusion conditions, He is co-implanted with damage ions to reproduce the correct He/dpa ratios in the desired or available depth range. However, the short-term fate of these He ions, i.e over the time scales of their own collisional phase, has not been yet unequivocally established. Here we present an atomistic study of the short-term evolution of He implantation in an Fe substrate to approximate the conditions encountered in dual ion-implantation studies in ferritic materials. Specifically, we calculate the fraction of He atoms that end up in substitutional sites shortly after implantation, i.e. before they contribute to long-term miscrostructural evolution. We find that fractions of at most 3% should be expected for most implantation studies. In addition, we carry out an exhaustive calculation of interstitial He migration energy barriers in the vicinity of matrix vacancies and find that they vary from approximately 20 to 60 meV depending on the separation and orientation of the He-vacancy pair.