Elucidating He-H assisted cavity evolution in alpha Cr under multiple ion beam irradiation

Dual and triple ion beam irradiations were performed on alpha-Cr using 5 MeV Fe++, 2.9 MeV He++ and/or 270 keV H+ to study the synergy of hydrogen and helium on cavity formation. Results indicate that co-implantation of helium with iron enhances cavity nucleation and co-implantation of hydrogen with iron augments cavity growth. Under triple beam irradiation, hydrogen also accelerates cavity nucleation via stabilizing initial embryos. First-principles calculations reveal that the presence of helium increases the maximum number of hydrogen atoms that can be captured by a vacancy from 6 to 9, providing an atomistic explanation to the triple beam synergy. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.