Vacancy loops in Breakaway Irradiation Growth of zirconium: Insight from atomistic simulations

Irradiation of zirconium alloys by neutrons causes dimensional changes associated with the formation of dislocation loops. In undeformed single crystals of this hexagonal close-packed material, an expansion in the crystallographic < c >-direction and a contraction in the < a >-direction are observed as a function of neutron fluence consisting of three stages, namely a rapid initial change, followed by a plateau, and then an accelerated "breakaway" growth whilst the volume remains constant. Molecular dynamics simulations suggest an atomic-level explanation: the initial dimensional changes are related to the formation of < a >-type nano-clusters of self-interstitial atoms (SlAs) while vacancies remain mostly isolated. In the plateau-region, formation of < a >-type vacancy loops compensates for the effect of growing SIA clusters and SIA < a >-loops. Upon further irradiation, vacancy < c > loops grow while the anisotropically diffusing SIAs anneal preferentially vacancy < a >-loops. By having a preferential vacancy sink on the basal plane and a net flow of interstitials to < a >-loops, the dimensional changes accelerate, thus leading to breakaway growth. The present simulations yield values for the thermodynamic stability, strain fields, and their effect on dimensional changes for vacancy < a > and < c >-loops as a function of their size, thus providing critical input for continuum models of radiation-induced growth of zirconium. (C) 2019 Elsevier B.V. All rights reserved.