Vacancy Loops in Breakaway Irradiation Growth of Zirconium: Insight from Atomistic Simulations
Mikael Christensen, Walter Wolf, Clive Freeman, Erich Wimmer, R. B. Adamson, M. Griffiths, E. V. Mader
Journal of Nuclear Materials
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 <a>-direction and a contraction in the <c>-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 (SIAs) 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.
Atomic models of defect free Zr, and vacancy <a>-loops and <c>-loops with diameters d = 2 nm, 6 nm, 14 nm, and 24 nm are available in MedeA .sci, and .cif formats and can be downloaded here:
Structural and energetic data for computation of loop energy of formation and dimensional changes are summarized here: