Investigating iodine-stress corrosion cracking of zirconium using quantum mechanics and atomistic simulations
Creators: name: Podgurschi, Vlad
Publisher: name: Imperial College London
Year: 2021

description: This thesis focuses on investigating iodine stress corrosion cracking using multiple modelling techniques. After performing some preliminary simulations using a hybrid quantum mechanics/molecular mechanics technique, fun- damental processes were then analysed using an empirical potential and density functional theory calculations. Molecular dynamics simulations of single crystal zirconium fracture were performed in order to study the deforma- tion mechanisms active on the basal and prismatic planes under the effects of temperature (0 to 300 K) and strain rate (10^8 – 10^10 s−1). On the basal plane, fracture behaviour involved twinning ({11 ̄21}&lt;1 ̄126&gt;) at higher strain rates and the emission of type dislocations that then dissociated into partial dislocations around pyramidal II I2 stacking faults at lower strain rates. The addition of iodine was found to decrease the formation energy of this stacking fault from 394 to 326 mJ m−2. The emission of edge type dislocations was prevalent on the prismatic plane and was found to be strongly affected by temperature, with higher dislocation densities and mobility at higher temperatures and consequently larger crack growth. Crack growth on the basal plane was then further investigated with irradiation damage seen to have an effect on it. While the direction of the primary knock on atom did not have a significant impact on the number of equilibrium defects, it did affect their spatial distribution relative to the crack tip. If the point defects created were in the region of stacking fault growth, the extent to which the initial crack front was faulted was lower. Consequently, the crack was seen to grow larger. Density functional theory calculations were used to investigate the interaction of iodine and oxygen with bulk zirconium under the effect of hydrostatic strain between -2 % and 3 % to approximate crack tip conditions. The formation energy of a single iodine octahedral interstitial atom in zirconium was found to decrease with increasing hydrostatic strain becoming favourable at 3 %. The formation energy of an iodine substitutional defect in zirconium was found to be relatively insensitive to strain. In order to analyse the effect of oxygen on the formation energies of iodine interstitial and substitutional defects, various ordered zirconium-oxygen suboxides were considered (Zr6O, Zr3O, Zr2O and ZrO). As the oxygen content increased, the formation energy of an iodine interstitial defect increased from 1.03 eV to 8.61 eV supporting the idea that oxygen has a protective effect at the crack tip. At the same time, a 3 % applied tensile hydrostatic strain caused the iodine interstitial defect formation energy to decrease more in structures with higher oxygen content: 4.56 eV decrease in ZrO compared to 1.47 eV decrease for pure zirconium. Although substitutional defects may form preferentially to octahedral interstitial defects in the ordered suboxides, a 3 % tensile strain significantly narrows the gap and higher strains that are perfectly plausible on the atomic scale could cause interstitial defects to form preferentially.