Segregation of tin in Zircaloy-2 under proton irradiation

Simulating neutron irradiation with charged particles would allow materials to be tested for possible deleterious irradiation effects before being used in reactors. The higher damage rates from charged particle irradiation as compared to neutron irradiation make it possible to achieve comparable doses in significantly smaller times. In this study, tin precipitates between 0.1 and 0.5 μm in diameter were observed at the surface of Zircaloy-2 thin foils after 5.5-MeV proton irradiation. The samples were irradiated to a dose of 1 dpa, at a temperature of 360 K, and a dose rate of 10^-4 dpa·s^-1. X-ray analysis showed the precipitates to be almost pure tin, which was confirmed by diffraction analysis that showed the particles to be β-tin. A simple model was developed that explains the observations by a preferential solute-interstitial interaction, leading to a defect-induced solute flux to the surface, with consequent precipitation from supersaturated solid solution. The calculation of the solute-interstitial interaction is based on an equilibrium between the two types of interstitials, solute and solvent. The interstitials are created in stoichiometric proportions and then allowed to convert from one type to the other with a difference of δ in the energy barriers for the two conversions. Applying the model to the observations with δ = 0.1 eV yields between two and five monolayers of solute segregated to the surface during the irradiation time.