Using atomistic simulation to understand microstructural evolution under proton irradition

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Abstract

This thesis describes an investigation into how the interaction of protons with microstructural features in Zr and Zr alloys may affect the amount of irradiation damage created in proton-irradiated materials compared with neutron-irradiated materials. Three key microstructural features were examined: secondary phase particles (SPPs), the Zr lattice and grain boundaries (GBs). These features were chosen to determine the effect of localised composition on the damage created by protons compared with neutrons and the effect of proton channelling on the energy deposited by protons within single crystals and grains separated by GBs. The displacement per atom (dpa) rate for protons and neutrons interacting with bulk Zr and SPPs was calculated using the computer programs SPECTRA-PKA and SRIM. Whilst dpa rates produced by protons and neutrons were similar in bulk Zr compared with SPPs, the dpa rate of protons decreased with increasing atomic density of the SPPs, contrasting to the damage created by neutrons. Correlating the damage in a proton-irradiated sample with that of a neutron-irradiated sample using the dpa rate will therefore underestimate the damage in proton-irradiated SPPs. Molecular dynamics was used to simulate proton channelling in single crystal Zr and bi-crystals with twist, tilt and mixed GBs. The highest degree of proton channelling occurred in crystal orientations near low-index crystal directions like the <11-20> zone axis. The variability of channelling from grain to grain changed the energy deposited by protons up to 40.6%. The inclusion of grains separated by GBs increased the average energy loss rate of protons by 9.8% compared with a single crystal, with twist GBs reducing the proton energy loss rate by 37.5% compared with tilt or mixed GBs. The energy loss rate was strongly affected by the relative crystal orientation of the lower grain to the upper grain. The energy deposited by protons is therefore not only affected by the orientation of a grain but also by the relative orientation of a grain to the surrounding grains. To compensate for channelling effects and grains of different orientations, the quoted damage in a polycrystalline sample should be an average of the damage across many grains with different orientations.

Keyword(s)

Channelling; Grain boundaries; Irradiation damage; Neutron irradiation; Proton irradition; Secondary phase particle; Zirconium

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