Fotopoulos Fotis, Vasileios; (2024) Atomistic Modelling of Defects and Impurities in Polycrystalline Copper. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Atomistic simulations using density functional theory (DFT) and empirical force fields were employed to study the structure and properties of defects and impurities, and mechanisms of void formation in polycrystalline copper (Cu) under thermal stress conditions. These simulations have demonstrated that vacancy formation energies and diffusion barriers are much lower at grain boundaries (GBs) than in the bulk of Cu. The influence of metallic and non-metallic impurities on the strength of Cu GBs is explored, with DFT indicating that, while Ti, V, Zr, and Ag enhance the strength, others, such as H, O, S, F, and P, lead to weakening of Cu. The study of the mechanical properties of Cu-Ti alloys shows that, even a low concentration, the presence of Ti markedly enhances the yield strength and elastic modulus of Cu by impeding dislocation activity. The investigation of hydrogen embrittlement (HE) demonstrates that hydrogen promotes a reduction of vacancy formation energy, especially at GBs. In addition, it was demonstrated that H prefers to aggregate in GBs and has low diffusion barriers (0.2 eV) within these regions. Simulations quantified the detrimental effect of hydrogen on the mechanical resilience of bi-crystalline Cu and suggested that the formation of stacking faults and voids can be significantly increased near GBs in polycrystalline Cu under strain at hydrogen concentrations of 20 mass ppm or higher. The thesis also examined the thermodynamics of the interaction of Cu vacancies and hydrogen interstitials, suggesting that vacancy clustering and void formation can occur only at high-temperature and high-pressure hydrogen conditions. Therefore, additional processes must be considered to facilitate the formation of vacancy clusters and voids. To address that, interface stress effects were examined using the embedded atom method (EAM) and DFT simulations to determine the FCC/BCC interface properties and the relationship between the interfacial lattice mismatch and formed dislocations.

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