Molecular Dynamics Study of Radiation and Creep Response of Nanotwinned FCC Metals
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Research over the past decade has provided compelling evidence that nanotwinned structures may be optimal motifs for the design of high-strength high-ductility materials. This dissertation presents our atomistic study of the deformation mechanisms governing the radiation tolerance, high temperature creep, and fracture response of nanotwinned face-centered cubic (fcc) metals. We employ molecular dynamics (MD) to elucidate the synergistic role of grain boundaries (GBs) and coherent twin boundaries (CTBs) in the radiation tolerance of nanotwinned Cu. While GBs are known to be excellent sinks for point defects, CTBs do not absorb point defects. A beneficial corollary is that the structural integrity of CTBs remains intact as radiation-induced defects pass through them and get absorbed into GBs. Thus, our tension simulations reveal that nanotwinned metals continue to exhibit high strength even after being subjected to radiation damage. We also perform atomistic simulations of cyclic nanoindentation to complement experimental studies of cyclic nano- and micro-indentation, along with indentation creep, on nanotwinned Cu and Ag. Taken together, the studies provide evidence that nanotwinned fcc structures are more stable than their nanocrystalline counterparts. Inspired by the excellent mechanical stability of nanotwinned metals during indentation creep, we investigate high temperature creep in polycrystalline nanotwinned Cu using MD. The simulations reveal that the nanotwinned metals exhibit greater creep resistance with decreasing twin boundary (TB) spacing at all applied stresses. Nanotwinned metals with very high density of TBs exhibit a new creep deformation mechanism at high stresses governed by TB migration. This is in contrast to nanocrystalline and nanotwinned metals with larger twin spacing, which exhibit a more conventional transition from GB diffusion and sliding to dislocation nucleation. Finally, our investigation of the crack propagation along CTBs in a range of fcc metals with various crack and sample geometries indicates that the alternating brittle-ductile behavior of CTBs observed perviously is sensitive to the material, and crack length. In summary, our results furnish insights into the role of TBs in governing the remarkable mechanical stability, creep resistance and radiation tolerance of nanotwinned metals, making them strong candidates for future structural materials for extreme environments.