Browsing by Author "Jang, Seonhee"

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    Molecular Dynamics Simulations of Plastic Deformation in Nanocrystalline Metal and Alloy
    (2007-10-24) Jang, Seonhee; Ronald O. Scattergood, Committee Chair; Donald W. Brenner, Committee Member; Carl C. Koch, Committee Member; Korukonda L. Murty, Committee Member
    Nanocrystalline metals have different mechanical properties from conventional grain sized metals. Hardness and yield strength have been found to increase with decreasing grain size in the nanocrystalline regime down to at least 15 nm on the basis of Hall-Petch mechanisms. Below grain sizes of ˜10 nm, the strength decreases with further grain refinement, leading to the inverse Hall-Petch effect. Although the experimental evidence has found these deformation responses in nanocrystalline materials, the underlying mechanisms are not well identified. Molecular dynamics simulations were carried out for uniaxial tensile straining of two-dimensional columnar microstructures of aluminum (Al) and aluminum-lead (Al-Pb) alloys. Pure Al has a critical grain size at dc ≈ 15 to 20 nm, the crossover from "normal" to "inverse" Hall-Petch effect, accompanied with intra-grain mechanisms by partial dislocations and twins as grain sizes increases. With increasing grain size there exists a transition in plastic deformation mechanism from inter-grain processes to one that consists of both inter-grain and intra-grain processes. For Al-Pb alloys with a 10 nm grain size, Pb segregates completely to the grain boundaries and the grain boundaries become wider and more disorganized as the Pb content increases. A softening effect was observed in agreement with, but less than that found experimentally. As the Pb content increases, partial dislocation nucleation at grain boundaries is completely suppressed and the plastic strain is accommodated by mechanisms other than dislocation slip. As the grain sizes increase up to 15 or 20 nm, dislocation generation at grain boundaries is also suppressed. However, dislocation generation is not entirely suppressed at 3 equivalent at% Pb, compared to the 10 nm grain size showing complete suppression.