Priya Vashishta
University of Southern California

Advanced materials and devices with nanometer grain/feature sizes are being developed to achieve higher strength and toughness in ceramic materials and greater speeds in electronic devices. Below 100 nm, however, continuum description of materials and devices must be supplemented by atomistic descriptions. Current state of the art atomistic simulations involve 10 million – 1 billion atoms. Pressure-induced structural transformations in spherical and faceted Gallium Arsenide nanocrystals of various shapes and sizes are investigated with a parallel molecular dynamics (MD) approach. The simulation results reveal that the pressure for zinc blende to rocksalt structural transformation depends on the nanocrystal size and all nanocrystals undergo non-uniform deformation during the transformation. Spherical nanocrystals above a critical diameter transform with grain boundaries. Faceted nanocrystals of comparable size have grain boundaries in 60% of the cases, whereas the other 40% are free of grain boundaries. Oxidation of aluminum nanoclusters is investigated with an approach based on dynamic charge transfer among atoms. Structural and dynamical correlations and local stresses reveal significant charge transfer and stress variations which cause rapid diffusion of Al and O on the nanocluster surface. The formation of a stable oxide scale of thickness 40A has been elucidated through the percolation of an OAl4 tetrahedral network. Two hundred and nine million atom MD simulation of hypervelocity projectile impact in aluminum nitride has also been studied. The simulations reveal strong interplay between shock-induced structural phase transformation, plastic deformation and brittle cracks. The shock wave splits into an elastic precursor and a wurtzite-to-rocksalt structural transformation wave. When the elastic wave reflected from the boundary of the sample interacts with the transformation wave front, nanocavities are generated along the penetration path of the projectile and dislocations in adjacent regions. The nanocavities coalesce to form mode I brittle cracks while dislocations generate kink bands that give rise to mode II cracks. These simulations provide a microscopic view of defects associated with simultaneous tensile and shear cracking at the structural phase transformation boundary due to shock impact in high-strength ceramics. Multiresolution algorithms that combine MD with finite element method and quantum simulations along with efficient visualization of billion atom data sets will also be discussed.

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