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High-resolution electron microscope observations of shock-loaded boron carbide have revealed the formation of nanoscale intragranular amorphous bands that occur parallel to specific crystallographic planes and contiguously with apparent cleaved fracture surfaces. This damage mechanism explains the measured, but not previously understood, decrease in the(More)
We report transmission electron microscope observations that provide evidence of deformation twinning in plastically deformed nanocrystalline aluminum. The presence of these twins is directly related to the nanocrystalline structure, because they are not observed in coarse-grained pure aluminum. We propose a dislocation-based model to explain the preference(More)
In crystalline materials, plastic deformation occurs by the motion of dislocations, and the regions between individual crystallites, called grain boundaries, act as obstacles to dislocation motion. Grain boundaries are widely envisaged to be mechanically static structures, but this report outlines an experimental investigation of stress-driven grain(More)
In high-resolution electron microscopy (HREM), dislocation core structures are examined by tilting the dislocation end-on along the appropriate zone axis. For end-on screw dislocations diffraction contrast is largely due to surface relaxation in the form of the Eshelby twist. In this paper, simulated, many-beam images of end-on, 1/2<111> Mo screw(More)
Recent advances in topological optimization methodologies for design of internal material architecture, coupled with the emergence of micro-and nanoscale fabrication processes, 3D imaging, and micron scale testing methodologies, now make it possible to design, fabricate, and characterize lattice materials with unprecedented control. This talk will describe(More)
Findings of laser-assisted atom probe tomography experiments on boron carbide elucidate an approach for characterizing the atomic structure and interatomic bonding of molecules associated with extraordinary structural stability. The discovery of crystallographic planes in these boron carbide datasets substantiates that crystallinity is maintained to the(More)
Recent observations of planar defects in boron carbide have been shown to deviate from perfect mirror symmetry and are referred to as "asymmetric twins." Here, we demonstrate that these asymmetric twins are really phase boundaries that form in stoichiometric B(4)C (i.e., B(12)C(3)) but not in B(13)C(2). TEM observations and ab initio simulations have been(More)
The theoretical strength of a material is the minimum stress to deform or fracture the perfect single crystal material that has no defects. This theoretical strength is considered as an upper bound on the attainable strength for a real crystal. In contradiction to this expectation, we use quantum mechanics (QM) simulations to show that for the boron carbide(More)