331 research outputs found
Nanoscale gold pillars strengthened through dislocation starvation
It has been known for more than half a century that crystals can be made stronger by introducing defects into them, i.e., by strain-hardening. As the number of defects increases, their movement and multiplication is impeded, thus strengthening the material. In the present work we show hardening by dislocation starvation, a fundamentally different strengthening mechanism based on the elimination of defects from the crystal. We demonstrate that submicrometer sized gold crystals can be 50 times stronger than their bulk counterparts due to the elimination of defects from the crystal in the course of deformation
Electronic-Mechanical Coupling in Graphene from in situ Nanoindentation Experiments and Multiscale Atomistic Simulations
We present the in situ nanoindentation experiments performed on suspended graphene devices to introduce homogeneous tensile strain,
while simultaneously carrying out electrical measurements. We find that
the electrical resistance shows only a marginal change even under
severe strain, and the electronic transport measurement confirms that
there is no band gap opening for graphene under moderate uniform
strain, which is consistent with our results from the first-principles
informed molecular dynamics simulation
Catastrophic vs Gradual Collapse of Thin-Walled Nanocrystalline Ni Hollow Cylinders As Building Blocks of Microlattice Structures
Lightweight yet stiff and strong lattice structures are attractive for various engineering applications, such as cores of sandwich shells and components designed for impact mitigation. Recent breakthroughs in manufacturing enable efficient fabrication of hierarchically architected microlattices, with dimensional control spanning seven orders of magnitude in length scale. These materials have the potential to exploit desirable nanoscale-size effects in a macroscopic structure, as long as their mechanical behavior at each appropriate scale – nano, micro, and macro levels – is properly understood. In this letter, we report the nanomechanical response of individual microlattice members. We show that hollow nanocrystalline Ni cylinders differing only in wall thicknesses, 500 and 150 nm, exhibit strikingly different collapse modes: the 500 nm sample collapses in a brittle manner, via a single strain burst, while the 150 nm sample shows a gradual collapse, via a series of small and discrete strain bursts. Further, compressive strength in 150 nm sample is 99.2% lower than predicted by shell buckling theory, likely due to localized buckling and fracture events observed during in situ compression experiments. We attribute this difference to the size-induced transition in deformation behavior, unique to nanoscale, and discuss it in the framework of “size effects” in crystalline strength
Ordering and dimensional crossovers in metallic glasses and liquids
The atomic-level structures of liquids and glasses are amorphous, lacking
long-range order. We characterize the atomic structures by integrating radial
distribution functions (RDF) from molecular dynamics (MD) simulations for
several metallic liquids and glasses: Cu46Zr54, Ni80Al20, Ni33.3Zr66.7, and
Pd82Si18. Resulting cumulative coordination numbers (CN) show that metallic
liquids have a dimension of d = 2.55 +/- 0.06 from the center atom to the first
coordination shell and metallic glasses have d = 2.71 +/- 0.04, both less than
3. Between the first and second coordination shells, both phases crossover to a
dimension of d = 3, as for a crystal. Observations from discrete atom
center-of-mass position counting are corroborated by continuously counting Cu
glass- and liquid-phase atoms on an artificial grid, which accounts for the
occupied atomic volume. Results from Cu grid analysis show short-range d = 2.65
for Cu liquid and d = 2.76 for Cu glass. Cu grid structures crossover to d = 3
at {\xi}~8 {\AA} (~3 atomic diameters). We study the evolution of local
structural dimensions during quenching and discuss its correlation with the
glass transition phenomenon.Comment: 15 pages, 6 figures in main tex
BMQ
BMQ: Boston Medical Quarterly was published from 1950-1966 by the Boston University School of Medicine and the Massachusetts Memorial Hospitals
Fractal atomic-level percolation in metallic glasses
Metallic glasses are metallic alloys that exhibit exotic material properties. They may have fractal structures at the atomic level, but a physical mechanism for their organization without ordering has not been identified. We demonstrated a crossover between fractal short-range (<2 atomic diameters) and homogeneous long-range structures using in situ x-ray diffraction, tomography, and molecular dynamics simulations. A specific class of fractal, the percolation cluster, explains the structural details for several metallic-glass compositions. We postulate that atoms percolate in the liquid phase and that the percolating cluster becomes rigid at the glass transition temperature
Percolation structure in metallic glasses and liquids
The atomic-level structures of liquids and glasses are similar, obscuring any structural basis for the glass transition. To delineate structural differences between them, we characterized the atomic structures using the integrated radial distribution functions (RDF) from molecular dynamics (MD) simulations for several metallic liquids and glasses: Cu_(46)Zr_(54), Ni_(80)Al_(20),
Ni_(33.3)Zr_(66.7), and Pd_(82)Si_(18). We find that the integrated RDF leads to cumulative coordination numbers (CN) that are similar for all four metallic glasses and for all four liquids, but are consistently different between the liquid and glass phases. We find that metallic liquids have a fractal dimension of df = 2.54 ± 0.06 from the center atom to the first coordination shell whereas the metallic glasses have d_f = 2.66 ± 0.04, which suggests the development of weak ordering during the glass transition. Beyond the second coordination shell, the CN indicates a dimension of d = 3 as for a crystal. Crossovers in dimension from d_f~2.54-2.66 to d = 3 between the first and second coordination shells imply an underlying percolation structure in metallic liquids and glasses
Electronic−Mechanical Coupling in Graphene from in situ Nanoindentation Experiments and Multiscale Atomistic Simulations
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