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Porous metals represent a class of materials where the interplay of ligament length, width, node structure, and local geometry/curvature offers a rich parameter space for the study of critical length scales on mechanical behavior. Colloidal crystal templating of three-dimensionally ordered macroporous (3DOM, i.e., inverse opal) tungsten provides a unique structure to investigate the mechanical behavior at small length scales across the brittle–ductile transition. Micropillar compression tests show failure at 50 MPa contact pressure at 30 °C, implying a ligament yield strength of approximately 6.1 GPa for a structure with 5% relative density. In situ SEM frustum indentation tests with in-plane strain maps perpendicular to loading indicate local compressive strains of approximately 2% at failure at 30 °C. Increased sustained contact pressure is observed at 225 °C, although large (20%) nonlocal strains appear at 125 °C. The elevated-temperature mechanical performance is limited by cracks that initiate on planes of greatest shear under the indenter.
In this work, the deformation mechanisms underlying the room temperature deformation of the pseudomorphic body centered cubic (BCC) Mg phase in Mg/Nb nanolayered composites are studied. Nanolayered composites comprised of 50% volume fraction of Mg and Nb were synthesized using physical vapor deposition with the individual layer thicknesses h of 5, 6.7, and 50 nm. At the lower layer thicknesses of h = 5 and 6.7 nm, Mg has undergone a phase transition from HCP to BCC such that it formed a coherent interface with the adjoining Nb phase. Micropillar compression testing normal and parallel to the interface plane shows that the BCC Mg nanolayered composite is much stronger and can sustain higher strains to failure than the HCP Mg nanolayered composite. A crystal plasticity model incorporating confined layer slip is presented and applied to link the observed anisotropy and hardening in the deformation response to the underlying slip mechanisms.
We examine the development of stable bimetal interfaces in nanolayered composites in severe plastic deformation. Copper-niobium multilayers of varying layer thicknesses from several micrometers to 10 nanometers (nm) were fabricated via accumulative roll bonding (ARB). Investigation of their 5-parameter character and atomic scale structure finds that when layer thicknesses refine well below one micrometer, the interfaces self-organize to a few interface orientation relationships. With atomic scale and crystal plasticity modeling, we identify that the two controlling factors that determine whether an interface is stable under high strain rolling are orientation stability of the bicrystal and interface formation energy. A figure-of-merit is introduced that not only predicts the development of the prevailing interfaces but also explains why other interfaces did not develop. Through a suite of nanomechanical and bulk test results, we show that ARB composites containing these stable interfaces are found to have exceptional hardness (∼4.5 GPa) and strength (∼2 GPa).
Microcompression tests were performed on the Al/Nb multilayers of incoherent interfaces with the layer thicknesses of 5 nm Al/5 nm Nb and 50 nm Al/50 nm Nb. The Al-Nb multilayers showed increase in strength as the layer thickness was reduced; the average flow stresses at 5% plastic strain from the 5 nm Al/5 nm Nb and 50 nm Al/50 nm Nb layer thickness specimens were determined to be 2.1 GPa and 1.4 GPa respectively. The results from this Al-Nb microcompression study were compared with those of the previous report on Cu-Nb multilayer microcompression results that indicated that the flow stresses of the Al-Nb multilayer are lower than those of Cu-Nb with the same bilayer spacing. The observed difference in strength was attributed to a potential difference in the interfacial strength of the two incoherent multilayer systems.
In November 2000, exit poll interviews with voters in Florida
indicated that Al Gore won the state. As a result, many television
networks declared Gore the winner of Florida, a pivotal state to
winning the presidency in 2000. Only a few hours later, the first
vote tallies from the Florida Secretary of State's office revealed
that George W. Bush was in fact leading in Florida. After 45 days of
recounts and lawsuits, it was clear that the exit polls were wrong;
Bush had won the state by the narrowest of margins. As a result of
the flawed exit poll the media and pollsters scoured and reanalyzed
the methodology used in 2000 to prepare and correct for the 2004
presidential election. The old system, Voter News Service (VNS) was
scrapped entirely, and Edison-Mitofsky Research was chosen to
implement a new and more accurate national exit poll in 2004 by a
consortium of news organizations retained by the Associated Press
called the National Election Pool (NEP). What happened? Exit poll
results from Edison-Mitofsky showed John Kerry ahead in Ohio,
Florida, and New Mexico—all states which he lost to Bush in
2004.Author names are listed
alphabetically. The co-authors were also the co-principal
investigators of the Loyola Marymount University 2005 Los
Angeles Mayoral Exit Poll. Thanks to Salvador Paniagua and Haven
Perez for their tremendous research assistance in implementing
this project and to the more than 120 student researchers who
participated in the exit polling and data entry. Robert Aguinaga
and Antonio Gonzalez of the Southwest Voter Registration and
Education Project also provided valuable assistance in
implementing the poll. Mark Blumenthal, of mysterypollster.com
was instrumental in tracking down exit poll archives.
Elevated temperature tensile tests of different microstructures arising from different heat treatments of the Fe-based metallic glass Vitroperm (Fe73.5Cu1Nb3Si15.5B7) are presented. An anneal at 600°C for 1h yields a single phase μ-Fe microstructure with equiaxed, randomly oriented 15 nm grains, which is an ideal candidate for study of material properties at diminishing length scale. This microstructure has good stability during tensile testing at 600°C, showing a strain rate exponent correlating to grain boundary sliding (m=0.5), but little ductility, and strengths to 1250 MPa. The brittle behavior could be attributed to the lack of dislocation activity at such length scales. At temperatures up to 725°C, grain growth occurs, leading to elongations as large as 65% at flow stresses of 250 MPa. Precipitation of a second Nb-rich phase accompanies the grain growth. This investigation is supported by NSF, Division of Materials Research, grant NSF-DMR-0240144.
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