628 research outputs found

    Evaluation of two thermal neutron detection units consisting of ZnS/6{}^6LiF scintillating layers with embedded WLS fibers read out with a SiPM

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    Two single channel detection units for thermal neutron detection are investigated in a neutron beam. They consist of two ZnS/6{}^6LiF scintillating layers sandwiching an array of WLS fibers. The pattern of this units can be repeated laterally and vertically in order to build up a one dimensional position sensitive multi-channel detector with the needed sensitive surface and with the required neutron absorption probability. The originality of this work arises from the fact that the WLS fibers are read out with SiPMs instead of the traditionally used PMTs or MaPMTs. The signal processing system is based on a photon counting approach. For SiPMs with a dark count rate as high as 0.7 MHz, a trigger efficiency of 80% is achieved together with a system background rate lower than 103{10}^{-3} Hz and a dead time of 30 μ\mus. No change of performance is observed for neutron count rates of up to 3.6 kHz.Comment: Submitted to Nuclear Instruments and Methods

    The use of Laue microdiffraction to study small-scale plasticity

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    Micromechanics is a booming research area experiencing the development of new advanced testing methods at small dimensions. A relatively young but very popular technique involves uniaxial compressing micrometer and sub-micrometer sized objects, usually in the shape of pillars. Research in this field has focused mainly on exploring size effects in single crystal metals. This article demonstrates that Laue microdiffraction allows exploring in-situ the evolving microstructure in the transition regime from elasticity to plasticity, a feature that is not accessible with other techniques but which is essential for the understanding of small-scale plasticit

    The bandstructure of gold from many-body perturbation theory

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    The bandstructure of gold is calculated using many-body perturbation theory (MBPT). Different approximations within the GW approach are considered. Standard single shot G0W0 corrections shift the unoccupied bands up by ~0.2 eV and the first sp-like occupied band down by ~0.4 eV, while leaving unchanged the 5d occupied bands. Beyond G0W0, quasiparticle self-consistency on the wavefunctions lowers the occupied 5d bands by 0.35 eV. Globally, many-body effects achieve an opening of the interband gap (5d-6sp gap) of 0.35 to 0.75 eV approaching the experimental results. Finally, the quasiparticle bandstructure is compared to the one obtained by the widely used HSE (Heyd, Scuseria, and Ernzerhof) hybrid functional

    Measurement and prediction of the transformation strain that controls ductility and toughness in advanced steels

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    New-generation multi-phase martensitic steels derive their high strength from the body-centered cubic (BCC) phase and high toughness from transformation of the metastable face-centered cubic (FCC) austenite that transforms into martensite upon loading. In spite of its critical importance, the in-situ transformation strain (or "shape deformation" tensor), which controls ductility and toughness, has never been measured in any alloy where the BCC lath martensite forms and has never been connected to underlying material properties. Here, we measure the in-situ transformation strain in a classic Fe-Ni-Mn alloy using high-resolution digital image correlation (HR-DIC). The experimentally obtained results can only be interpreted using a recent theory of lath martensite crystallography. The predicted in-situ transformation strain agrees with the measurements, simultaneously demonstrating the method and validating the theory. Theory then predicts that increasing the FCC to BCC lattice parameter ratio substantially increases the in-situ transformation strain magnitude. This new correlation is demonstrated using data on existing steels. These results thus establish a new additional basic design principle for ductile and tough alloys: control of the lattice parameter ratio by alloying. This provides a new path for development of even tougher advanced high-strength steels. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd

    Neutron diffraction and diffraction contrast imaging for mapping the TRIP effect under load path change

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    The transformation induced plasticity (TRIP) effect is investigated during a load path change using a cruciform sample. The transformation properties are followed by in-situ neutron diffraction derived from the central area of the cruciform sample. Additionally, the spatial distribution of the TRIP effect triggered by stress concentrations is visualized using neutron Bragg edge imaging including, e.g., weak positions of the cruciform geometry. The results demonstrate that neutron diffraction contrast imaging offers the possibility to capture the TRIP effect in objects with complex geometries under complex stress states.Fil: Polatidis, Efthymios. Paul Scherrer Institute; SuizaFil: Morgano, Manuel. Paul Scherrer Institute; SuizaFil: Malamud, Florencia. Comision Nacional de Energia Atomica. Gerencia D/area Invest y Aplicaciones No Nucleares. Departamento Haces de Neutrones del Ra10 - Cab.; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Bacak, Michael. Paul Scherrer Institute; SuizaFil: Panzner, Tobias. Paul Scherrer Institute; Suiza. Swissneutronics; SuizaFil: Van Swygenhoven, Helena. Paul Scherrer Institute; Suiza. École Polytechnique Fédérale de Lausanne; SuizaFil: Strobl, Markus. Paul Scherrer Institute; Suiz

    Atomic-scale modeling of the deformation of nanocrystalline metals

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    Nanocrystalline metals, i.e. metals with grain sizes from 5 to 50 nm, display technologically interesting properties, such as dramatically increased hardness, increasing with decreasing grain size. Due to the small grain size, direct atomic-scale simulations of plastic deformation of these materials are possible, as such a polycrystalline system can be modeled with the computational resources available today. We present molecular dynamics simulations of nanocrystalline copper with grain sizes up to 13 nm. Two different deformation mechanisms are active, one is deformation through the motion of dislocations, the other is sliding in the grain boundaries. At the grain sizes studied here the latter dominates, leading to a softening as the grain size is reduced. This implies that there is an ``optimal'' grain size, where the hardness is maximal. Since the grain boundaries participate actively in the deformation, it is interesting to study the effects of introducing impurity atoms in the grain boundaries. We study how silver atoms in the grain boundaries influence the mechanical properties of nanocrystalline copper.Comment: 10 pages, LaTeX2e, PS figures and sty files included. To appear in Mater. Res. Soc. Symp. Proc. vol 538 (invited paper). For related papers, see http://www.fysik.dtu.dk/~schiotz/publist.htm
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