The Experts below are selected from a list of 6432 Experts worldwide ranked by ideXlab platform
Gerhard Dehm - One of the best experts on this subject based on the ideXlab platform.
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dislocation plasticity and detwinning under thermal stresses in nanotwinned ag thin films
Social Science Research Network, 2020Co-Authors: Maya K Kini, Claudia Merola, Benjamin Breitbach, Dennis Klapproth, Bastian Philippi, Jeanbaptiste Molin, Christoph Kirchlechner, Gerhard DehmAbstract:Wafer curvature measurements reported in literature for polycrystalline (often textured) and epitaxial Fcc Metal thin films on hard substrates show a characteristic "signature" in the stress-temperature evolution for either type of films. While epitaxial films reveal characteristic elastic - ideal plastic deformation with no dislocation storage and highly repeatable cycles, polycrystalline films show considerable hardening upon cooling in addition to the relaxation by diffusional creep at elevated temperatures. In the present study, we study the deformation characteristics of an electron beam deposited epitaxial nanotwinned Ag on Si (111) substrate. The twin spacing λ of the nanotwinned Ag is controlled by suitable heat treatment and the "signature" thermomechanical deformation curves by wafer curvature measurements are recorded for twin spacings varying from 20 nm to 1 μm. Further, deformation is compared to other small scale deformation studies on Fcc Metals such as epitaxial bicrystal films, bicrystal micropillars containing a coherent twin boundary and nanotwinned micropillars.
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Insight into indentation-induced plastic flow in austenitic stainless steel
Journal of Materials Science, 2020Co-Authors: Gerhard Dehm, Steffen BrinckmannAbstract:The indentation-induced plasticity and roughness have been investigated intensively by experiments and simulations during the last decades. However, the precise mechanisms of how dislocation flow leads to pile-up formation are still not completely understood, although this is one of the initial steps causing surface roughening in tribological contacts at low loads. In this work, $$\{001\}$$ { 001 } -, $$\{101\}$$ { 101 } - and $$\{111\}$$ { 111 } -grain orientations in an austenite stainless steel [(face-centered cubic (Fcc) phase]) are indented with varying load forces. By using scanning electron-based methods and slip plane analysis, we reveal: (1) how slip-steps show the change of pile-up formation, (2) how the slip-plane inclination determines the dislocation flow and (3) how slip-plane interactions result in the final pile-up shape during indentation. We find that the flow direction transforms from the forward flow to the sideway at a transition angle of $$55^\circ{-}58^\circ $$ 55 ∘ - 58 ∘ between the slip-plane and the surface. We use large displacement finite element method simulations to validate an inversion of the resolved shear stress at this transition angle. We provide insights into the evolution of plasticity in dislocation-mediated Fcc Metal indentations, with the potential application of this information for indentation simulations and for understanding the initial stage of scratching during tribology in the future.
Harold S Park - One of the best experts on this subject based on the ideXlab platform.
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a surface stacking fault energy approach to predicting defect nucleation in surface dominated nanostructures
Journal of The Mechanics and Physics of Solids, 2013Co-Authors: Jinwu Jiang, Harold S Park, Ken Gall, Austin M Leach, Timon RabczukAbstract:Abstract We present a surface stacking fault (SSF) energy approach to predicting defect nucleation from the surfaces of surface-dominated nanostructure such as Fcc Metal nanowires. The approach leads to a criterion that predicts the initial yield mechanism via either slip or twinning depending on whether the unstable twinning energy or unstable slip energy is smaller as determined from the resulting SSF energy curve. The approach is validated through a comparison between the SSF energy calculation and low-temperature classical molecular dynamics simulations of copper nanowires with different axial and transverse surface orientations, and cross sectional geometries. We focus on the effects of the geometric cross section by studying the transition from slip to twinning previously predicted in moving from a square to rectangular cross section for 〈 100 〉 / { 100 } nanowires, and also for moving from a rhombic to truncated rhombic cross sectional geometry for 〈 110 〉 nanowires. We also provide the important demonstration that the criterion is able to predict the correct deformation mechanism when full dislocation slip is considered concurrently with partial dislocation slip and twinning. This is done in the context of rhombic 〈110〉 aluminum nanowires which do not show a tensile reorientation due to full dislocation slip. We show that the SSF energy criterion successfully predicts the initial mode of surface-nucleated plasticity at low temperature, while also discussing the effects of strain and temperature on the applicability of the criterion.
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a continuum model for the mechanical behavior of nanowires including surface and surface induced initial stresses
International Journal of Solids and Structures, 2011Co-Authors: F Song, Harold S Park, Guoliang Huang, Xiaoning LiuAbstract:The continuum modeling of the mechanical behavior of nanowires has recently attracted much attention due to its simplicity and efficiency. However, there are still some critical issues to be solved. In this paper, we demonstrate the importance of accounting for the effects of initial stresses in the nanowires that are caused by deformation due to surface stresses; we note that such initial stresses have previously been neglected in most existing continuum models. By considering the local geometrical nonlinearity of strains during the incremental flexural motion, a new formulation of the Euler–Bernoulli beam model for nanowires is developed through the incremental deformation theory, in which effects of the surface stress, the surface-induced initial stress and surface elasticity are naturally incorporated. It is found through comparisons to existing experimental and computational results for both Fcc Metal and ceramic nanowires that the surface-induced initial stresses, which are neglected in the Young–Laplace model, can significantly influence the overall mechanical properties of nanowires. We additionally demonstrate and quantify the errors induced by using the Young–Laplace model due to its approximation of surface stresses acting on only the top and bottom surfaces of nanowires.
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surface stress effects on the bending properties of Fcc Metal nanowires
Physical Review B, 2009Co-Authors: Geng Yun, Harold S ParkAbstract:The major purpose of this work is to investigate surface stress effects on the bending behavior and properties of $⟨100⟩/{100}$ gold nanowires with both fixed/fixed and fixed/free boundary conditions. The results are obtained through utilization of the recently developed surface Cauchy-Born model, which captures surface stress effects on the elastic properties of nanostructures through a three-dimensional, nonlinear finite element formulation. There are several interesting findings in the present work. First, we quantify the stress and displacement fields that result in the nanowires due to bending deformation. In doing so, we find that regardless of boundary condition, the stresses that are present in the nanowires due to deformation induced by surface stresses prior to any applied bending deformation dominate any stresses that are generated by the bending deformation unless very large $(\ensuremath{\approx}5%)$ bending strains are applied. In contrast, when the stresses and displacements induced by surface stresses prior to bending are subtracted from the stress and displacement fields of the bent nanowires, we find that the bending stresses and displacements do match the solutions expected from bulk continuum beam theory, but only within the nanowire bulk, and not at the nanowire surfaces. Second, we find that the deformation induced by surface stresses also has a significant impact on the nanowire Young's modulus that is extracted from the bending simulations, where a strong boundary-condition dependence is also found. By comparing all results to those that would be obtained using various linear surface-elastic theories, we demonstrate that a nonlinear, finite deformation formulation that captures changes in both bulk- and surface-elastic properties resulting from surface stress-induced deformation is critical to reproducing the experimentally observed boundary-condition dependence in Young's modulus of Metal nanowires. Furthermore, we demonstrate that linear surface-elastic theories based solely on the surface energy erroneously predict an increase in Young's modulus with decreasing nanowire size regardless of boundary condition. In contrast, while the linear surface-elastic theories based upon the Gurtin and Murdoch formalism can theoretically predict elastic softening with decreasing size, we demonstrate that, regardless of boundary condition, the stiffening due to the surface stress dominates the softening due to the surface stiffness for the range of nanowire geometries considered in the present work. Finally, we determine that the nanowire Young's modulus is essentially identical when calculated via either bending or resonance for both boundary conditions, indicating that surface effects have a similar impact on the elastic properties of nanowires for both loading conditions.
T G Nieh - One of the best experts on this subject based on the ideXlab platform.
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in situ neutron diffraction study of deformation behavior of a multi component high entropy alloy
Applied Physics Letters, 2014Co-Authors: W H Liu, Xunli Wang, A D Stoica, T G NiehAbstract:Deformation behavior of a high-entropy alloy (HEA) was investigated by in situ tensile deformation with neutron diffraction. It was found that the face-centered cubic (Fcc) HEA alloy showed strong crystal elastic and plastic anisotropy, and the evolution of its lattice strains and textures were similar to those observed in conventional Fcc Metals and alloys. Our results demonstrated that, in spite of chemical complexity, the multi-component HEA behaved like a simple Fcc Metal and the deformation was caused by the motion of mixed dislocations.
Akihiko Machida - One of the best experts on this subject based on the ideXlab platform.
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neutron diffraction study on the deuterium composition of nickel deuteride at high temperatures and high pressures
Physica B-condensed Matter, 2020Co-Authors: Hiroyuki Saitoh, Toyoto Sato, Akihiko Machida, Takanori Hattori, Asami Sanofurukawa, Kenichi Funakoshi, Shinichi Orimo, Katsutoshi AokiAbstract:Abstract The site occupancy of deuterium (D) atoms in face-centered-cubic nickel (Fcc Ni) was measured along a cooling path from 1073 to 300 K at an initial pressure of 3.36 GPa via in situ neutron powder diffraction. Deuterium atoms predominantly occupy the octahedral (O) sites and slightly occupy the tetrahedral (T) sites of the Fcc Metal lattice. The O-site occupancy increases from 0.4 to 0.85 as the temperature is lowered from 1073 to 300 K. Meanwhile, the T-site occupancy remains ~0.02. The temperature-independent behavior of the T-site occupancy is unusual, and its process is not yet understood. From the linear relation between the expanded lattice volume and D content, a D-induced volume expansion of 2.09 (13) A3/D atom was obtained. This value is in agreement with the values of 2.14–2.2 A3/D-atom previously reported for Ni and Ni0·8Fe0.2 alloy.
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site dependent hardening of the lanthanum Metal lattice by hydrogen absorption
Solid State Communications, 2011Co-Authors: Akihiko Machida, Ayako Ohmura, Tetsu Watanuki, K Aoki, Satoshi Nakano, T Ikeda, Kenichi TakemuraAbstract:Abstract The compressibility of lanthanum (La) Metal and its hydrides were measured at room temperature by high pressure synchrotron X-ray diffraction. La Metal pressurized in a hydrogen medium forms a hydride with an Fcc Metal lattice, which likely contains hydrogen at a concentration close to 3.0 and persists over the measured pressure span up to 21 GPa. Equations of state have been determined by helium compression experiments for LaH2 with tetrahedral interstitial sites fully occupied with hydrogen atoms and for LaH2.46 with octahedral interstitial sites partially occupied with hydrogen atoms and tetrahedral sites fully occupied. Both hydrides possess Fcc Metal lattices. The bulk modulus values B 0 are 66.7 ± 1.2 GPa for LaH2 and 68.4±1.0 GPa for LaH2.46. These values are three times larger than that of La Metal and are very close to each other despite the difference in hydrogen occupation states. The hardening of the Metal lattice by hydrogenation is attributed predominantly to hydrogen–Metal interactions at the tetrahedral sites and is most pronounced for La, which has the largest ionic radius among rare-earth Metals.
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pressure induced structural change from hexagonal to Fcc Metal lattice in scandium trihydride
Journal of Alloys and Compounds, 2007Co-Authors: Ayako Ohmura, Akihiko Machida, Tetsu Watanuki, K Aoki, Satoshi Nakano, K TakemuraAbstract:Abstract We synthesized scandium hydrides by hydrogenation of a scandium foil with hydrogen fluid under high pressure at ambient temperature. Scandium dihydride (ScH 2 ) and trihydride (ScH 3 ) were prepared near 4 and 5 GPa, respectively. The hydrogenation process and pressure-induced structural changes in ScH 3 were investigated by synchrotron radiation X-ray diffraction measurements up to 54.7 GPa. A structural transition from hexagonal to the Fcc lattice began at 30 GPa and was completed at 46 GPa via an intermediate state similar to those reported for other hexagonal trihydrides. The intermediate state was not interpreted in terms of a coexisting state for the low-pressure hexagonal and the high-pressure Fcc structures. The onset transition pressure of ScH 3 supported the previously proposed relation that the hexagonal–Fcc transition pressure is inversely proportional to the ionic radius of the trihydride.
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infrared spectroscopic study of the band gap closure in yh 3 at high pressure
Physical Review B, 2006Co-Authors: Ayako Ohmura, Akihiko Machida, Tetsu Watanuki, K Aoki, Satoshi Nakano, Kenichi TakemuraAbstract:Infrared vibrational absorption spectra are measured for yttrium trihydride at room temperature and pressures up to $30\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. The spectral change that begins near $12\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ is interpreted in terms of a hcp-Fcc structural transition, which agrees with previous x-ray diffraction measurements. For the hydrogen vibrations in the low-pressure hcp phase, the mode Gr\"uneisen parameters are derived from the observed peak frequency shifts with pressure and the reported bulk modulus. The value of 1.91 for the octahedral-site vibration is three times larger than those for the tetrahedral-site vibrations, suggesting hybridization between the hydrogen $1s$ and yttrium $4d$ orbitals at the octahedral site. The infrared transmission spectra collapse when the high-pressure Fcc phase is compressed beyond $23\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. The band gap abruptly closes without a structural change in the Fcc Metal lattice. The experimental results are in contrast to the previous theoretical calculations predicting the electronic transition either with hcp-Fcc structural change or in the hcp low-pressure phase without structural change. The transition mechanism is still inconclusive.
B Rauschenbach - One of the best experts on this subject based on the ideXlab platform.
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anisotropic strain in nitrided austenitic stainless steel
Journal of Applied Physics, 2000Co-Authors: S Mandl, B RauschenbachAbstract:Nitrogen treatment of austenitic stainless steels at temperatures around 300–400 °C leads to the formation of a hard and corrosion resistant phase called expanded austenite. The most puzzling feature of this phase is an anisotropic lattice expansion where the expansion along the (100) is about 20% larger than for the (111) direction. In this report an investigation of the lattice expansion of austenitic stainless steel after nitrogen plasma immersion ion implantation by x-ray diffraction is presented. Pole figure measurements as well as θ/2θ scans at different polar angles were performed. Comparing the data with three different structural models, Fcc, tetragonal, and triclinic, it can be concluded that none of them yields good agreement with the data, so that the crystallographic characterization is still open. However, a model for the atomic origin of the observed lattice expansion based on the anisotropic Young’s modulus, which is found for every Fcc Metal, is presented, showing good agreement with the ...
