Towards an unbiased comparison of CC, BCC, and FCC lattices in terms of prealiasing
Vad, Viktor
2014-06-01
In the literature on optimal regular volume sampling, the Body-Centered Cubic (BCC) lattice has been proven to be optimal for sampling spherically band-limited signals above the Nyquist limit. On the other hand, if the sampling frequency is below the Nyquist limit, the Face-Centered Cubic (FCC) lattice was demonstrated to be optimal in reducing the prealiasing effect. In this paper, we confirm that the FCC lattice is indeed optimal in this sense in a certain interval of the sampling frequency. By theoretically estimating the prealiasing error in a realistic range of the sampling frequency, we show that in other frequency intervals, the BCC lattice and even the traditional Cartesian Cubic (CC) lattice are expected to minimize the prealiasing. The BCC lattice is superior over the FCC lattice if the sampling frequency is not significantly below the Nyquist limit. Interestingly, if the original signal is drastically undersampled, the CC lattice is expected to provide the lowest prealiasing error. Additionally, we give a comprehensible clarification that the sampling efficiency of the FCC lattice is lower than that of the BCC lattice. Although this is a well-known fact, the exact percentage has been erroneously reported in the literature. Furthermore, for the sake of an unbiased comparison, we propose to rotate the Marschner-Lobb test signal such that an undue advantage is not given to either lattice. © 2014 The Eurographics Association and John Wiley & Sons Ltd. Published by John Wiley & Sons Ltd.
Lattice dynamics and thermodynamics of bcc iron at pressure: first-principles linear response study
Sha, Xianwei; Cohen, R. E.
2005-01-01
We compute the lattice-dynamical and thermal equation of state properties of ferromagnetic bcc iron using the first principles linear response linear-muffin-tin-orbital method in the generalized-gradient approximation. The calculated phonon dispersion and phonon density of states, both at ambient and high pressures, show good agreement with inelastic neutron scattering data. We find the free energy as a function of volume and temperature, including both electronic excitations and phonon contr...
Dislocations in C11.sub.B./sub. and BCC lattices
Paidar, Václav; Vitek, V.
Budapest: Eötvös University, 2012, s. 23-28. ISBN 978-615-5270-01-7. [International conference on fundamental properties of dislocations /4./. Budapest (HU), 27.08.2012-31.08.2012] R&D Projects: GA AV ČR IAA100100920 Institutional support: RVO:68378271 Keywords : dislocation cores * bcc metals * C11 B intermetallics Subject RIV: BM - Solid Matter Physics ; Magnetism
Lattice defects in V–Ti BCC alloys before and after hydrogenation
Matsuda, Junko, E-mail: junko.matsuda@i2cner.kyushu-u.ac.jp [International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395 (Japan); Akiba, Etsuo [International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395 (Japan); Department of Mechanical Engineering, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395 (Japan)
2013-12-25
Highlights: •Strain contrast with spacing smaller than 20 nm is observed in as-cast and hydrogenated V–Ti alloys. •Twin boundaries and stacking faults are introduced into the V–Ti alloys after hydrogenation. •These planar defects are formed due to accommodation of shear stress during hydrogenation. •Densities of twin boundaries are higher in the alloys with smaller contents of V. •Effective absorbing capacities at ambient condition depend on the densities of twin boundaries. -- Abstract: Microstructures of V–Ti BCC alloys before and after hydrogenation/dehydrogenation are investigated using transmission electron microscopy. Strain contrast with spacing smaller than 20 nm is observed both in the as-cast alloys and hydrogenated alloys. Twin boundaries and stacking faults with spacing of 5–20 nm are introduced into the V–Ti BCC alloys after hydrogenation. These are parallel to {11"¯1} planes of FCC hydrides. The twin boundaries and stacking faults are formed due to the accommodation of shear stress during hydrogenation accompanied by large anisotropic expansion along the c-axis direction in V–Ti BCC alloys. Density of twin boundaries is high in the alloys with small contents of V, which have a small effective hydrogen capacity at ambient condition. Dependence of microstructure evolution in the V–Ti alloys during hydrogenation on the V/Ti ratio is also discussed with the thermodynamic stabilities from measurements of Pressure–Composition isotherms and mechanical properties.
In this work we study the critical behavior of the quantum spin-1/2 anisotropic Heisenberg antiferromagnet in the presence of a longitudinal field on a body centered cubic (bcc) lattice as a function of temperature, anisotropy parameter (Δ) and magnetic field (H), where Δ=0 and 1 correspond the isotropic Heisenberg and Ising models, respectively. We use the framework of the differential operator technique in the effective-field theory with finite cluster of N=4 spins (EFT-4). The staggered ms=(mA−mB)/2 and total m=(mA+mB)/2 magnetizations are numerically calculated, where in the limit of ms→0 the critical line TN(H,Δ) is obtained. The phase diagram in the T−H plane is discussed as a function of the parameter Δ for all values of H∈[0,Hc(Δ)], where Hc(Δ) correspond the critical field (TN=0). Special focus is given in the low temperature region, where a reentrant behavior is observed around of H=Hc(Δ)≥Hc(Δ=1)=8J in the Ising limit, results in accordance with Monte Carlo simulation, and also was observed for all values of Δ∈[0,1]. This reentrant behavior increases with increase of the anisotropy parameter Δ. In the limit of low field, our results for the Heisenberg limit are compared with series expansion values. - Highlights: ► In the lat decade there has been a great interest in the physics of the quantum phase transition in spins system. ► Effective-field theory in cluster with N=4 spins is generalized to treat the quantum spin-1/2 Heisenberg model. ► We have obtained phase diagram at finite temperature for the quantum spin-1/2 antiferromagnet Heisenberg model as a bcc lattice.
Neto, Minos A., E-mail: minos@pq.cnpq.br [Departamento de Fisica, Universidade Federal do Amazonas, 3000, Japiim, Manaus, 69077-000 AM (Brazil); Roberto Viana, J., E-mail: vianafisica@bol.com.br [Departamento de Fisica, Universidade Federal do Amazonas, 3000, Japiim, Manaus, 69077-000 AM (Brazil); Ricardo de Sousa, J., E-mail: jsousa@edu.ufam.br [Departamento de Fisica, Universidade Federal do Amazonas, 3000, Japiim, Manaus, 69077-000 AM (Brazil); National Institute of Science and Technology for Complex Systems, 3000, Japiim, Manaus, 69077-000 AM (Brazil)
2012-08-15
In this work we study the critical behavior of the quantum spin-1/2 anisotropic Heisenberg antiferromagnet in the presence of a longitudinal field on a body centered cubic (bcc) lattice as a function of temperature, anisotropy parameter ({Delta}) and magnetic field (H), where {Delta}=0 and 1 correspond the isotropic Heisenberg and Ising models, respectively. We use the framework of the differential operator technique in the effective-field theory with finite cluster of N=4 spins (EFT-4). The staggered m{sub s}=(m{sub A}-m{sub B})/2 and total m=(m{sub A}+m{sub B})/2 magnetizations are numerically calculated, where in the limit of m{sub s}{yields}0 the critical line T{sub N}(H,{Delta}) is obtained. The phase diagram in the T-H plane is discussed as a function of the parameter {Delta} for all values of H Element-Of [0,H{sub c}({Delta})], where H{sub c}({Delta}) correspond the critical field (T{sub N}=0). Special focus is given in the low temperature region, where a reentrant behavior is observed around of H=H{sub c}({Delta}){>=}H{sub c}({Delta}=1)=8J in the Ising limit, results in accordance with Monte Carlo simulation, and also was observed for all values of {Delta} Element-Of [0,1]. This reentrant behavior increases with increase of the anisotropy parameter {Delta}. In the limit of low field, our results for the Heisenberg limit are compared with series expansion values. - Highlights: Black-Right-Pointing-Pointer In the lat decade there has been a great interest in the physics of the quantum phase transition in spins system. Black-Right-Pointing-Pointer Effective-field theory in cluster with N=4 spins is generalized to treat the quantum spin-1/2 Heisenberg model. Black-Right-Pointing-Pointer We have obtained phase diagram at finite temperature for the quantum spin-1/2 antiferromagnet Heisenberg model as a bcc lattice.
The scalar-product cluster variation method (SP-CVM) for calculation of antiphase boundary (APB) energies has been extended for application to {001} APBs in the body-centered cubic lattice using the irregular tetrahedron cluster approximation. In order to do so, the proof of the SP-CVM relation has been updated to include the cases where the domain interface consists of more than one plane of atoms (i.e., it consists of a layer of atoms). The algorithm has been developed for the cases of thermal APBs with APB vectors a0A2 and a0A2/2. It is then applied, as an illustration, to the determination of APB energies in isothermal calculations in the Fe-Al system
Moya, Javier A., E-mail: jmoya.fi.uba@gmail.com [Grupo Interdisciplinario en Materiales-IESIING, Universidad Católica de Salta, INTECIN UBA-CONICET, Salta (Argentina); Gamarra Caramella, Soledad; Marta, Leonardo J. [Grupo Interdisciplinario en Materiales-IESIING, Universidad Católica de Salta, INTECIN UBA-CONICET, Salta (Argentina); Berejnoi, Carlos [Universidad Nacional de Salta, Facultad de Ingeniería, Salta (Argentina)
2015-05-15
Highlights: • A method for determining composition in ternary nanocrystals is presented. • X-ray diffraction and Mössbauer spectroscopy data were employed. • We perform theoretical charts for lattice parameter of Fe-rich ternary alloys. • A linear relationship in lattice parameter for binary alloys is evaluated. • A parabolic relationship is proposed for the Fe–Co–Si alloy. - Abstract: Charts containing lattice parameters of Fe{sub 1−x}(M,N){sub x} ternary systems with M and N = Si, Al, Ge or Co, and 0 ⩽ x ⩽ ∼0.3, were developed by implementing a linear relationship between the respective binary alloys with the same solute content of the ternary one. Charts were validated with experimental data obtained from literature. For the Fe–Co–Si system, the linear relationship does not fit the experimental data. For the other systems (except the Fe–Co–Ge one where no experimental data was found), the lineal relationship constitute a very good approximation. Using these charts and the lattice parameter data obtained from X-ray diffraction technique combining with the solute content data obtained from Mössbauer spectroscopy technique it is possible to determine the chemical composition of nanograins in soft magnetic nanocomposite materials and some examples are provided.
Properties of Zr – 12.5 % Nb and Zr – 25 % Nb Alloys with hcp and bcc Lattices: ab-initio Modeling
V.O. Kharchenko
2012-06-01
Full Text Available Ab-initio calculations of the electronic structure for Zr – 12.5 % Nb and Zr – 25 % Nb alloys are done using density functional theory, method of linearized augmented plane-wave approximation and generalized gradient approximation. The structural and energetic properties, such as the electron densities, densities of states, energetically and X-ray spectra in the case of the hcp and bcc lattices are calculated. Optimal values for the lattice constants are found. The most favorable structure for each studied alloy is defined.
A hierarchical microstructure is obtained in an alloy with composition Fe–8.1Al–12.2Cr–1.9Mo–18.2Ni–2.0Ti (wt.%) processed by melt-spinning. The evolution of the precipitation pathways is investigated using transmission electron microscopy (TEM) techniques, atom probe tomography (APT) and first-principles thermodynamic calculations. As-solidified ribbons exhibit a random dispersion of B2-ordered precipitates (NiAl-type) in an Fe-based matrix. Subsequent aging at 700 °C yields nucleation and growth of the L21-phase (Ni2TiAl-type) within the primary B2-precipitates, leading to a microstructure exhibiting three types of hierarchy: (i) a structural hierarchy due to chemical ordering, with a chemically disordered matrix of bcc-Fe (A2), the nearest-neighbor (NN) ordered B2-precipitates (NiAl-type) and the next nearest-neighbor (NNN) ordered L21-precipitates (Ni2TiAl-type) within B2, (ii) a dimensional hierarchy with a continuous bcc-Fe matrix, coherently embedded B2-precipitates, with a size range of 60–200 nm and the coherent precipitate substructure, with L21-phase and dimensions of 15–20 nm. (iii) A spatial hierarchy where B2-precipitates are embedded in the bcc-Fe matrix and L21-precipitates nucleate and grow only within B2-precipitates. In addition, it is verified that the interface between B2 and L21 is coherent and adopts a diffuse structural profile. Monte-Carlo simulations reproduce these observations and it is found that interface energies of B2 and L21 reduce from 50 mJ/m2 at 0 K to 11 mJ/m2 at 973 K. Kinetic-Monte-Carlo simulations support the interpretation of the experimental results that the L21 nucleates within the B2 phase
Gas-bubble superlattice formation in bcc metals
The spatial ordering of small helium bubbles (diameter 1.5-2.0 nm) produced in the bcc metals V, W, Mo, Cr, Fe and Ta by ion implantation with 30-50 keV He+ is studied using transmission electron microscopy (TEM) and earlier studies are reappraised. Implantations are at temperatures in the range 300-773 K. Gas-bubble ordering onto {110} planes is observed in all the metals studied. For V, W and Mo, the bubbles order fully in three dimensions to form a bcc gas-bubble superlattice oriented parallel with the metal lattice. A similar conclusion is reached also for Cr, Fe and Ta although the study of these metals is less detailed. The bubble lattice parameters are found from bubble spacings determined from electron diffraction patterns. The results, confirmed from bright-field micrographs, are as follows: V, similar 3.9 nm at 300 K; W, similar 6.2 nm at 775 K; Mo, similar 6.2 nm at 675 K; Cr, similar 5.1 nm at 300 K; Fe, similar 6.0 nm at 300 K; and Ta, similar 6.4 nm at 575 K. The results suggest that ordered bcc bubble superlattices are a consistent feature in bcc metals following high-dose implantation with helium at temperatures ≅0.2Tm, where Tm is the melting temperature of the metal. ((orig.))
Atomic displacements in bcc dilute alloys
Hitesh Sharma; S Prakash
2007-04-01
We present here a systematic investigation of the atomic displacements in bcc transition metal (TM) dilute alloys. We have calculated the atomic displacements in bcc (V, Cr, Fe, Nb, Mo, Ta and W) transition metals (TMs) due to 3d, 4d and 5d TMs at the substitutional site using the Kanzaki lattice static method. Wills and Harrison interatomic potential is used to calculate the atomic force constants, the dynamical matrix and the impurity-induced forces. We have thoroughly investigated the atomic displacements using impurities from 3d, 4d and 5d series in the same host metal and the same impurity in different hosts. We have observed a systematic pattern in the atomic displacements for Cr-, Fe-, Nb-, Mo-, Ta- and W-based dilute alloys. The atomic displacements are found to increase with increase in the number of d electrons for all alloys considered except for V dilute alloys. The 3d impurities are found to be more easily dissolved in the 3d host metals than 4d or 5d TMs whereas 4d and 5d impurities show more solubility in 4d and 5d TMs. In general, the relaxation energy calculation suggests that impurities may be easily solvable in 5d TM hosts when compared to 3d or 4d TMs.
First-principles thermoelasticity of bcc iron under pressure
Sha, Xianwei; Cohen, R. E.
2006-01-01
We investigate the elastic and isotropic aggregate properties of ferromagnetic bcc iron as a function of temperature and pressure by computing the Helmholtz free energies for the volume-conserving strained structures using the first-principles linear response linear-muffin-tin-orbital method and the generalized-gradient approximation. We include the electronic excitation contributions to the free energy from the band structures, and phonon contributions from quasi-harmonic lattice dynamics. W...
bcc cobalt: Metastable phase or forced structure?
Liu, Amy Y.; Singh, David J.
1993-05-01
General potential linearized augmented plane wave calculations of the elastic and magnetic properties of body-centered-cubic (bcc) Co are presented and discussed. Particular attention is given to the nature of the stability of bcc Co films that have been grown epitaxially on GaAs substrates. The bcc structural phase of Co, which is not found in nature, is calculated to be unstable with respect to a tetragonal distortion that transforms it continuously into the face-centered-cubic (fcc) phase. This means that bcc Co is not a true metastable phase of bulk Co. The thinner films of bcc Co that have been synthesized are more properly termed forced structures. We speculate that the few existing thick (up to 357 Å) samples of bcc Co are stabilized by the presence of impurities or other defects. This is further supported by discrepancies between the measured and calculated magnetic moments in bcc Co.
Elzain, M.E. [Sultan Qaboos Univ., Al-Khod (Oman). Dept. of Phys.; Yousif, A.A. [Sultan Qaboos Univ., Al-Khod (Oman). Dept. of Phys.
1994-11-01
The magnetic moment {mu}, hyperfine field B{sub hf} and isomer shift IS at the Fe site in bcc Fe-Al alloys were calculated from first principle. Contrary to the belief that Al atoms reside in the iron lattice as magnetic holes, it was found that the local magnetic moment of Fe is decreased when Al is at a nearest neighbour site (NN), while it increases if Al is at a next-nearest neighbour site (NNN). Consequently, the average {mu} per Fe atom was found to be, initially, independent of Al content. Assuming a linear dependence of {mu}{sub Fe} on the number of Al atoms at NN and NNN sites, we calculated the average {mu}, which was found to agree with experimental results of cold worked alloys for disordered Fe-Al alloy. On the other hand, antiferromagnetic coupling appears in the CsCl ordered structures. The average B{sub hf} was also calculated and compared to experimental data and the trends in the IS are considered. (orig.)
Ohtake, Mitsuru, E-mail: ohtake@futamoto.elect.chuo-u.ac.jp [Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551 (Japan); Higuchi, Jumpei; Yabuhara, Osamu [Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551 (Japan); Kirino, Fumiyoshi [Graduate School of Fine Arts, Tokyo National University of Fine Arts and Music, 12-8 Ueno-koen, Taito-ku, Tokyo 110-8714 (Japan); Futamoto, Masaaki [Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551 (Japan)
2011-09-30
Permalloy (Py) single-crystal films with bcc structure were obtained on GaAs(011){sub B3} single-crystal substrates by ultra high vacuum rf magnetron sputtering. The film growth and the detailed film structures were investigated by refection high energy electron diffraction and pole figure X-ray diffraction. bcc-Py films epitaxially grow on the substrates in the orientation relationship of Py(011)[011-bar]{sub bcc} || GaAs(011)[011-bar]{sub B3}. The lattice constant of bcc-Py film is determined to be a = 0.291 nm. With increasing the film thickness, parts of the bcc crystal transform into more stable fcc structure by atomic displacement parallel to the bcc{l_brace}011{r_brace} close-packed planes. The resulting film thus consists of a mixture of bcc and fcc crystals. The phase transformation mechanism is discussed based on the experimental results. The in-plane magnetization properties reflecting the magnetocrystalline anisotropy of bcc-Py crystal are observed for the Py films grown on GaAs(011){sub B3} substrates.
Permalloy (Py) single-crystal films with bcc structure were obtained on GaAs(011)B3 single-crystal substrates by ultra high vacuum rf magnetron sputtering. The film growth and the detailed film structures were investigated by refection high energy electron diffraction and pole figure X-ray diffraction. bcc-Py films epitaxially grow on the substrates in the orientation relationship of Py(011)[011-bar]bcc || GaAs(011)[011-bar]B3. The lattice constant of bcc-Py film is determined to be a = 0.291 nm. With increasing the film thickness, parts of the bcc crystal transform into more stable fcc structure by atomic displacement parallel to the bcc{011} close-packed planes. The resulting film thus consists of a mixture of bcc and fcc crystals. The phase transformation mechanism is discussed based on the experimental results. The in-plane magnetization properties reflecting the magnetocrystalline anisotropy of bcc-Py crystal are observed for the Py films grown on GaAs(011)B3 substrates.
THE CHANGES OF BARRIER ENERGY IN FCCâBCC PHASE TRANSFORMATION BY SHEAR STRESSES
Kazanç, Sefa; ÖZGEN, Soner
2010-01-01
ABSTRACT The Lattice energy of a cubic nickel crystal has been calculated by using the embedded atom method. The embedding energy has been determined by means of quantum mechanical approximations. The lattice energy changes of the static structure including 864 atoms with Bain and shear stresses have been obtained. The energies of the fcc and bcc phases caused by Bain stress have been compared. The variation of the barrier energy required for the transition between these structures has ...
First-principles study of interactions between substitutional solutes in bcc iron
Gorbatov, O. I.; Delandar, A. Hosseinzadeh; Gornostyrev, Yu N.; Ruban, A. V.; Korzhavyi, P. A.
2016-07-01
Using density functional theory based calculations, employing the locally self-consistent Green's function method and the projected augmented wave method, we develop a database of solute-solute interactions in dilute alloys of bcc Fe. Interactions within the first three coordination shells are computed for the ferromagnetic state as well as for the paramagnetic (disordered local moment) state of the iron matrix. The contribution of lattice relaxations to the defect interaction energy is investigated in the ferromagnetic state. Implications of the obtained results for modeling the phenomena of point defect clustering and phase precipitation in bcc Fe-based alloys and steel are discussed.
Displacive processes in systems with bcc patent lattice
Paidar, Václav
2011-01-01
Roč. 56, č. 6 (2011), s. 841-851. ISSN 0079-6425 R&D Projects: GA AV ČR IAA100100920 Institutional research plan: CEZ:AV0Z10100520 Keywords : diffusionless phase transformations * displacive processes * gamma surfaces Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 18.216, year: 2011
Extended Finnis-Sinclair potential for bcc and fcc metals and alloys
We propose an extended Finnis-Sinclair (FS) potential by extending the repulsive term into a sextic polynomial for enhancing the repulsive interaction and adding a quartic term to describe the electronic density function. It turns out that for bcc metals the proposed potential not only overcomes the 'soft' behaviour of the original FS potential, but also performs better than the modified FS one by Ackland et al, and that for fcc metals the proposed potential is able to reproduce the lattice constants, cohesive energies, elastic constant, vacancy formation energies, equations of state, pressure-volume relationships, melting points and melting heats. Moreover, for some fcc-bcc systems, e.g. the Ag-refractory metal systems, the lattice constants, cohesive energies and elastic constants of some alloys are reproduced by the proposed potential and are quite compatible with those directly determined by ab initio calculations
Microstructural studies of hydrogen and deuterium in bcc refractory metals
Over the past four years this research has been principally concerned with uncovering the microstructural atomic arrangements in alloys of hydrogen and deuterium with bcc refractory metals. Because these are interstitial phases in which the host metal lattice is substantially deformed by the incorporation of the H(D) atoms, there are pronounced x-ray scattering effects. X-ray diffraction has, therefore, been the main structural tool. A main objective of the project has been to determine the degree to which phase relations and solid solution properties in metal-hydride alloys depend upon the hydrogen-hydrogen interaction via the displacement field of the metal atoms. This has often included the elucidation of subtle thermodynamic properties which are revealed in structural studies
Diffusion simulation of Cr-Fe bcc systems at atomic level using a random walk alogorithm
San Sebastián, I; Aldazabal, J.; Capdevila, Carlos; García de Andrés, Carlos
2008-01-01
This paper proposes a model to simulate the diffusion of impurities in bcc atomic lattices. It works with threedimensional volume divided in small cubic elements (voxels), containing more than one atomic cell each. Once the domain is discretized, impurities jump for one voxel to another according to certain probability that takes into account the composition and geometry of the target voxel. In present work, model was applied to prismatic volume, and in order to ded...
An alternative explanation for epitaxial growth. The case of fcc(111) on bcc(110)
Grey, F.; Bohr, J.
Starting from basic observations concerning commensurate structures, a simple explanation is given for the existence of energetically stable states of an fcc(111) adsorbate on a bcc(110) substrate, as a function of the mismatch and relative orientation of the two lattices. Numerical simulations a...... used to show how epitaxially rotated phases are favoured due to finite-size effects, and how adsorbate relaxation stabilizes higher-order commensurate structures....
Bcc-fcc structure transition of Te
Using the synchrotron radiation x-ray powder diffraction technique, the structure phase transition of Te has been investigated at pressure up to 330 GPa and at 298 K. The phase transition from the bcc (Te-V) to a new high-pressure phase (Te-VI) was found at 100 GPa and the structure of the new phase was suggested to be a superlattice of the fcc structure. The Te-VI phase further transformed to the fcc phase (Te-VII) at 255 GPa with disappearance of satellite peaks. Since for group 16 elements, it has been considered that the structure of the highest pressure phase is bcc, the present results provided new information about the high-pressure behaviour of these elements.
Monte Carlo simulation of atomic aggregates formation in model bcc binary alloys. Preliminary report
By means of the Monte Carlo simulation an atomistic description of the structure of model bcc binary alloys was made. We used ABV model of the alloy where the approach of pair interaction to first neighbours with constant ordering energy is assumed. The dynamics was introduced by means of a vacancy that interchanges of place with nearest neighbouring atoms. The simulations were made in a bcc lattice with 128, 1024, 8192 and 16000 sites, applying periodic boundary conditions to avoid edge effects. We calculate the formation probabilities of different atomic aggregate A9-m Bm (m = 0, 1, 2,... 9) as function of concentration of the components and the temperature. In some regions of temperature and concentration, compositional and thermal polymorphism of aggregates is observed. (author)
Atomic mechanism of homogeneous melting of bcc Fe at the limit of superheating
Atomic mechanism of homogeneous melting of bcc Fe is studied via monitoring spatiotemporal arrangements of the liquid-like atoms, which are detected by the Lindemann criterion of melting, during the heating process. Calculations are performed by molecular dynamics (MD) simulations. Calculations show that liquid-like atoms occur randomly in the crystalline matrix at temperature far below the melting point due to local instability of the crystalline lattice. Number of liquid-like atoms increases with increasing temperature and they have a tendency to form clusters. Subsequently, a single percolated liquid-like cluster is formed in the crystalline model and at the melting point 99% atoms in the model become liquid-like to form a liquid phase. Melting is also accompanied by the sudden changes in various static and thermodynamic quantities. However, total melting is reached just at the point above the melting one. Three characteristic temperatures of the homogeneous melting of bcc Fe are determined.
Behaviour of bcc technical superconductors under dynamic mechanical stress
The behavior of bcc technical superconductors NbTi, NbZr and Nb under dynamic mechanical stress was investigated using two measuring techniques. In shot-sample training experiments the load was measured at which, in wire samples with a transport current applied, normal transitions occured in succesive straining cycles. Complementary, the acoustic emission from superconductors was monitored during strain at 4.2 K. A mechanism based on the formation of a stress induced shear transformation was proposed to account for the training behavior. This partially reversible shearing of the lattice is not sufficient to furnish the required energy for transition in the normal state but it may be detected by means of acoustic emission. On the other hand, a time correlation between acoustic emission and normal transition was found although training behavior and emission responded differently to the same metallurgical treatment. The experiments strongly indicate that the mechanism directly responsible for local energy release is microyielding, induced by the shear transformation. The stress relaxation which accompanies this transformation results in an increased load on the rest of the sample cross-section. Consequently microyielding caused by the transformation could occur in that region of the sample without being detected on the stress-strain curve. This would result in a sufficient release of energy to increase the sample temperature above its critical value. (orig./HP)
Titanium's high-temperature elastic constants through the hcp-bcc phase transformation
The five independent elastic constants of hexagonal monocrystal titanium were determined up to the phase-transformation temperature, and the two isotropic elastic constants of polycrystalline titanium were determined beyond, up to 1300 K. Anomalous temperature dependences were observed just below the phase-transformation temperature: C11 and C66 increase with increasing temperature whereas C33 and C44 remarkably decrease, for example. To determine the Cij, we used the free-vibration resonance frequencies obtained by electromagnetic acoustic resonance. After the phase transformation, the resonance frequencies changed little with the temperature increase, showing that the bcc-phase elastic constants change little with temperature. The polycrystalline elastic constants remained unchanged up to 1300 K after the phase transformation. The anomalous temperature dependence near the transformation is interpreted in terms of the small c/a ratio of the hcp phase and change of the atomic distances to meet the Burgers lattice relationship. Temperature-insensitive elastic constants in the bcc phase suggest the stabilizing of the bcc phase with increasing temperature
Hot electron spin attenuation lengths of bcc Fe34Co66-Room temperature Magnetocurrent of 1200%
We investigate spin-dependent hot electron transport through metallic epitaxial spin valves by ballistic electron magnetic microscopy (BEMM). By variation of the thickness of one of the ferromagnetic layers we determine the spin dependent attenuation lengths which reflect hot electron transport along the vicinity of the [1 0 0]-axis of the bcc Fe34Co66 lattice. The majority spin attenuation length is more than 6 times larger than that of the minority spins within the measured energy interval of 1.3 up to 2 eV above the Fermi level. Consequently a Magnetocurrent effect exceeding 1200% accompanied by a monotonic bias voltage behavior is observed at room temperature.
The Material Orientation Relationship for the BCC-HCP Transition
Kashchenko, M. P.; Chashchina, V. G.
2007-01-01
The dynamical model of forming of martensitic crystals for the bcc-hcp transition is offered. It is shown that all macroscopic morphological characters (the habit plane, the macroshear and the orientational relationship) are expressed through elastic moduluses Cij of an initial bcc phase.
Entropies and enthalpies of vacancy formation and diffusion in BCC iron are calculated for each temperature directly from free-energies using phase-space trajectories obtained from spin–lattice dynamics simulations. Magnon contributions are found to be particularly substantial in the temperature regime near the α−β (ferro/para-magnetic) transition. Strong temperature dependence and singular behavior can be seen in this temperature regime, reflecting magnon softening effects. Temperature dependence of the lattice component in this regime is also much more significant compared to previous estimations based on Arrhenius-type fitting. Similar effects on activation processes involving other irradiation-produced defects in magnetic materials are expected
Slip transmission in bcc FeCr polycrystal
Patriarca, Luca, E-mail: luca.patriarca@polimi.it [Politecnico di Milano, Department of Mechanical Engineering, Via La Masa 34, I-20156 Milano (Italy); Abuzaid, Wael; Sehitoglu, Huseyin [Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, 1206W. Green St., Urbana, IL 61801 (United States); Maier, Hans J. [Institut für Werkstoffkunde, Leibniz Universität Hannover, An der Universität 2, D-30823 Garbsen (Germany)
2013-12-20
Grain boundaries induce heterogeneities in the deformation response of polycrystals. Studying these local variations in response, measured through high resolution strain measurement techniques, is important and can improve our understanding of fatigue damage initiation in the vicinity of grain boundaries and material hardening. In this work, strain fields across grain boundaries were measured using advanced digital image correlation techniques. In conjunction with strain measurements, grain orientations from electron back-scattered diffraction were used to establish the dislocation reactions at each boundary, providing the corresponding residual Burgers vectors due to slip transmission across the interfaces. A close correlation was found between the magnitude of the residual Burgers vector and the local strain change across the boundary. When the residual Burgers vector magnitude (with respect to the lattice spacing) exceeds 1.0, the high strains on one side of the boundary are paired with low strains across the boundary, indicating the difficulties for slip dislocations to penetrate the grain interfaces. When the residual Burgers vector approaches zero, the strain fields vary smoothly across the boundary due to limited resistance to slip transmission. The results suggest that the residual Burgers vector magnitude, which relates to the GB (Grain Boundary) resistance to slip transmission, enables a quantitative analysis of the accumulation of strain at the microstructural level and the development of strain heterogeneities across grain boundaries. The results are presented for FeCr bcc alloy which exhibits single slip per grain making the measurements and dislocation reactions rather straightforward. The work points to the need to incorporate details of slip dislocation–grain boundary interaction (slip transmission) in modeling research.
A mostly single bcc phase with nanoscale grain sizes of 10 to 20 nm was found to form by annealing amorphous Fe-Zr-B, Fe-Hf-B, and Fe-M-B-Cu(M=Ti, Zr, Hf, Nb, and Ta) alloys for 3.6 ks in the range of 723 to 923 K. The high permeability (μe) above 10 000 at 1 kHz combined with high saturation magnetization (Bs) above 1.5 T was obtained for the bcc alloys. The highest μe and Bs values reach 14 000 and 1.7 T for Fe91Zr7B2, 20 000 and 1.55 T for Fe87Zr7B5Cu1, and 48 000 and 1.52 T for Fe86Zr7B6Cu1. Magnetostriction (λs) decreases significantly by the phase transition from amorphous to bcc phase and is measured to be 1 x 10-6 for the bcc Fe86Zr7B6Cu1 alloy. The small λs as well as the small grain size is concluded to be the reason for the good soft magnetic properties. The lattice parameter of this bcc phase is 0.2870 nm being larger than that of pure α-Fe. The small λs seems to be achieved by the dissolution of solute elements above an equilibrium solubility limit. The bcc Fe86Zr7B6Cu1 alloy also shows the low core loss of 0.066 W/kg at 1 T and 50 Hz, which is considerably smaller than that of amorphous Fe78Si9B13 and bcc Fe-3.5mass%Si alloys in practical uses as core materials in transformer
The nanostructure and hydrogenation reaction of Mg50Co50 BCC alloy prepared by ball-milling
Matsuda, J.; Shao, H.; Nakamura, Y.; Akiba, E.
2009-05-01
Mg50Co50 alloy before and after hydrogenation was investigated by means of transmission electron microscopy (TEM). Mg50Co50 alloy before hydrogenation was found to contain crystals not larger than 5 nm in size. Selected-area electron diffraction patterns (SAEDPs) revealed that these nanocrystals have a body-centered cubic (BCC) structure with a lattice parameter of about 0.3 nm. Distribution of Mg and Co elements in the Mg50Co50 alloy was uniform, indicated by energy dispersive x-ray spectroscopy (EDS) analysis. Crystallization and decomposition occurred in the Mg50Co50 alloy during hydrogenation. A large number of crystals larger than 10 nm were observed in the hydrogenated sample. The SAEDPs showed polycrystalline rings corresponding to the BCC phase and the Co metal phase. The existence of Mg-rich Mg-Co crystals and Co particles was also confirmed by TEM-EDS analysis.
First-principles calculation on the diffusion of hydrogen along a screw dislocation core in BCC iron
The new boundary condition for a single screw dislocation core in BCC iron has been devised to calculate the diffusion of hydrogen using the first principles calculation. It is found that the core structure of a screw dislocation changes drastically by extension due to the presence of high density hydrogen atoms in the core region. Also, we searched the diffusion path of a hydrogen atom from the surrounding trapping lattice site to the core of a screw dislocation, and found that there is the large energy barrier of 0.25eV. We conclude that the diffusion of hydrogen atoms along the core of a screw dislocation is very unlikely to occur in BCC iron. (author)
Diffusion simulation of Cr-Fe bcc systems at atomic level using a random walk algorithm
San Sebastian, I.; Aldazabal, J. [CEIT and Tecnun (University of Navarra), San Sebastian (Spain); Capdevila, C.; Garcia-Mateo, C. [MATERALIA Research Group, Department of Physical Metallurgy, Centro Nacional de Investigaciones Metalurgicas (CENIM-CSIC), Avda. Gregorio del Amo 8, 28040 Madrid (Spain)
2008-06-15
This paper proposes a model to simulate the diffusion of impurities in bcc atomic lattices. It works with three-dimensional volume, divided in small cubic elements (voxels), containing more than one atomic cell each. Once the domain is discretized, impurities jump from one voxel to another according to certain probability that takes into account the composition and geometry of the target voxel. In the present work, a model was applied to a prismatic volume and in order to deduce the relationship between the atomic jumping frequency and the temperature two different cases were studied. One consists of a Fe matrix with Cr impurities, and the other is based on a Cr matrix with Fe impurities. Results obtained from these simulations were compared with profiles obtained by Dictra software. Results for the atomic jumping frequencies were fitted to an Arrhenius type equation, as shown in following expressions: From these equations it is possible to obtain an activation energy for the atomic jumping phenomenon of {proportional_to}306 kJ/mol and {proportional_to}411 kJ/mol for the Fe-matrix and Cr-matrix systems, respectively. These energies match the empirical measured values for the diffusion of Cr and Fe impurities, 250 kJ/mol and 407 kJ/mol, respectively. Results obtained in this work assure that the proposed model is suitable for simulating the three-dimensional diffusion of substitutional impurities in Cr and Fe bcc systems. It could be easily expanded to other bcc matrix systems. (copyright 2008 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
Theoretical elastic moduli of ferromagnetic bcc Fe alloys.
Zhang, Hualei; Punkkinen, Marko P J; Johansson, Börje; Vitos, Levente
2010-07-14
The polycrystalline elastic parameters of ferromagnetic Fe(1-x)M(x) (M = Al, Si, V, Cr, Mn, Co, Ni, Rh; 0 ≤ x ≤ 0.1) random alloys in the body centered cubic (bcc) crystallographic phase have been calculated using first-principles alloy theory in combination with statistical averaging methods. With a few exceptions, the agreement between the calculated and the available experimental data for the polycrystalline aggregates is satisfactory. All additions considered here decrease the bulk modulus (B) and Poisson's ratio (ν) of bcc Fe. The complex composition dependence of the C(44) single-crystal elastic constant is reflected in the polycrystalline shear modulus (G), Young's modulus (E), and Debye temperature (Θ). The polycrystalline anisotropy of bcc Fe is increased by all additions, and Al, Si, Ni, and Rh yield the largest alloying effects. PMID:21399255
The binding energy of an interstitial hydrogen atom at various lattice sites around the a0/2[111] screw dislocation core in BCC iron has been determined using the first-principles calculation. The calculation was based on the core structure of a screw dislocation with symmetric displacement field, which was obtained using the large-scale supercell containing 231 atoms and 1 x 1 x 4 k-point samplings. The binding or trapping energy of a hydrogen atom at both the t-site (tetrahedral site) and o-site (octahedral site) near a core is found to be approximately 0.2 eV. (author)
The energetic and structural properties of bcc NiCu, FeCu alloys: a first-principles study
Xie, Yao-Ping; Zhao, Shi-Jin
2011-01-01
Using special quasirandom structures (SQS's), we perform first-principles calculations studying the metastable bcc NiCu and FeCu alloys which occur in Fe-Cu-Ni alloy steels as precipitated second phase. The mixing enthalpies, density of state, and equilibrium lattice parameters of these alloys are reported. The results show that quasi-chemical approach and vegard rule can well predict the energetic and structural properties of FeCu alloys but fail to yield that of NiCu. The reason rests with ...
Hot electron spin attenuation lengths of bcc Fe34Co66—Room temperature Magnetocurrent of 1200%
Heindl, E.; Kefes, C.; Soda, M.; Vancea, J.; Back, C. H.
2009-11-01
We investigate spin-dependent hot electron transport through metallic epitaxial spin valves by ballistic electron magnetic microscopy (BEMM). By variation of the thickness of one of the ferromagnetic layers we determine the spin dependent attenuation lengths which reflect hot electron transport along the vicinity of the [1 0 0]-axis of the bcc Fe34Co66 lattice. The majority spin attenuation length is more than 6 times larger than that of the minority spins within the measured energy interval of 1.3 up to 2 eV above the Fermi level. Consequently a Magnetocurrent effect exceeding 1200% accompanied by a monotonic bias voltage behavior is observed at room temperature.
Crystallographic Lattice Boltzmann Method
Namburi, Manjusha; Krithivasan, Siddharth; Ansumali, Santosh
2016-01-01
Current approaches to Direct Numerical Simulation (DNS) are computationally quite expensive for most realistic scientific and engineering applications of Fluid Dynamics such as automobiles or atmospheric flows. The Lattice Boltzmann Method (LBM), with its simplified kinetic descriptions, has emerged as an important tool for simulating hydrodynamics. In a heterogeneous computing environment, it is often preferred due to its flexibility and better parallel scaling. However, direct simulation of realistic applications, without the use of turbulence models, remains a distant dream even with highly efficient methods such as LBM. In LBM, a fictitious lattice with suitable isotropy in the velocity space is considered to recover Navier-Stokes hydrodynamics in macroscopic limit. The same lattice is mapped onto a cartesian grid for spatial discretization of the kinetic equation. In this paper, we present an inverted argument of the LBM, by making spatial discretization as the central theme. We argue that the optimal spatial discretization for LBM is a Body Centered Cubic (BCC) arrangement of grid points. We illustrate an order-of-magnitude gain in efficiency for LBM and thus a significant progress towards feasibility of DNS for realistic flows. PMID:27251098
Crystallographic Lattice Boltzmann Method
Namburi, Manjusha; Krithivasan, Siddharth; Ansumali, Santosh
2016-06-01
Current approaches to Direct Numerical Simulation (DNS) are computationally quite expensive for most realistic scientific and engineering applications of Fluid Dynamics such as automobiles or atmospheric flows. The Lattice Boltzmann Method (LBM), with its simplified kinetic descriptions, has emerged as an important tool for simulating hydrodynamics. In a heterogeneous computing environment, it is often preferred due to its flexibility and better parallel scaling. However, direct simulation of realistic applications, without the use of turbulence models, remains a distant dream even with highly efficient methods such as LBM. In LBM, a fictitious lattice with suitable isotropy in the velocity space is considered to recover Navier-Stokes hydrodynamics in macroscopic limit. The same lattice is mapped onto a cartesian grid for spatial discretization of the kinetic equation. In this paper, we present an inverted argument of the LBM, by making spatial discretization as the central theme. We argue that the optimal spatial discretization for LBM is a Body Centered Cubic (BCC) arrangement of grid points. We illustrate an order-of-magnitude gain in efficiency for LBM and thus a significant progress towards feasibility of DNS for realistic flows.
Shear instabilities in perfect bcc crystals during simulated tensile tests
Černý, M.; Šesták, P.; Pokluda, J.; Šob, Mojmír
2013-01-01
Roč. 87, č. 1 (2013), 014117/1-014117/4. ISSN 1098-0121 R&D Projects: GA ČR(CZ) GAP108/12/0311 Institutional support: RVO:68081723 Keywords : instabilities * tensile test * bcc metals * ab initio calculations Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 3.664, year: 2013
Structure and energetics of nanoclusters in bcc-Fe containing copper, nickel and vacancies
Al-Motasem, Ahmed Tamer; Bergner, Frank; Birkenheuer, Uwe [Institute of Safety Research (Germany); Posselt, Matthias [Institute of Ion Beam and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf (Germany)
2011-07-01
Reactor pressure vessel (RPV) steels consist of polycrystalline bcc-Fe containing Cu, Ni and other foreign atoms. The continuous irradiation by fast neutrons leads to supersaturation of vacancies and self-interstitials and enhances the diffusion of Cu and Ni which occurs via the vacancy mechanism. These processes favor the formation of nanoclusters consisting of vacancies, Cu and Ni. The interaction of dislocations with these precipitates is considered to be the main cause of hardening and embrittlement of the RPV steels. In order to model the evolution of the precipitates under irradiation by rate theory, the energetics and thermodynamics of the clusters must be known. These data are hardly obtainable by experiments, however, they can be provided by atomic-level computer simulations. In the present work a combination of on-lattice Monte Carlo simulations and off-lattice Molecular Dynamics calculations is employed to determine structure and energetics of the nanoclusters. The atomistic simulations show that ternary clusters exhibit a shell structure with a core consisting of vacancies followed by a shell of Cu and an outer shell of Ni. Binary vacancy-Cu and Ni-Cu clusters show a similar shell structure, whereas the atomic configuration of vacancy-Ni agglomerates is completely different.
Ab initio calculations of grain boundaries in bcc metals
Scheiber, Daniel; Pippan, Reinhard; Puschnig, Peter; Romaner, Lorenz
2016-03-01
In this study, we compute grain boundary (GB) properties for a large set of GBs in bcc transition metals with a special focus on W, Mo and Fe using ab initio density functional theory (DFT) and semi-empirical second nearest neighbour modified embedded atom method (2NN-MEAM) potentials. The GB properties include GB energies, surface energies, GB excess volume and work of separation, which we analyse and then compare to experimental data. We find that the used 2NN-MEAM potentials can predict general trends of GB properties, but do not always reproduce the GB ground state structure and energy found with DFT. In particular, our results explain the experimental finding that W and Mo prefer intergranular fracture, while other bcc metals prefer transgranular cleavage.
Formation of dislocation loops during He clustering in bcc Fe
Gao, N.; Van Swygenhoven, H.; Victoria, M.; Chen, J.
2011-11-01
The clustering of helium in bcc (body centered cubic) iron and the growth of a helium bubble are simulated at the atomistic level for the helium-rich vacancy-poor condition. It is shown that a \\frac{1}{2}\\langle 111\\rangle dislocation loop is formed as a sequential collection of crowdions, the latter being the most stable self-interstitial atom configuration in the presence of a He cluster.
Formation of dislocation loops during He clustering in bcc Fe
Gao, N; Chen, J [NES-High Temperature Materials, Paul Scherrer Institute, CH-5232 Villigen PSI (Switzerland); Van Swygenhoven, H [NUM/ASQ-Materials Science and Simulation, Paul Scherrer Institute, CH-5232 Villigen PSI (Switzerland); Victoria, M, E-mail: helena.vanswygenhoven@psi.ch [Lawrence Livermore National Laboratory, PO Box 808 L-370, Livermore, CA 94550 (United States)
2011-11-09
The clustering of helium in bcc (body centered cubic) iron and the growth of a helium bubble are simulated at the atomistic level for the helium-rich vacancy-poor condition. It is shown that a 1/2 <111> dislocation loop is formed as a sequential collection of <111> crowdions, the latter being the most stable self-interstitial atom configuration in the presence of a He cluster. (fast track communication)
BCC single crystal plasticity modeling and its experimental identification
A crystal plasticity model for body-centered-cubic (BCC) single crystals, taking into account the plastic anisotropy due to non-planar spreading of screw dislocation cores is presented. In view of the longstanding contradictory statements on the deformation of BCC single crystals and their macroscopic slip planes, recent insights and developments are reported and included in this model. The flow stress of BCC single crystals shows a pronounced dependence on the crystal orientation and the temperature, mostly due to non-planar spreading of a/2(1 1 1) type screw dislocation cores. The main consequence here is the well-known violation of Schmid's law in these materials, resulting in an intrinsic anisotropic effect which is not observed in, for example, FCC materials. Experimental confrontations at the level of a single crystal are generally missing in the literature. To remedy this, uniaxial tension simulations are done at material point level for α-Fe, Mo and Nb single crystals and compared with reported experiments. Material parameters, including non-Schmid parameters, are calibrated from experimental results using a proper identification method. The model is validated for different crystal orientations and temperatures, which was not attempted before in the open literature
Quasicontinuum simulation of crack propagation in bcc-Fe
Highlights: → Multiscale simulation and literature study of fracture in bcc-Fe with 4 orientations. → Investigation of anisotropic vs. isotropic formulations of boundary conditions. → Systematical study of the influence of T-stress on crack tip mechanisms. - Abstract: We have investigated fracture in bcc-Fe through multiscale simulations. The quasicontinuum (QC) method with an embedded atom method (EAM) interatomic potential is applied. The analyses have been carried out assuming different crystallographic orientations and different T-stress under Mode I loading. Both anisotropic and isotropic formulations of the modified boundary layer (MBL) approach has here been investigated and compared. The results show that the mechanisms at the crack tip and the critical stress intensity factor KIc are sensitive to both the crystallographic orientation and whether or not the formulation of the boundary conditions are isotropic or anisotropic. Mechanisms such as cleavage crack propagation, twinning, and dislocation emission are observed in the analyses. A short literature review on atomistic and multiscale simulations of fracture in bcc-Fe has been performed and evaluated, and also compared with the current results.
Quaternionic Representations of the Pyritohedral Group, Related Polyhedra and Lattices
Koca, Nazife Ozdes; Koca, Mehmet; Al-Mukhaini, Aida; Al-Qanobi, Amal
2015-01-01
We construct the fcc (face centered cubic), bcc (body centered cubic) and sc (simple cubic) lattices as the root and the weight lattices of the affine Coxeter groups W(D3) and W(B3)=Aut(D3). The rank-3 Coxeter-Weyl groups describing the point tetrahedral symmetry and the octahedral symmetry of the cubic lattices have been constructed in terms of quaternions. Reflection planes of the Coxeter-Dynkin diagrams are identified with certain planes of the unit cube. It turns out that the pyritohedral...
Strain relief of heteroepitaxial bcc-Fe(001) films
Wedler, G.; Schneider, C. M.; Trampert, A.; Koch, R.
2004-01-01
The strain relief of heteroepitaxial bcc-Fe(001) films, deposited at 520-570 K onto MgO(001), has been investigated by scanning tunneling microscopy. In accordance with real-time stress measurements, the tensile misfit strain is relieved during coalescence of flat, mainly 2-3 monolayers (ML) high Fe islands at the high thickness of similar to20 ML. To accommodate the misfit between merging strain-relaxed islands, a network of 1/2[111] screw dislocations is formed. A strong barrier for disloca...
Transonic twins in 3D bcc iron crystal
Spielmannová, Alena; Machová, Anna; Hora, Petr
2010-01-01
Roč. 48, č. 2 (2010), s. 296-302. ISSN 0927-0256 R&D Projects: GA AV ČR KJB200760802; GA ČR GA101/09/1630 Institutional research plan: CEZ:AV0Z20760514 Keywords : transonic twins * bcc iron * molecular dynamic simulation Subject RIV: JG - Metallurgy Impact factor: 1.458, year: 2010 http://apps.isiknowledge.com/full_record.do?product=WOS&search_mode=GeneralSearch&qid=3&SID=V1mj77dMKmjeKefm7Db&page=1&doc=1
X-Ray Microdiffraction Characterization of Deformation Heterogeneities in BCC Crystals
The deformation behavior of BCC metals is being investigated by x-ray microdiffraction measurements (mu XRD) for the purpose of characterizing the dislocation structure that results from uniaxial compression experiments. The high brilliance synchrotron source at the Advanced Light Source (Lawrence Berkeley National Lab) and the micron resolution of the focusing optics allow for the mapping of Laue diffraction patterns across a sample. These measurements are then analyzed in order to map the distribution of residual stresses in the crystal. An important finding is the observation of Laue spot ''streaking'', which indicates localized rotations in the lattice. These may represent an accumulation of same-sign dislocations. Theoretical modeling of the diffraction response for various slip systems is presented, and compared to experimental data. Preliminary results include orientation maps from a highly strained Ta bicrystal and a less highly strained Mo single crystal. The orientation maps of the bicrystal indicate a cell-like structure of dense dislocation walls. This deformation structure is consistent with previous OIM studies of the same crystal. The results suggest that mu XRD may be a particularly useful tool for microscale studies of deformation patterns in a multi-scale investigation of the mechanisms of deformation that ranges from macroscopic deformation tests to high resolution TEM studies of dislocation structures
Molecular dynamics simulation of hydrogen-edge dislocation interaction in BCC iron
Nedelcu, S. [Department of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ (United Kingdom); Kizler, P. [Staatliche Materialpruefungsanstalt (MPA), Universitaet Stuttgart, Pfaffenwaldring 32, 70569 Stuttgart (Germany)
2002-09-16
Hydrogen embrittlement in metals is a complex multi-causal phenomenon, which, on the atomic scale, comprises H diffusion, decohesion, cavity nucleation as well as the interaction between H and dislocations. The present atomic-level simulation uses the embedded atom method (EAM) to describe the energetics of a hydrogen atom during the interaction with a moving edge dislocation in a bcc iron crystal. Particularly, in the chosen framework it is found that an H atom taking a site in the Fe lattice can block the movement of an edge dislocation, while the stress exerted on the dislocation can reach a maximum value of 15.5 MPa. However, it is also found that an interstitial H atom does not impede the movement of the dislocation and exerts a maximum stress of 38.2 MPa at the time when the moving dislocation passes by. These quantitative findings are correlated with the macroscopically observed mechanical behavior of hydrogen charged iron, which had pointed out that hydrogen might cause an increase of the flow stress and/or a local plasticity enhancement. (Abstract Copyright [2002], Wiley Periodicals, Inc.)
He-induced vacancy formation in bcc Fe solid from first-principles simulation
Using first-principles calculations, we investigated the He and H effects on vacancy formation in bcc Fe. From energetic point of view, the presence of interstitial He (H) atom reduces vacancy formation energy and single He prefers to occupy vacancy center. One monovacancy can accommodate at least 20 He (or 5 H) atoms, with He–He (H–H) distances of 1.5–1.7 Å (1.8–2.38 Å) in the Hem–vacancy (Hm–vacancy) complexes. The shorter He–He distance and larger expansion of vacancy space after multiple He insertion may account for more number of He atoms trapped in vacancy with regard to H. Moreover, second vacancy formation energy around the He–vacancy complex decreases remarkably with increasing amount of He atoms. Thus, trapping of multiple He in a Fe monovacancy can induce formation of new vacancy at nearby lattice site, and several vacancy–SIA pairs would form concurrently
Heindl, E. [Department of Physics, University of Regensburg, 93040 Regensburg (Germany)], E-mail: Emanuel.Heindl@physik.uni-r.de; Kefes, C.; Soda, M.; Vancea, J.; Back, C.H. [Department of Physics, University of Regensburg, 93040 Regensburg (Germany)
2009-11-15
We investigate spin-dependent hot electron transport through metallic epitaxial spin valves by ballistic electron magnetic microscopy (BEMM). By variation of the thickness of one of the ferromagnetic layers we determine the spin dependent attenuation lengths which reflect hot electron transport along the vicinity of the [1 0 0]-axis of the bcc Fe{sub 34}Co{sub 66} lattice. The majority spin attenuation length is more than 6 times larger than that of the minority spins within the measured energy interval of 1.3 up to 2 eV above the Fermi level. Consequently a Magnetocurrent effect exceeding 1200% accompanied by a monotonic bias voltage behavior is observed at room temperature.
Over the past four years this research has been principally concerned with uncovering the microstructural atomic arrangements in alloys of hydrogen and deuterium with bcc refractory metals. Because these are interstitial phases in which the host metal lattice is substantially deformed by the incorporation of the H(D) atoms, there are pronounced x-ray scattering effects. X-ray diffraction has, therefore, been the main structural tool. A main objective of the project has been to determine the degree to which phase relations and solid solution properties in metal-hydride alloys depend upon the hydrogen-hydrogen interaction via the displacement field of the metal atoms. This has often included the elucidation of subtle thermodynamic properties which are revealed in structural studies
Dislocation Dynamics in a Crystal Lattice (Peierls-Nabarro) Relief
Petukhov, B. V.
2007-01-01
The theory of the dislocation motion in the periodic potential relief of the crystal lattice (the Peierls-Nabarro barriers) is reviewed. On the basis of the kink mechanism the temperature dependence of the flow stress is described for a wide class of materials. The theory of quantum mechanical dislocation tunnelling through the Peierls-Nabarro barriers is extended and compared with experimental data on the plasticity of alkali halides, BCC and HCP metals at low temperatures. The behavior of t...
Strain ordering in BCC metals and the associated anelasticity
The BCC to BCT transformation is thought to occur as a consequence of strain ordering due to the interaction between impurity interstitials. A Hamiltonian is given, which involves the interaction energies between the strain fields of the interstitials belonging to three distinct sublattices. In the BCT phase, one of the sublattices is preferentially occupied. The free energy of the system is calculated in the mean field approximation. In this, the BCC to BCT transformation is found to be a first-order transition at a temperature Tsub(p) that is proportional to the concentration of the interstitials and certain basic interaction parameters. The anelastic behaviour of the interacting interstitials is then studied in the region T > Tsub(p). From the anelastic strain, which is proportional to the order parameter associated with the phase transition, the static compliance is obtained. The latter obeys a Curie-Weiss type of law. The creep function, which determines the response to a constant applied stress, is found to exhibit viscous behaviour near Tsub(p). From the creep function, the frequency-dependent compliance and the internal friction are evaluated. The results predict a shift and a broadening of the internal friction peak as Tsub(p) is approached from above. The features show qualitative resemblance with the recent data on Ta-O. (author)
Alling, B.; Körmann, F.; Grabowski, B.; Glensk, A.; Abrikosov, I. A.; Neugebauer, J.
2016-06-01
We study the impact of lattice vibrations on magnetic and electronic properties of paramagnetic bcc and fcc iron at finite temperature, employing the disordered local moments molecular dynamics (DLM-MD) method. Vibrations strongly affect the distribution of local magnetic moments at finite temperature, which in turn correlates with the local atomic volumes. Without the explicit consideration of atomic vibrations, the mean local magnetic moment and mean field derived magnetic entropy of paramagnetic bcc Fe are larger compared to paramagnetic fcc Fe, which would indicate that the magnetic contribution stabilizes the bcc phase at high temperatures. In the present study we show that this assumption is not valid when the coupling between vibrations and magnetism is taken into account. At the γ -δ transition temperature (1662 K), the lattice distortions cause very similar magnetic moments of both bcc and fcc structures and hence magnetic entropy contributions. This finding can be traced back to the electronic densities of states, which also become increasingly similar between bcc and fcc Fe with increasing temperature. Given the sensitive interplay of the different physical excitation mechanisms, our results illustrate the need for an explicit consideration of vibrational disorder and its impact on electronic and magnetic properties to understand paramagnetic Fe. Furthermore, they suggest that at the γ -δ transition temperature electronic and magnetic contributions to the Gibbs free energy are extremely similar in bcc and fcc Fe.
Li, Xiaojie; Schönecker, Stephan; Li, Ruihuan; Li, Xiaoqing; Wang, Yuanyuan; Zhao, Jijun; Johansson, Börje; Vitos, Levente
2016-07-01
To examine the effect of neutron transmutation on tungsten as the first wall material of fusion reactors, the elastic properties of W1‑x‑y Re x Os y (0 ⩽ x, y ⩽ 6%) random alloys in body centered cubic (bcc) structure are investigated systematically using the all-electron exact muffin-tin orbitals (EMTO) method in combination with the coherent-potential approximation (CPA). The calculated lattice constant and elastic properties of pure W are consistent with available experiments. Both Os and Re additions reduce the lattice constant and increase the bulk modulus of W, with Os having the stronger effect. The polycrystalline shear modulus, Young’s modulus and the Debye temperature increase (decrease) with the addition of Re (Os). Except for C 11, the other elastic parameters including C 12, C 44, Cauchy pressure, Poisson ratio, B/G, increase as a function of Re and Os concentration. The variations of the latter three parameters and the trend in the ratio of cleavage energy to shear modulus for the most dominant slip system indicate that the ductility of the alloy enhances with increasing Re and Os content. The calculated elastic anisotropy of bcc W slightly increases with the concentration of both alloying elements. The estimated melting temperatures of the W–Re–Os alloy suggest that Re or Os addition will reduce the melting temperature of pure W solid. The classical Labusch–Nabarro model for solid-solution hardening predicts larger strengthening effects in W1‑y Os y than in W1‑x Re x . A strong correlation between C‧ and the fcc–bcc structural energy difference for W1‑x‑y Re x Os y is revealed demonstrating that canonical band structure dictates the alloying effect on C‧. The structural energy difference is exploited to estimate the alloying effect on the ideal tensile strength in the [0 0 1] direction.
A new method for development of bond-order potentials for transition bcc metals
A new development of numerical bond-order potentials (BOPs) for the non-magnetic transition metals V, Nb, Ta, Cr, Mo and W is presented. The principles on which the BOPs have been set up are the same as in earlier developments (Aoki et al 2007 Prog. Mater. Sci. 52 154). However, the bond integrals are based on the recently advanced method of parametrization of tight-binding from DFT calculations (Madsen et al 2011 Phys. Rev. B 83 4119, Urban et al 2011 Phys. Rev. B 84 155119) and do not require any screening. At the same time, the functional form of the environmentally dependent repulsion is identified with the functional form of the repulsion arising from the overlap of s and p electrons in argon as proposed in Aoki and Kurokawa (2007 J. Phys.: Condens. Matter 19 136228). This is justified by the same physical origin of the environment dependent repulsion, which in transition metals arises from the overlap of s electrons that are being squeezed into the ion core regions under the influence of the strong covalent d bonds. The testing of the developed BOPs involves investigation of alternative higher energy structures, transformation paths connecting the bcc structure with other structures via continuously distorted configurations, evaluation of the vacancy formation energy and calculation of phonon spectra. In all cases, the BOP calculations are in more than satisfactory agreement with either DFT calculations and/or available experimental data. The calculated γ-surfaces for {1 0 1} planes all suggest that the core of 1/2〈1 1 1〉 screw dislocations is non-degenerate in the transition metals. This is also in full agreement with available calculations that account fully for the quantum-mechanical nature of the d electrons that provide the bulk of the bonding in transition metals. The testing of developed BOPs clearly demonstrates that they are transferable to structures well outside the regime of the ideal bcc lattice and are suitable for investigating the
The so-called doubling problem in the lattice description of fermions led to a proof that under certain circumstances chiral gauge theories cannot be defined on the lattice. This is called the no-go theorem. It implies that if Γ/sub/A is defined on a lattice then its infrared limit, which should correspond to the quantum description of the classical action for the slowly varying fields on lattice scale, is inevitably a vector like theory. In particular, if not circumvented, the no-go theorem implies that there is no lattice formulation of the Standard Weinberg-Salam theory or SU(5) GUT, even though the fermions belong to anomaly-free representations of the gauge group. This talk aims to explain one possible attempt at bypassing the no-go theorem. 20 refs
The effects of next-to-nearest-neighbour hopping on Bose–Einstein condensation in cubic lattices
G K Chaudhary; R Ramakumar
2010-01-01
In this paper, we present results of our calculations on the effects of next-to-nearest-neighbour boson hopping (′) energy on Bose–Einstein condensation in cubic lattices. We consider both non-interacting and repulsively interacting bosons moving in the lowest Bloch band. The interacting bosons are studied using Bogoliubov method. We find that the Bose condensation temperature is enhanced by increasing ′ for bosons in a simple cubic (sc) lattice and decreases for bosons in body-centred cubic (bcc) and face-centred cubic (fcc) lattices. We also find that interaction-induced depletion of the condensate is reduced for bosons in an sc lattice while it is enhanced for bosons in bcc and fcc lattices.
The physical and mechanical metallurgy of advanced O+BCC titanium alloys
Cowen, Christopher John
This thesis comprises a systematic study of the microstructural evolution, phase transformation behavior, elevated-temperature creep behavior, room-temperature and elevated-temperature tensile behavior, and room-temperature fatigue behavior of advanced titanium-aluminum-niobium (Ti-Al-Nb) alloys with and without boron additions. The specific alloys studied were: Ti-5A1-45Nb (at%), Ti-15Al-33Nb (at%), Ti-15Al-33Nb-0.5B (at%), Ti-15Al-33Nb-5B (at%), Ti-21Al-29Nb (at%), Ti-22Al-26Nb (at%), and Ti-22Al-26Nb-5B (at%). The only alloy composition that had been previously studied before this thesis work began was Ti-22Al-26Nb (at%). Publication in peer-reviewed material science journals of the work performed in this thesis has made data available in the scientific literature that was previously non-existent. The knowledge gap for Ti-Al-Nb phase equilibria over the compositional range of Ti-23Al-27Nb (at%) to Ti-12Al-38Nb (at%) that existed before this work began was successfully filled. The addition of 5 at% boron to the Ti-15Al-33Nb alloy produced 5-9 volume percent boride phase needles within the microstructure. The chemical composition of the boride phase measured by electron microprobe was determined to be approximately B 2TiNb. The lattice parameters of the boride phase were simulated through density functional theory calculations by collaborators at the Air Force Research Laboratory based on the measured composition. Using the simulated lattice parameters, electron backscatter diffraction kikuchi patterns and selected area electron diffraction patterns obtained from the boride phase were successfully indexed according to the space group and site occupancies of the B27 orthorhombic crystal structure. This suggests that half the Ti (c) Wyckoff positions are occupied by Ti atoms and the other half are occupied by Nb atoms in the boride phase lattice. Creep deformation behavior is the main focus of this thesis and in particular understanding the dominant creep
An Overview of BCC Climate System Model Development and Application for Climate Change Studies
WU Tongwen; WU Fanghua; LIU Yiming; ZHANG Fang; SHI Xueli; CHU Min; ZHANG Jie; FANG Yongjie; WANG Fang; LU Yixiong; LIU Xiangwen; SONG Lianchun; WEI Min; LIU Qianxia; ZHOU Wenyan; DONG Min; ZHAO Qigeng; JI Jinjun; Laurent LI; ZHOU Mingyu; LI Weiping; WANG Zaizhi; ZHANG Hua; XIN Xiaoge; ZHANG Yanwu; ZHANG Li; LI Jianglong
2014-01-01
This paper reviews recent progress in the development of the Beijing Climate Center Climate System Model (BCC-CSM) and its four component models (atmosphere, land surface, ocean, and sea ice). Two recent versions are described: BCC-CSM1.1 with coarse resolution (approximately 2.8125◦×2.8125◦) and BCC-CSM1.1(m) with moderate resolution (approximately 1.125◦×1.125◦). Both versions are fully cou-pled climate-carbon cycle models that simulate the global terrestrial and oceanic carbon cycles and include dynamic vegetation. Both models well simulate the concentration and temporal evolution of atmospheric CO2 during the 20th century with anthropogenic CO2 emissions prescribed. Simulations using these two versions of the BCC-CSM model have been contributed to the Coupled Model Intercomparison Project phase fi ve (CMIP5) in support of the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5). These simulations are available for use by both national and international communities for investigating global climate change and for future climate pro jections. Simulations of the 20th century climate using BCC-CSM1.1 and BCC-CSM1.1(m) are presented and validated, with particular focus on the spatial pattern and seasonal evolution of precipitation and surface air temperature on global and continental scales. Simulations of climate during the last millennium and pro jections of climate change during the next century are also presented and discussed. Both BCC-CSM1.1 and BCC-CSM1.1(m) perform well when compared with other CMIP5 models. Preliminary analyses in-dicate that the higher resolution in BCC-CSM1.1(m) improves the simulation of mean climate relative to BCC-CSM1.1, particularly on regional scales.
Lattice relaxations and hyperfine fields of heavy impurities in Fe
Korhonen, T.; Settels, A.; Papanikolaou, N.; Zeller, R.; Dederichs, P. H.
2000-01-01
We present first-principles calculations of the lattice relaxations and hyperfine fields of heavy impurities in bcc Fe. We consider impurities of the 5sp and 6sp series, containing the largest atoms in the periodic table. As an application we calculate the hyperfine fields of these impurities and in particular the effects of lattice relaxations on these fields. The calculations are based on a full-potential Korringa-Kohn-Rostoker Green's-function method for defects and employ the local spin-d...
Cubic to tetragonal crystal lattice reconstruction during ordering or decomposition
Cheong, Byung-kl [Carnegie-Mellon Univ., Pittsburgh, PA (United States). Dept. of Materials Science and Engineering
1992-09-01
This thesis studied thermodynamic stability and morphology of product phases in diffusional phase transformations involving cubic-to-tetragonal crystal lattice reconstructions. Two different kinds of diffusional transformations were examined: L1{sub 0} ordering (fcc to fct lattice change) and decomposition of off-stoichiometric B2 ordering alloys accompanying bcc to fcc Bain transformation. In the first case, Fe-45 at.% Pd alloys were studied by TEM; in the second, the Bain strain relaxation during decomposition of hyper-eutectoid Cu-9.04 wt% Be alloy was studied. CuAu and InMg were also studied.
Plastic anisotropy and dislocation trajectory in BCC metals.
Dezerald, Lucile; Rodney, David; Clouet, Emmanuel; Ventelon, Lisa; Willaime, François
2016-01-01
Plasticity in body-centred cubic (BCC) metals at low temperatures is atypical, marked in particular by an anisotropic elastic limit in clear violation of the famous Schmid law applicable to most other metals. This effect is known to originate from the behaviour of the screw dislocations; however, the underlying physics has so far remained insufficiently understood to predict plastic anisotropy without adjustable parameters. Here we show that deviations from the Schmid law can be quantified from the deviations of the screw dislocation trajectory away from a straight path between equilibrium configurations, a consequence of the asymmetrical and metal-dependent potential energy landscape of the dislocation. We propose a modified parameter-free Schmid law, based on a projection of the applied stress on the curved trajectory, which compares well with experimental variations and first-principles calculations of the dislocation Peierls stress as a function of crystal orientation. PMID:27221965
Atomistic simulations for multiscale modeling in bcc metal
Belak, J.; Moriarty, J.A.; Soderlind, P.; Xu, W.; Yang, L.H.; Zhu
1998-09-25
Quantum-based atomistic simulations are being used to study fundamental deformation and defect properties relevant to the multiscale modeling of plasticity in bcc metals at both ambient and extreme conditions. Ab initio electronic-structure calculations on the elastic and ideal-strength properties of Ta and Mo help constrain and validate many-body interatomic potentials used to study grain boundaries and dislocations. The predicted C(capital Sigma)5 (310)[100] grain boundary structure for Mo has recently been confirmed in HREM measurements. The core structure, (small gamma) surfaces, Peierls stress, and kink-pair formation energies associated with the motion of a/2(111) screw dislocations in Ta and Mo have also been calculated. Dislocation mobility and dislocation junction formation and breaking are currently under investigation.
Atomistic modeling of carbon Cottrell atmospheres in bcc iron
Atomistic simulations with an EAM interatomic potential were used to evaluate carbon-dislocation binding energies in bcc iron. These binding energies were then used to calculate the occupation probability of interstitial sites in the vicinity of an edge and a screw dislocation. The saturation concentration due to carbon-carbon interactions was also estimated by atomistic simulations in the dislocation core and taken as an upper limit for carbon concentration in a Cottrell atmosphere. We obtained a maximum concentration of 10 ± 1 at.% C at T = 0 K within a radius of 1 nm from the dislocation lines. The spatial carbon distributions around the line defects revealed that the Cottrell atmosphere associated with an edge dislocation is denser than that around a screw dislocation, in contrast with the predictions of the classical model of Cochardt and colleagues. Moreover, the present Cottrell atmosphere model is in reasonable quantitative accord with the three-dimensional atom probe data available in the literature.
A definition of lattice BRS invariance is given. The requirement of lattice BRS invariance successfully replaces that of local gauge invariance as a principle for selecting allowed actions. This replacement also works to any finite order in perturbation theory, but, on the nonperturbative level one encounters an obstacle reflecting the existence of an even number of solutions to the gauge fixing problem. The problem of latticizing the classical action for open bosonic strings discovered by Witten is discussed and a possible direction for dealing with it is pointed out. 3 refs
Non-perturbative phenomena are essential to understanding quantum chromodynamics (QCD), the theory of the strong interactions. The particles observed are mesons and baryons, but the fundamental fields are quarks and gluons. Most properties of the hadrons are inaccessible in perturbation theory. Aside from their mere existence, the most blatant example is the mass spectrum. The lack of an accurate, reasonably precise, calculation of the mass spectrum is a major piece of unfinished business for theoretical particle physics. In addition, a wide variety of other non-perturbative calculations in QCD are necessary to interpret ongoing experiments. For example, it is impossible to extract the Cabibbo-Kobayashi-Maskawa angles without knowing matrix elements of operators in the K, D and B mesons. Furthermore, non-perturbative analyses of quarkonia can determine the strong coupling constant with uncertainties already comparable to perturbative analyses of high-energy data. These lectures cover lattice field theory, the only general, systematic approach that can address quantitatively the non-perturbative questions raised above. Sects. 2--8 explain how to formulate quantum field theory on a lattice and why lattice field theory is theoretically well-founded. Sect. 9 sketches some analytic calculations in scalar lattice field theory. They serve as an example of how lattice field theory can contribute to particle physics without necessarily using computers. Sect. 10 turns to the most powerful tool in lattice field theory: large-scale Monte Carlo integration of the functional integral. Instead of discussing algorithms in gory detail, the general themes of computational field theory are discussed. The methods needed for spectroscopy, weak matrix elements, and the strong coupling constant are reviewed. 52 refs., 7 figs., 1 tab
The finite-element method enables us to convert the operator differential equations of a quantum field theory into operator difference equations. These difference equations are consistent with the requirements of quantum mechanics and they do not exhibit fermion doubling, a problem that frequently plagues lattice treatments of fermions. Guage invariance can also be incorporated into the difference equations. On a finite lattice the operator difference equations can be solved in closed form. For the case of the Schwinger model the anomaly is computed and results in excellent agreement are obtained with the known continuum value
Isothermal compression of bcc transition metals to 100 kbar
Pressure-volume relationships for the bcc transition metals in the groups VB (V, Nb, and Ta) and VIB (Cr, Mo, and W) were determined under hydrostatic pressure to 100 kbar at room temperature by means of x-ray diffraction, employing diamond-anvil pressure cell and the ruby (R1 line) fluorescence calibration technique. Using the measured ultrasonic K'0 values, where K0 is the bulk modulus at ambient pressure, for five metals and the deduced shock-wave K'0 value for Cr, the values of K0 for V, Nb, Ta, Mo, W, and Cr are calculated to be 1.54, 1.71, 1.94, 2.67, 3.07, and 1.93 Mbar, respectively, by least-squares fit of the experimental P-V data to the Birch-Murnaghan equation. The K0 values thus determined are in good agreement with the ultrasonic and shock-wave values for V, Nb, Ta, Mo, and W. For Cr, the K0 value (1.93 Mbar) is in agreement with the shock-wave value (1.92 Mbar) but not with the ultrasonic value (1.65 Mbar). This discrepancy can be explained in view of the paramagnetic→antiferromagnetic transition in Cr in the temperature proximity of the ultrasonic measurements (Neel temperature T/sub N/approx.311 0K at 1 bar) and by the negative pressure dependence of T/sub N/
Cesium under pressure: First-principles calculation of the bcc-to-fcc phase transition
Carlesi, S.; Franchini, A.; Bortolani, V.; Martinelli, S.
1999-05-01
In this paper we present the ab initio calculation of the structural properties of cesium under pressure. The calculation of the total energy is done in the local-density approximation of density-functional theory, using a nonlocal pseudopotential including the nonlinear core corrections proposed by Louie et al. The calculation of the pressure-volume diagram for both bcc and fcc structures allows us to prove that the transition from bcc to fcc structure is a first-order transition.
Plasticity and Failure in Nanocrystalline BCC Metals via MD Simulation
Rudd, R E
2010-02-12
Advances in the ability to generate extremely high pressures in dynamic experiments such as at the National Ignition Facility has motivated the need for special materials optimized for those conditions as well as ways to probe the response of these materials as they are deformed. We need to develop a much deeper understanding of the behavior of materials subjected to high pressure, especially the effect of rate at the extremely high rates encountered in those experiments. Here we use large-scale molecular dynamics (MD) simulations of the high-rate deformation of nanocrystalline tantalum at pressures less than 100 GPa to investigate the processes associated with plastic deformation for strains up to 100%. We focus on 3D polycrystalline systems with typical grain sizes of 10-20 nm. We also study a rapidly quenched liquid (amorphous solid) tantalum. We apply a constant volume (isochoric), constant temperature (isothermal) shear deformation over a range of strain rates, and compute the resulting stress-strain curves to large strains for both uniaxial and biaxial compression. We study the rate dependence and identify plastic deformation mechanisms. The identification of the mechanisms is facilitated through a novel technique that computes the local grain orientation, returning it as a quaternion for each atom. This analysis technique is robust and fast, and has been used to compute the orientations on the fly during our parallel MD simulations on supercomputers. We find both dislocation and twinning processes are important, and they interact in the weak strain hardening in these extremely fine-grained microstructures. We also present some results on void growth in nanocrystalline BCC metals under tension.
Highlights: • Atomic mobilities of bcc-A2 and bcc-B2 Al–Ni–Fe phases were determined. • Experimental interdiffusivities and tracer diffusivities were critically evaluated. • Main diffusivities show minimum around section x(Al) = 50 at.%. • Cross diffusivities show different features for negative and positive values. • The results reveal the importance of full diffusion-distance measurement. - Abstract: The influence of composition and temperature on the atomic mobilities of Al, Ni and Fe in the bcc-A2 and bcc-B2 Al–Ni–Fe phases was investigated based on a recently developed phenomenological model, together with the newly published thermodynamic description. The experimental interdiffusivities and tracer diffusivities of Ni and Fe in various Al–Ni–Fe alloys available in the literature were critically evaluated. The calculated diffusivities can reasonably reproduce the experimental ones. The calculated iso-diffusivity lines in the B2 phase show a minimum around the quasi-binary section at 50 at.% Al for the main diffusivities of both Al and Ni, while for the cross diffusivities of Al and Ni, they show significantly different characteristics when changing from negative to positive values. The obtained atomic mobilities were further used to predict the concentration profiles and diffusion paths in a variety of B2 Al–Ni–Fe diffusion couples. The comparison reveals the importance of full diffusion-distance measurement for the accurate determination of diffusion coefficients
Shim, Jae Hyeok; Kim, Ji Woo; Lee, Gil Jae [KAIST, Daejeon (Korea, Republic of)
2007-02-15
We performed the MD simulations with modified version of the MDCASK code with many-body interatomic potentials by Ackland for the assesment of mobility of dislocation with temperature, the interaction between defects, SiA loops and Cu precipitate and dislocation in BCC iron alloys. The effect of a helium atom on the migration of self-interstitial dislocation loops with a Burgers vector 1/2<111> in bcc Fe has been investigated using molecular statics and molecular dynamics simulations. It is found that an interstitial He atom hinders the migration and coalescence behavior of dislocation loops by strongly binding to the loop on the edge sites. An unstable interstitial He atom on the loop plane easily moves to the stable edge sites by interstitial diffusion. A substitutional He atom does not significantly disturb the migration of dislocation loops, showing weak binding. Molecular dynamics simulations of the interaction between a screw dislocation and a coherent bcc Cu precipitate in bcc Fe indicate that the screw dislocation stress field assists a martensitic transformation into a close-packed structure for precipitate diameters larger than 1.8 nm, resulting in a stronger obstacle to dislocation glide. The observed martensitic transformation mechanism agrees with the Nishiyama-Kajiwara model. For coherent bcc Cu precipitates with diameter larger than 2.5 nm, the screw dislocation bypass mechanism becomes Orowan looping due to the coherency loss of the precipitates during the transformation
Vairavel, Mathayan; Sundaravel, Balakrishnan; Panigrahi, Binaykumar
2016-09-01
There are contradictory theoretical predictions of lattice location of oxygen interstitial atom at tetrahedral and octahedral interstices in bcc Fe. For validating these predictions, 300 keV O18 ions with fluence of 5 × 1015 ions/cm2 are implanted into bcc Fe single crystals at room temperature and annealed at 400 °C. The Rutherford backscattering spectrometry (RBS) and nuclear reaction analysis (NRA)/channeling measurements are carried out with 850 keV protons. The lattice location of implanted O18 is analysed using the α-particles yield from O18(p,α)N15 nuclear reaction. The tilt angular scans of α-particle yield along and axial directions are performed at room temperature. Lattice location of O18 is found to be at tetrahedral interstitial site by comparing the experimental scan with simulated scans using FLUX7 software.
Modeling of the magnetic free energy of self-diffusion in bcc Fe
Sandberg, N.; Chang, Z.; Messina, L.; Olsson, P.; Korzhavyi, P.
2015-11-01
A first-principles based approach to calculating self-diffusion rates in bcc Fe is discussed with particular focus on the magnetic free energy associated with diffusion activation. First, the enthalpies and entropies of vacancy formation and migration in ferromagnetic bcc Fe are calculated from standard density functional theory methods in combination with transition state theory. Next, the shift in diffusion activation energy when going from the ferromagnetic to the paramagnetic state is estimated by averaging over random spin states. Classical and quantum mechanical Monte Carlo simulations within the Heisenberg model are used to study the effect of spin disordering on the vacancy formation and migration free energy. Finally, a quasiempirical model of the magnetic contribution to the diffusion activation free energy is applied in order to connect the current first-principles results to experimental data. The importance of the zero-point magnon energy in modeling of diffusion in bcc Fe is stressed.
On the stability of rhenium up to 1 TPa pressure against transition to the bcc structure
A K Verma; P Ravindran; R S Rao; B K Godwal; R Jeanloz
2003-01-01
We have carried out electronic structure total energy calculations on rhenium in the hexagonal close packed (hcp) and body centred cubic (bcc) phases, by the full potential linear muffin–tin orbital method, in order to verify the stability of the ambient pressure hcp phase against transition to the bcc structure at high pressures. As per our results, no hcp to bcc structural transition can occur up to 1 TPa pressures. Moreover, our Bain path calculations show that face centred cubic and body centred tetragonal structures are also not energetically preferred over hcp in this pressure range. The axial ratio (/) of Re changes by less than 0.33% in the pressure range studied.
Photocatalytic degradation of gaseous toluene over bcc-In2O3 hollow microspheres
Graphical abstract: - Highlights: • The bcc-In2O3 hollow microspheres were synthesized as photocatalyst. • Structural and photocatalytic properties of samples are tested. • Degradation of gaseous toluene over the bcc-In2O3 hollow microspheres. - Abstract: In this work, the body-centered cubic indium oxide (bcc-In2O3) hollow microspheres were prepared via a P123-assisted solvothermal process. The structural properties of samples were investigated by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, UV–visible diffusive reflectance spectroscopy, and nitrogen adsorption-desorption isotherms. The photocatalytic effects of degrading gaseous toluene were evaluated by gas chromatography and in situ Fourier transform infrared (FTIR) spectra under a irradiation of a 500 W high pressure xenon lamp. The results indicated that the as-prepared bcc-In2O3 hollow microspheres exhibited a high degradation efficiency towards toluene within a short reaction time. Besides, the preliminary mechanism therein was inferred with the aid of in situ FTIR and electron spin-paramagnetic resonance techniques to understand the degradation process
In situ TEM straining of bcc twins – unexpected modes of slip transfer
Gemperle, Antonín; Gemperlová, Juliana; Zárubová, Niva; Dlabáček, Zdeněk
Villigen : Paul Scherrer Institute, 2005, s. 298-298. ISBN N. ISSN 1019-6447. [Dreiländertagung Microscopy Conference (MC 2005). Davos (CH), 28.08.2005-02.09.2005] Institutional research plan: CEZ:AV0Z10100520 Keywords : in situ TEM experiments * slip transfer * twins in bcc Subject RIV: BM - Solid Matter Physics ; Magnetism
Nancy Omar
2015-03-01
Minimal Inhibitory Concentration (MIC determining tests showed that only 11.5% were resistant to meropenem at MIC > 16 μg/ml, while 40% of the strains were resistant to ceftazidime at MIC > 32 μg/ml. Those results for the time being indicate that meropenem is the best therapeutic option for Bcc infections in AMUH.
NiFe epitaxial films with hcp and fcc structures prepared on bcc-Cr underlayers
Higuchi, Jumpei, E-mail: higuchi@futamoto.elect.chuo-u.ac.jp [Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551 (Japan); Ohtake, Mitsuru; Sato, Yoichi [Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551 (Japan); Kirino, Fumiyoshi [Graduate School of Fine Arts, Tokyo National University of Fine Arts and Music, 12-8 Ueno-koen, Taito-ku, Tokyo 110-8714 (Japan); Futamoto, Masaaki [Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551 (Japan)
2011-09-30
NiFe epitaxial films are prepared on Cr(211){sub bcc} and Cr(100){sub bcc} underlayers grown hetero-epitaxially on MgO single-crystal substrates by ultra-high vacuum rf magnetron sputtering. The film growth behavior and the crystallographic properties are studied by reflection high energy electron diffraction and pole figure X-ray diffraction. Metastable hcp-NiFe(11-bar 00) and hcp-NiFe(112-bar 0) crystals respectively nucleate on Cr(211){sub bcc} and Cr(100){sub bcc} underlayers, where the hcp-NiFe crystals are stabilized through hetero-epitaxial growth. The hcp-NiFe(11-bar 00) crystal is a single-crystal with the c-axis parallel to the substrate surface, whereas the hcp-NiFe(112-bar 0) crystal is a bi-crystal with the respective c-axes lying in plane and perpendicular each other. With increasing the film thickness, the hcp structure in the NiFe films starts to transform into more stable fcc structure by atomic displacement parallel to the hcp(0001) close packed plane. The resulting films consist of hcp and fcc crystals.
NiFe epitaxial films with hcp and fcc structures prepared on bcc-Cr underlayers
NiFe epitaxial films are prepared on Cr(211)bcc and Cr(100)bcc underlayers grown hetero-epitaxially on MgO single-crystal substrates by ultra-high vacuum rf magnetron sputtering. The film growth behavior and the crystallographic properties are studied by reflection high energy electron diffraction and pole figure X-ray diffraction. Metastable hcp-NiFe(11-bar 00) and hcp-NiFe(112-bar 0) crystals respectively nucleate on Cr(211)bcc and Cr(100)bcc underlayers, where the hcp-NiFe crystals are stabilized through hetero-epitaxial growth. The hcp-NiFe(11-bar 00) crystal is a single-crystal with the c-axis parallel to the substrate surface, whereas the hcp-NiFe(112-bar 0) crystal is a bi-crystal with the respective c-axes lying in plane and perpendicular each other. With increasing the film thickness, the hcp structure in the NiFe films starts to transform into more stable fcc structure by atomic displacement parallel to the hcp(0001) close packed plane. The resulting films consist of hcp and fcc crystals.
Mechanosynthesis of supersaturated solid solutions of Sn in near-equiatomic bcc FeCo
Highlights: ► (Fe50−x/2Co50−x/2)Snx mixtures are mechanically alloyed for x ≤ 33 at.%. ► As-ground powders are studied by X-ray diffraction and 57Fe and 119Sn Mössbauer spectroscopy. ► Supersaturated solutions of Sn in disordered bcc FeCo alloys are formed in our dynamical conditions up to x ≤ 20 at.%. ► This maximum Sn solubility found is much larger than the equilibrium solubility which is about 0.5 at.%. ► The mean 119Sn hyperfine field in the bcc alloys is essentially constant with x and equal to 9.6 T at room temperature. - Abstract: The mechanosynthesis of Fe–Co–Sn ternary alloys from initial powder mixtures of composition (Fe50−x/2Co50−x/2)Snx is studied for x ≤ 33 at.%. Disordered nanocrystalline bcc solid solutions are formed in that way up to Sn contents as large as ∼20 at.%. The dissolution of Sn in near-equiatomic bcc Fe–Co is unambiguously proven by X-ray diffraction and 57Fe and 119Sn Mössbauer spectroscopy.
LATTICE: an interactive lattice computer code
LATTICE is a computer code which enables an interactive user to calculate the functions of a synchrotron lattice. This program satisfies the requirements at LBL for a simple interactive lattice program by borrowing ideas from both TRANSPORT and SYNCH. A fitting routine is included
Tang, Zhi; Gao, Michael C.; Diao, Haoyan; Yang, Tengfei; Liu, Junpeng; Zuo, Tingting; Zhang, Yong; Lu, Zhaoping; Cheng, Yongqiang; Zhang, Yanwen; Dahmen, Karin A.; Liaw, Peter K.; Egami, Takeshi
2013-12-01
The crystal lattice type is one of the dominant factors for controlling the mechanical behavior of high-entropy alloys (HEAs). For example, the yield strength at room temperature varies from 300 MPa for the face-centered-cubic (fcc) structured alloys, such as the CoCrCuFeNiTi x system, to about 3,000 MPa for the body-centered-cubic (bcc) structured alloys, such as the AlCoCrFeNiTi x system. The values of Vickers hardness range from 100 to 900, depending on lattice types and microstructures. As in conventional alloys with one or two principal elements, the addition of minor alloying elements to HEAs can further alter their mechanical properties, such as strength, plasticity, hardness, etc. Excessive alloying may even result in the change of lattice types of HEAs. In this report, we first review alloying effects on lattice types and properties of HEAs in five Al-containing HEA systems: Al x CoCrCuFeNi, Al x CoCrFeNi, Al x CrFe1.5MnNi0.5, Al x CoCrFeNiTi, and Al x CrCuFeNi2. It is found that Al acts as a strong bcc stabilizer, and its addition enhances the strength of the alloy at the cost of reduced ductility. The origins of such effects are then qualitatively discussed from the viewpoints of lattice-strain energies and electronic bonds. Quantification of the interaction between Al and 3 d transition metals in fcc, bcc, and intermetallic compounds is illustrated in the thermodynamic modeling using the CALculation of PHAse Diagram method.
Kinyon, Michael
2012-01-01
Categorical skew lattices are a variety of skew lattices on which the natural partial order is especially well behaved. While most skew lattices of interest are categorical, not all are. They are characterized by a countable family of forbidden subalgebras. We also consider the subclass of strictly categorical skew lattices.
Irradiation tests have shown that U-Mo dispersion fuel suffers from interaction between the fuel and the aluminum matrix at higher temperature. A potential solution to mitigate these phenomena is to use a fuel alloy foil in place of the fuel-aluminum dispersion. This monolithic fuel provides a lower fuel-matrix interfacial surface area and a much higher uranium density than dispersion type fuel. Internal stresses will arise, during fabrication and operating conditions, in the interface between fuel and cladding materials due to the difference in the thermal expansion coefficients and elastic constants. Our aim is to ascertain these properties for the gamma phase in the U-Mo alloy from the knowledge of its internal energy as a function of deformation. Internal energy of the disordered phase is obtained as a cluster expansion from AB initio calculated interaction parameters in each of three deportation modes: Hydrostatic, tetragonal and trigonal. Preliminary results are presented that have served as a proof of method suitability. (author)
BCc1, the novel antineoplastic nanocomplex, showed potent anticancer effects in vitro and in vivo
Kalanaky S
2015-12-01
Full Text Available Somayeh Kalanaky,1,2 Maryam Hafizi,1–3 Saideh Fakharzadeh,1 Mohammad Vasei,4 Ladan Langroudi,5 Ehsan Janzamin,6 Seyed Mahmoud Hashemi,7 Maryam Khayamzadeh,2 Masoud Soleimani,6 Mohammad Esmaeil Akbari,2 Mohammad Hassan Nazaran1 1Department of Research and Development, Sodour Ahrar Shargh Company, Tehran, Iran; 2Cancer Research Centre, Shahid Beheshti University of Medical Sciences, Tehran, Iran; 3Stem Cell Technology Research Center, Tehran, Iran; 4Department of Pathology, Tehran University of Medical Sciences, Tehran, Iran; 5Department of Immunology, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Iran; 6Department of Haematology, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Iran; 7Department of Immunology, Faculty of Medicine, Shahid Beheshti University of Medical Sciences, Tehran, Iran Purpose: In spite of all the efforts and researches on anticancer therapeutics, an absolute treatment is still a myth. Therefore, it is necessary to utilize novel technologies in order to synthesize smart multifunctional structures. In this study, for the first time, we have evaluated the anticancer effects of BCc1 nanocomplex by vitro and in vivo studies, which is designed based on the novel nanochelating technology.Methods: Human breast adenocarcinoma cell line (MCF-7 and mouse embryonic fibroblasts were used for the in vitro study. Antioxidant potential, cell toxicity, apoptosis induction, and CD44 and CD24 protein expression were evaluated after treatment of cells with different concentrations of BCc1 nanocomplex. For the in vivo study, mammary tumor-bearing female Balb/c mice were treated with different doses of BCc1 and their effects on tumor growth rate and survival were evaluated.Results: BCc1 decreased CD44 protein expression and increased CD24 protein expression. It induced MCF-7 cell apoptosis but at the same concentrations did not have negative effects on mouse embryonic fibroblasts viability and protected
Ashokan, V.; Abou Ghantous, M.; Khater, A.
2015-12-01
Ferromagnetic nanojunctions … Fe[Fe1-cCoc ] ℓ Fe …, with ℓ is the number of layers which constitute the nanojunction, based on Fe/Co alloy are considered for the first time in this work. We model the salient magnetic properties of the layered ferromagnetic nanostructures between magnetically ordered iron leads. The effective field theory (EFT) Ising spin method is used to compute reliable Jav exchange values for the VCA Fe/Co alloy materials in comparison with experimental data and compared to existing DFT calculated exchange interactions. The new set of exchange interaction values between pairs of nearest neighbors atom in the alloy are deduced and agree with previous known measurement of lattice constant for this alloy. Using the combined EFT and mean field theory (MFT) spin methods, the sublattice magnetizations of the Fe and Co sites on the individual bcc basal planes of the layered nanostructures, are calculated and analyzed. The sublattice magnetizations, effective magnetic moments per site, and the possible ferromagnetic order of the layers [Fe1-cCoc ] ℓ on the individual bcc atomic planes of the embedded nanostructures for all temperatures and in particular for TcFe ≤ T ≤Tα→γ are presented as a function of temperature and thicknesses of the layered ferromagnetic nanostructures, for different stable concentrations c=0.25, 0.5 and 0.75. In the absence of first principles calculations for these basic physical variables for the layered nanostructures between iron leads, the combined EFT and MFT approach yields the only available information for them at present in the absence of a possible Curie temperature for these alloys. These variables are necessary for certain spin dynamic computations, as for the ballistic magnon transport across embedded nanojunctions in magnonics. The model is general, and may applied directly to other composite magnetic elements and embedded nanostructures.
Spin dependent transport of hot electrons in bcc Fe(100) and bcc Fe{sub 34}Co{sub 66}(100)
Heindl, Emanuel; Vancea, Johann; Back, Christian H. [Department of Physics, University of Regensburg (Germany)
2010-07-01
We investigate hot electron spin filtering in thin bcc Fe(100) and bcc Fe{sub 34}Co{sub 66}(100) layers using Ballistic Electron Emission Microscopy. An STM tip is used as a tunable constant current source of hot electrons being injected into single crystalline metallic spin valve structures. The subsequent ballistic hot electron transport is recorded and separated from thermalized electron transport by means of a metal semiconductor junction whose Schottky barrier acts as a spectrometer. Electron transport is carried out with the main transport axis along the-axis of the ferromagnetic layers at electron energies between 1 eV and 2.5 eV above the Fermi level. Parallel and antiparallel magnetization configurations of the spin valve are readily adjustable with an external magnetic field as revealed by Kerr effect and magnetocurrent measurements. When the Fe{sub 34}Co{sub 66} electrode is replaced by Fe the spin contrast drops by more than a factor of 5 in the studied energy interval. We interpret this observation to the spin asymmetry of unoccupied states and to the electron velocity being distinct for majority and minority spins. By cooling down from room temperature to 130 K ballistic currents become significantly enhanced for both materials in the parallel and the antiparallel magnetization configuration, while hot electron spin polarization is enhanced for Fe{sub 34}Co{sub 66}, only.
Analytic bond-order potentials for the bcc refractory metals Nb, Ta, Mo and W
Bond-order potentials (BOPs) are based on the tight-binding approximation for determining the energy of a system of interacting atoms. The bond energy and forces are computed analytically within the formalism of the analytic BOPs. Here we present parametrizations of the analytic BOPs for the bcc refractory metals Nb, Ta, Mo and W. The parametrizations are optimized for the equilibrium bcc structure and tested for atomic environments far from equilibrium that had not been included in the fitting procedure. These tests include structural energy differences for competing crystal structures; tetragonal, trigonal, hexagonal and orthorhombic deformation paths; formation energies of point defects as well as phonon dispersion relations. Our tests show good agreement with available experimental and theoretical data. In practice, we obtain the energetic ordering of vacancy, [1 1 1], [1 1 0], and [1 0 0] self-interstitial atom in agreement with density functional theory calculations. (paper)
First-principles calculation on Peierls stress of a screw dislocation in BCC molybdenum
Predicting atomistic properties of a dislocation is a first step toward an understanding of plastic behavior of materials, in particular BCC metals. The core structure and Peierls stress of a screw dislocation in BCC molybdenum have been studied over the years using the first-principles and empirical methods, however, their conclusions vary due to the inefficiency of the methods. We have executed the first-principles calculation based on the density functional method, employing the most accurate 1 × 1 × 20 k-point samplings, to determine Peierls stress of the a0/2[111] screw dislocation of molybdenum. We have determined the value of 1.8 GPa for the simple shear stress along (-110)<111>. (author)
Importance of Shear in the bcc-to-hcp Transformation in Iron
Caspersen, Kyle J.; Lew, Adrian; Ortiz, Michael; Carter, Emily A.
2004-09-01
Iron shows a pressure-induced martensitic phase transformation from the ground state ferromagnetic bcc phase to a nonmagnetic hcp phase at ≈13 GPa. The exact transformation pressure (TP) and pathway are not known. Here we present a multiscale model containing a quantum-mechanics-based multiwell energy function accounting for the bcc and hcp phases of Fe and a construction of kinematically compatible and equilibrated mixed phases. This model suggests that shear stresses have a significant influence on the bcc↔hcp transformation. In particular, the presence of modest shear accounts for the scatter in measured TPs. The formation of mixed phases also provides an explanation for the observed hysteresis in TP.
Multi-scale modeling of the iron bcc arrow hcp martensitic phase transformation
Caspersen, Kyle; Carter, Emily; Lew, Adrian; Ortiz, Michael
2004-03-01
Pressures exceeding 10 GPa induce a martensitic phase transformation in iron, where ferro-magnetic bcc transforms into non-magnetic hcp. The transition pressure is not known precisely, but is thought to depend strongly on shear. To investigate the properties of this transformation and the role of shear, we have developed a multi-scale iron model. This model contains a free energy derived from an ab-initio based non-linear elastic expansion, a kinematically compatible spinodal decomposition of phases, ab-initio based interfacial energies, and a dependence on the bcc rightarrow hcp transformation path(s). The model shows spinodal decomposition behavior (with a slight expected deviation) as well as predicting 10 GPa to be the transformation pressure. Additionally, the model predicted that the inclusion of shear facilitates the transformation, causing transformation pressure to decrease.
Tang, M; Kubin, L P
2001-05-01
In order to study the dislocation density evolution of body centered cubic (bcc) crystals at low temperature by dislocation dynamics (DD) simulations, we investigated carefully three different boundary conditions (BC) for DD, i.e., the quasi-free surface BC, the flux-balanced BC, and the periodic BC. The latter two BCs can account for the dislocation loss from the boundary of the finite simulation box. PBC can also eliminate the influence of surfaces and improve the line connectivity. We have found that the PBC provides a convenient and effective boundary condition for DD simulations and have applied it to the study of dislocation density evolution of bcc metals during stage 0 deformation at low temperature.
Computational study of atomic mobility for bcc phase in Ti-Al-Fe system
Chen, Yi; Li, Jinshan; Tang, Bin; Kou, Hongchao; Segurado Escudero, Javier; Cui, Yuwen
2014-01-01
Experimental diffusion data were critically assessed to develop the atomic mobility for the bcc phase of the Ti–Al–Fe system by using the DICTRA software. Good agreements were obtained from comprehensive comparisons made between the calculated and the experimental diffusion coefficients. The developed atomic mobility was then validated by well predicting the interdiffusion behavior observed from the diffusion-couple experiments in available literature.
Diffusion of hydrogen within idealised grains of bcc-Fe: A kinetic Monte Carlo study
Du, Yaojun A.; Rogal, Jutta; Drautz, Ralf
2012-01-01
Structural defects in materials such as vacancies, grain boundaries, and dislocations may trap hydrogen and a local accumulation of hydrogen at these defects can lead to the degradation of the materials properties. An important aspect in obtaining insight into hydrogen induced embrittlement on the atomistic level is to understand the diffusion of hydrogen in these materials. In our study we employ kinetic Monte Carlo (kMC) simulations to investigate hydrogen diffusion in bcc iron within diffe...
Critical currents in A-15 structure Nb3Al converted from cold-worked bcc structure
Nb3Al prepared in the ductile bcc phase by quenching, followed by mechanical working, followed by conversion to the A-15 structure is found to carry currents above 109 A/m2 in fields near 20 T. These critical currents are comparable to those found in Nb3Ge and V3Ga, the closest competing materials for use in high fields. Further enhancement of J/sub c/ is possible if thermal treatments are optimized
Stress dependence of the Peierls barrier of 1/2111 screw dislocations in BCC metals
Gröger, Roman; Vitek, V.
2013-01-01
Roč. 61, č. 17 (2013), s. 6362-6371. ISSN 1359-6454 R&D Projects: GA MŠk(CZ) ED1.1.00/02.0068; GA ČR GAP204/10/0255 Institutional support: RVO:68081723 Keywords : Screw dislocation * Peierls barrier * Nudged elastic band * BCC metals * Peierls stress Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 3.940, year: 2013
Which stresses affect the glide of screw dislocations in bcc metals?
Gröger, Roman
2014-01-01
Roč. 94, č. 18 (2014), s. 2021-2030. ISSN 1478-6435 R&D Projects: GA MŠk(CZ) ED1.1.00/02.0068 Grant ostatní: Marie Curie Actions(CZ) 247705 MesoPhysDel Institutional support: RVO:68081723 Keywords : Peierls stress * screw dislocation * bcc metal * non-glide stress * yield criterion Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 1.825, year: 2014
Crack-induced stress, dislocations and acoustic emission by 3-D atomistic simulation in bcc iron
Spielmannová, Alena; Machová, Anna; Hora, Petr
2009-01-01
Roč. 57, č. 14 (2009), s. 4065-4073. ISSN 1359-6454 R&D Projects: GA ČR GA101/09/1630; GA AV ČR KJB200760802; GA ČR(CZ) GA101/07/0789 Institutional research plan: CEZ:AV0Z20760514 Keywords : bcc iron * crack * dislocation emisision Subject RIV: JG - Metallurgy Impact factor: 3.760, year: 2009
Control of bcc and fcc phase formation during mechanical alloying of Ti-Al-Nb
A Ti-Al-Nb alloy was processed by ball milling or mechanical alloying in a high energy shaker mill in an attempt to produce a fine grained BCC alloy. Previous studies of this alloy resulted in the formation of an amorphous phase followed by a 100% FCC alloy (probably a nitride phase). In the present study, ball milling was conducted in two different laboratories with nitride- and oxide-free starting powders in each location. Two types of starting powders were used: pre-alloyed powders and mixed elemental powders of the same composition. The production of a 90% BCC/10% FCC alloy was accomplished indicating that the production of 100% BCC alloy may be possible. The methods used to prevent the formation of nitrides and oxides of these very reactive constituents during mechanical alloying are discussed and x-ray diffraction results of the mechanically alloyed powders milled by various techniques are presented. The most important factor leading to amorphization and FCC phase formation appears to be contamination associated with periodic sampling of the alloy during ball milling even when dry, inert gas gloveboxes are used for powder transfer
Ab initio study of Cr interactions with point defects in bcc Fe
Full text of publication follows. Ferritic martensitic steels are candidate structural materials for fast neutron reactors, and in particular high-Cr reduced-activation steels. In Fe-Cr alloys, Cr plays a major role in the radiation-induced evolution of the mechanical properties. Using ab initio calculations based on density functional theory, the properties of Cr in α-Fe have been investigated. The intrinsic point defect formation energies were found to be larger in model bcc Cr as compared to those in ferromagnetic bcc Fe. The interactions of Cr with point defects (vacancy and self interstitials) have been characterised. Single Cr atoms interact weakly with vacancies but significantly with self-interstitial atoms. Mixed interstitials of any interstitial symmetry are bound. Configurations where two Cr atoms are in nearest neighbour position are generally unfavourable in bcc Fe except when they are a part of a interstitial complex. Mixed interstitials do not have as strong directional stability as pure Fe interstitials have. The effects on the results using the atom description scheme of either the ultrasoft pseudo-potential (USPP) or the projector augmented wave (PAW) formalisms are connected to the differences in local magnetic moments that the two methods predict. As expected for the Fe-Cr system, the results obtained using the PAW method are more reliable than the ones obtained with USPP. (authors)
Hydrogen storage in TiCr1.2(FeV)x BCC solid solutions
The Ti-V-based BCC solid solutions have been considered attractive candidates for hydrogen storage due to their relatively large hydrogen absorbing capacities near room temperature. In spite of this, improvements of some issues should be achieved to allow the technological applications of these alloys. Higher reversible hydrogen storage capacity, decreasing the hysteresis of PCI curves, and decrease in the cost of the raw materials are needed. In the case of vanadium-rich BCC solid solutions, which usually have large hydrogen storage capacities, the search for raw materials with lower cost is mandatory since pure vanadium is quite expensive. Recently, the substitutions of vanadium in these alloys have been tried and some interesting results were achieved by replacing vanadium by commercial ferrovanadium (FeV) alloy. In the present work, this approach was also adopted and TiCr1.2(FeV)x alloy series was investigated. The XRD patterns showed the co-existence of a BCC solid solution and a C14 Laves phase in these alloys. SEM analysis showed the alloys consisted of dendritic microstructure and C14 colonies. The amount of C14 phase increases when the amount of (FeV) decreases in these alloys. Concerning the hydrogen storage, the best results were obtained for the TiCr1.2(FeV)0.4 alloy, which achieved 2.79 mass% of hydrogen storage capacity and 1.36 mass% of reversible hydrogen storage capacity
Using ab initio calculations in designing bcc Mg-Li alloys for ultra-lightweight applications
Ab initio calculations are becoming increasingly useful to engineers interested in designing new alloys, because these calculations are able to accurately predict basic material properties only knowing the atomic composition of the material. In this paper, single crystal elastic constants of 11 bcc Mg-Li alloys are calculated using density functional theory (DFT) and compared with available experimental data. Based on DFT determined properties, engineering parameters such as the ratio of bulk modulus over shear modulus (B/G) and the ratio of Young's modulus over mass density (Y/ρ) are calculated. Analysis of B/G and Y/ρ shows that bcc Mg-Li alloys with 30-50 at.% Li offer the most potential as lightweight structural material. Compared with fcc Al-Li alloys, bcc Mg-Li alloys have a lower B/G ratio, but a comparable Y/ρ ratio. An Ashby map containing Y/ρ vs B/G shows that it is not possible to increase both Y/ρ and B/G by changing only the composition of a binary alloy
无
2003-01-01
A plastic deformation model for bcc metals is proposed in consideration of reaction stresses. The shear strains and the corresponding reaction stresses induced by the activation of dislocations are calculated in the model, which will influence the following dislocation activation. The rolling texture in bcc metals is simulated up to 80% reduction, while the ratio of critical resolved shear stresses between the dislocations slipping on the {110} and {112} planes is chosen as 0.95. The corresponding calculation is also conducted with the activation of second dislocation, if the difference between the orientation factor of the two dislocations with maximal orientation factors is lower than 5%. It is shown that the simulated texture is closer to that of the 80% rolled interstitial free steels than other modeling. It is believed that the new model can give more attention to both of the strain and stress continuities during the plastic deformation of polycrystalline metals, and therefore approaches closer to the real deformation process in bcc metals.
Diffusion behavior of Cr diluted in bcc and fcc Fe: Classical and quantum simulation methods
We characterize the atomic mobility behavior driven by vacancies, in bcc and fcc Fe−Cr diluted alloys, using a multi-frequency model. We calculate the full set of the Onsager coefficients and the tracer self and solute diffusion coefficients in terms of the mean jump frequencies. The involved jump frequencies are calculated using a classical molecular static (CMS) technique. For the bcc case, we also perform quantum calculations based on the density functional theory (DFT). There, we show that, in accordance with Bohr's correspondence principle, as the size of the atomic cell (total number of atoms) is increased, quantum results with DFT recover the classical ones obtained with CMS calculations. This last ones, are in perfect agreement with available experimental data for both, solute and solvent diffusion coefficients. For high temperatures, in the fcc phase where no experimental data are yet available, our CMS calculations predict the expected solute and solvent diffusion coefficients. - Graphical abstract: Display Omitted - Highlights: • Comparison of diffusion coefficients obtained from classical and quantum methods. • We perform our calculations in diluted bcc/fcc Fe–Cr alloy. • Magnetic and phonon effects must be taken into account. • Classical calculations are in perfect agreement with experimental data
Diffusion behavior of Cr diluted in bcc and fcc Fe: Classical and quantum simulation methods
Ramunni, Viviana P., E-mail: vpram@cnea.gov.ar [CONICET, Avda. Rivadavia 1917, Cdad. de Buenos Aires C.P. 1033 (Argentina); Comisión Nacional de Energía Atómica, Gerencia Materiales, Av. Del Libertador 8250, C1429BNP Ciudad de Buenos Aires (Argentina); Rivas, Alejandro M.F. [CONICET, Avda. Rivadavia 1917, Cdad. de Buenos Aires C.P. 1033 (Argentina); Comisión Nacional de Energía Atómica, Departamento de Física Teórica, Tandar, Av. Del Libertador 8250, C1429BNP Ciudad de Buenos Aires (Argentina)
2015-07-15
We characterize the atomic mobility behavior driven by vacancies, in bcc and fcc Fe−Cr diluted alloys, using a multi-frequency model. We calculate the full set of the Onsager coefficients and the tracer self and solute diffusion coefficients in terms of the mean jump frequencies. The involved jump frequencies are calculated using a classical molecular static (CMS) technique. For the bcc case, we also perform quantum calculations based on the density functional theory (DFT). There, we show that, in accordance with Bohr's correspondence principle, as the size of the atomic cell (total number of atoms) is increased, quantum results with DFT recover the classical ones obtained with CMS calculations. This last ones, are in perfect agreement with available experimental data for both, solute and solvent diffusion coefficients. For high temperatures, in the fcc phase where no experimental data are yet available, our CMS calculations predict the expected solute and solvent diffusion coefficients. - Graphical abstract: Display Omitted - Highlights: • Comparison of diffusion coefficients obtained from classical and quantum methods. • We perform our calculations in diluted bcc/fcc Fe–Cr alloy. • Magnetic and phonon effects must be taken into account. • Classical calculations are in perfect agreement with experimental data.
Reactor lattice transport calculations
The present lecture is a continuation of the lecture on Introduction to the Neutron Transport Phenomena. It comprises three aspects of lattice calculations. First the idea of a reactor lattice is introduced. Then the main definitions used in reactor lattice analysis are given, and finally two basic methods applied for solution of the transport equations are defined. Several remarks on secondary results from lattice transport calculations are added. (author)
Sober Topological Molecular Lattices
张德学; 李永明
2003-01-01
A topological molecular lattice (TML) is a pair (L, T), where L is a completely distributive lattice and r is a subframe of L. There is an obvious forgetful functor from the category TML of TML's to the category Loc of locales. In this note,it is showed that this forgetful functor has a right adjoint. Then, by this adjunction,a special kind of topological molecular lattices called sober topological molecular lattices is introduced and investigated.
Querying Relational Concept Lattices
Azmeh, Zeina; Huchard, Marianne; Napoli, Amedeo; Rouane Hacene, Amine Mohamed; Valtchev, Petko
2011-01-01
Relational Concept Analysis (RCA) constructs conceptual abstractions from objects described by both own properties and inter-object links, while dealing with several sorts of objects. RCA produces lattices for each category of objects and those lattices are connected via relational attributes that are abstractions of the initial links. Navigating such interrelated lattice family in order to find concepts of interest is not a trivial task due to the potentially large size of the lattices and t...
Marichal, Jean-Luc
2007-01-01
We define the concept of weighted lattice polynomial functions as lattice polynomial functions constructed from both variables and parameters. We provide equivalent forms of these functions in an arbitrary bounded distributive lattice. We also show that these functions include the class of discrete Sugeno integrals and that they are characterized by a median based decomposition formula.
Zakrzewski, W J
2004-01-01
We consider some lattices and look at discrete Laplacians on these lattices. In particular we look at solutions of the equation $\\triangle(1)\\phi = \\triangle(2)Z$ where $\\triangle(1)$ and $\\triangle(2)$ are two such laplacians on the same lattice. We discuss solutions of this equation in some special cases.
Effect of lattice relaxations on the hyperfine fields of heavy impurities in Fe
Korhonen, T. E-mail: tko@fyslab.hut.fi; Settels, A.; Papanikolaou, N.; Zeller, R.; Dederichs, P.H
2001-05-01
We present first-principles calculations of hyperfine fields of heavy impurities in BCC Fe. In particular, the effect of lattice relaxations on the calculated hyperfine fields are studied. The calculations are based on a full-potential Korringa-Kohn-Rostoker Green's function method for defects and employ the local spin-density approximation for the exchange and correlation effects. The non-spherical parts of the potential and the charge density are treated correctly, while the forces are calculated by an ionic version of the Hellmann-Feynman theorem. Lattice statics methods are used to describe the longer ranged relaxations. The calculated hyperfine fields of 5sp and 6sp elements are compared with the available experimental data and it is shown that the inclusion of lattice relaxations in the calculation improves the agreement with experiments.
Calculation of thermodynamic equilibrium between bcc disordered solid solutions U and Mo
There is actually an interest to develop a new fuel with higher density for research reactors. Fuel plates would be obtained by dispersion, a method that requires both a very dense fuel dispersant (>15.0 g U/cm3 ) and a very high volume loading of the dispersant (>55%). Dispersants based in gamma (BCC) stabilized uranium alloys are being investigated, as they are able to reach uranium densities of 17.0 g U/cm3. Among them, we focus in U(Mo) bcc solid solutions with the addition of ternary elements to stabilize gamma phase. Transition metals, 4d and 5d, of groups VII and VIII are good candidates for the ternary alloy U - Mo - X. Their relative power to stabilize gamma phase seems to be in close relation with bonding energies between atoms in the alloy. A first approach to the calculation of these energies has been performed by the semi empiric method of Miedema where only bonds between pairs are considered, neglecting ternary and quaternary bonds. There is also a lack of information concerning solubilities of the ternary elements in the ternary cubic phase. In this work we aim to calculate bonding energies between atoms in the alloy using a cluster expansion of the formation energy (T=0 K) of a series of bcc ordered compounds in the systems U-Mo-X. Then the calculation of the equilibrium phase diagram by the Cluster Variation Method will be done (CVM). We show here the first part of the investigation devoted to calculation of phases equilibria in the U Mo system Formation energies of the ordered compounds were obtained by the first principles methods TB-LMTO-ASA and FP-LAPW. Another set of bonding energies was calculated in order to fit the known experimental diagram and new formation energies for the ordered compounds were derived from them. Discrepancies between both sets are discussed. (author)
Simulation of He embrittlement at grain boundaries in bcc transition metals
Suzudo, Tomoaki; Yamaguchi, Masatake
2015-10-01
To investigate what atomic properties largely determine vulnerability to He embrittlement at grain boundaries (GB) of bcc metals, we introduce a computational model composed of first principles density functional theory and a He segregation rate theory model. Predictive calculations of He embrittlement at the first wall of the future DEMO fusion concept reactor indicate that variation in the He embrittlement originated not only from He production rate related to neutron irradiation, but also from the He segregation energy at the GB that has a systematic trend in the periodic table.
Phase stability of ternary fcc and bcc Fe-Cr-Ni alloys
Wrobel, Jan S.; Nguyen-Manh, Duc; Lavrentiev, Mikhail Yu.; Muzyk, Marek; Dudarev, Sergei L.
2014-01-01
The phase stability of fcc and bcc magnetic binary Fe-Cr, Fe-Ni, Cr-Ni alloys and ternary Fe-Cr-Ni alloys is investigated using a combination of density functional theory (DFT), Cluster Expansion (CE) and Magnetic Cluster Expansion (MCE). Energies, magnetic moments, and volumes of more than 500 alloy structures are evaluated using DFT, and the most stable magnetic configurations are compared with experimental data. Deviations from the Vegard law in fcc Fe-Cr-Ni alloys, associated with non-lin...
Superconductivity and the b.c.c. to A-15 transformation in Nb-Au alloys
Nb-Au alloys near the composition Nb3Au can be quenched from the high temperature α solid solution phase field, retaining the b.c.c. structure. Results are reported on their superconducting, mechanical and physical properties as a function of composition. Short low temperature anneals transform these materials to the equilibrium A-15 structure with Tsub(c) up to 11 K. Results are reported on the superconducting and physical properties of the transformed A-15 structure materials and the way in which the properties of the transformed materials are affected by annealing temperature and time is described. (author)
Study of BCC and FCC crystal atomic structure under instant plastic deformation
Evolution of atomic structure of BCC and FCC crystals under conditions of pulsed external loads and great plastic deformations on the basis of computerized experiments is studied. Deformation of crystals was carried out stage-by-stage up to 32%. The results of computerized experiments showed that by instantaneous external loads plastic deformation, depending on its stage, may proceed either on the account of partial dislocations motion, or on account of twinning , or by means of atomic planes turn and shift. Regularities of the system potential energy turn angle of atomic planes change in dependence on the value of the crystal plastic deformation are determined
A numerical study of crack initiation in a bcc iron system based on dynamic bifurcation theory
Crack initiation under dynamic loading conditions is studied under the framework of dynamic bifurcation theory. An atomistic model for BCC iron is considered to explicitly take into account the detailed molecular interactions. To understand the strain-rate dependence of the crack initiation process, we first obtain the bifurcation diagram from a computational procedure using continuation methods. The stability transition associated with a crack initiation, as well as the connection to the bifurcation diagram, is studied by comparing direct numerical results to the dynamic bifurcation theory [R. Haberman, SIAM J. Appl. Math. 37, 69–106 (1979)].
A numerical study of crack initiation in a bcc iron system based on dynamic bifurcation theory
Li, Xiantao
2014-10-01
Crack initiation under dynamic loading conditions is studied under the framework of dynamic bifurcation theory. An atomistic model for BCC iron is considered to explicitly take into account the detailed molecular interactions. To understand the strain-rate dependence of the crack initiation process, we first obtain the bifurcation diagram from a computational procedure using continuation methods. The stability transition associated with a crack initiation, as well as the connection to the bifurcation diagram, is studied by comparing direct numerical results to the dynamic bifurcation theory [R. Haberman, SIAM J. Appl. Math. 37, 69-106 (1979)].
A numerical study of crack initiation in a bcc iron system based on dynamic bifurcation theory
Li, Xiantao, E-mail: xli@math.psu.edu [Pennsylvania State University, University Park, Pennsylvania 16802 (United States)
2014-10-28
Crack initiation under dynamic loading conditions is studied under the framework of dynamic bifurcation theory. An atomistic model for BCC iron is considered to explicitly take into account the detailed molecular interactions. To understand the strain-rate dependence of the crack initiation process, we first obtain the bifurcation diagram from a computational procedure using continuation methods. The stability transition associated with a crack initiation, as well as the connection to the bifurcation diagram, is studied by comparing direct numerical results to the dynamic bifurcation theory [R. Haberman, SIAM J. Appl. Math. 37, 69–106 (1979)].
Radhakrishnan, Balasubramaniam [ORNL; Gorti, Sarma B [ORNL
2008-01-01
Thermo-mechanical processing to produce optimum grain structure and texture is key to the successful utilization of commercial aluminum alloys and steels as sheet products. Several modeling techniques have been developed in the past with a reasonably good predictive capability for bulk deformation textures. However, prediction of texture evolution during recrystallization remains very challenging because of uncertainties involved in predicting the mechanisms that lead to nuclei formation and crystallographic orientations of the nuclei, and the uncertainties involved in predicting the grain boundary properties that determine the growth kinetics of the nuclei. We present some of our recent work in modeling the recrystallization textures following hot deformation in polycrystalline BCC and FCC metals.
Quantum-well states and induced magnetism in Fe/CuN/Fe bcc (001) trilayers
Niklasson, A.M.N.; Mirbt, S.; Skriver, Hans Lomholt;
1996-01-01
profiles of two single Fe/Cu interfaces. The small deviations from this simple superposition are shown to be a consequence of quantum-well states confined within the paramagnetic spacer. This connection is confirmed by direct calculation of the state density. The results are of conceptual interest for the......We have used a first-principles Green's function technique to investigate the formation of magnetic moments in Fe/Cu-N/Fe bcc (001) trilayers. We show that the magnetic moment in the paramagnetic spacer material to a first approximation may be described as a linear superposition of the magnetic...
Investigating the geographical distribution of skin cancer (BCC type) in Ardabil province via GIS
Alireza Mohebbipour; Saeid Alipour; Saeid Sadeghiyeh Ahari; Firouz Amani; Esmaeil Farzaneh
2015-01-01
Background: Skin cancer is the most common type of cancer in most countries and it holds the first or the second place in terms of frequency in different areas of the country. BCC is the most usual type of tumor in the white skinned people, and its incidence rate rises as individuals get older, especially after age 40.Thistype of skin cancer mostly occurs in the white skinned people and, in85% of cases; it develops on the head and neck. This study aimed at examining the geographical distribut...
Zuo, Jinqing; Ren, Hong-Li; Wu, Jie; Nie, Yu; Li, Qiaoping
2016-09-01
The subseasonal variability and predictability of the Arctic Oscillation/North Atlantic Oscillation (AO/NAO) is evaluated using a full set of hindcasts generated from the Beijing Climate Center Atmospheric General Circulation Model version 2.2 (BCC_AGCM2.2). It is shown that the predictability of the monthly mean AO/NAO index varies seasonally, with the highest predictability during winter (December-March) and the lowest during autumn (August-November), with respect to both observations and BCC_AGCM2.2 results. As compared with the persistence prediction skill of observations, the model skillfully predicts the monthly mean AO/NAO index with a one-pentad lead time during all winter months, and with a lead time of up to two pentads in December and January. During winter, BCC_AGCM2.2 exhibits an acceptable skill in predicting the daily AO/NAO index of ∼9 days, which is higher than the persistence prediction skill of observations of ∼4 days. Further analysis suggests that improvements in the simulation of storm track activity, synoptic eddy feedback, and troposphere-stratosphere coupling in the Northern Hemisphere could help to improve the prediction skill of subseasonal AO/NAO variability by BCC_AGCM2.2 during winter. In particular, BCC_AGCM2.2 underestimates storm track activity intensity but overestimates troposphere-stratosphere coupling, as compared with observations, thus providing a clue to further improvements in model performance.
Drawing annealed cylindric 18/8 Cr Ni steels, which are originally free of textures, produces the transformed phases - hcp and bcc - both showing major texture contributions with increasing stretching of the cylindric specimens. After stretching the original fcc-phase shows two orientations: [100]fcc vertical stroke vertical stroke cylinder axis and [111]fcc vertical stroke vertical stroke cylinder axis, i.e. direction of stress. In both cases the martensitic phase is produced by gliding and shear in the sequence fcc → hcp → bcc by Nishiyama-Wasserman (N-W) or Kurdjumov-Sachs (K-S) transformation in the (111)fcc planes, which enclose a small angle with direction of stress, i.e. cylinder axis. The calculated orientation distributions of the (110)bcc reflex are compared with the distribution measured by neutron diffraction to get information on the bulk material. The special K-S transformation with only 6 (110)bcc orientations shows relatively good agreement with the measured distribution, except at small angles ω between the cylinder axis and the scattering vector. This might be caused by the isotropic fraction of the fcc phase producing an anisotropic (110)bcc orientation distribution. (orig.)
Effects of shrinkage/expansion of crystal lattice on the diffusivity and the solubility of hydrogen isotopes in bcc-metals were investigated. In order to obtain systematic and detailed information, an energy was decomposed into two parts: potential-energy-of-hydrogen and elastic-energy. Both of the migration barrier and the solution energy of hydrogen were increased by lattice shrinkage and were decreased by lattice expansion, basically. Change of migration barrier was mainly subject to that of elastic-energy. However, when the crystal lattice was strongly shrunk, e.g. 5% shrinkage, the migration barrier was decreased due to an abrupt decrease of potential-energy-of-hydrogen, which resulted from increasing interaction between the hydrogen atom and the second-nearest neighbor metal atoms. (author)
In the last few years lattice gauge theory has become the primary tool for the study of nonperturbative phenomena in gauge theories. The lattice serves as an ultraviolet cutoff, rendering the theory well defined and amenable to numerical and analytical work. Of course, as with any cutoff, at the end of a calculation one must consider the limit of vanishing lattice spacing in order to draw conclusions on the physical continuum limit theory. The lattice has the advantage over other regulators that it is not tied to the Feynman expansion. This opens the possibility of other approximation schemes than conventional perturbation theory. Thus Wilson used a high temperature expansion to demonstrate confinement in the strong coupling limit. Monte Carlo simulations have dominated the research in lattice gauge theory for the last four years, giving first principle calculations of nonperturbative parameters characterizing the continuum limit. Some of the recent results with lattice calculations are reviewed
Lattice theory for nonspecialists
These lectures were delivered as part of the academic training programme at the NIKHEF-H. These lectures were intended primarily for experimentalists, and theorists not specializing in lattice methods. The goal was to present the essential spirit behind the lattice approach and consequently the author has concentrated mostly on issues of principle rather than on presenting a large amount of detail. In particular, the author emphasizes the deep theoretical infra-structure that has made lattice studies meaningful. At the same time, he has avoided the use of heavy formalisms as they tend to obscure the basic issues for people trying to approach this subject for the first time. The essential ideas are illustrated with elementary soluble examples not involving complicated mathematics. The following subjects are discussed: three ways of solving the harmonic oscillator problem; latticization; gauge fields on a lattice; QCD observables; how to solve lattice theories. (Auth.)
Lattice Dynamical Interpretation of the Structure of \\zeta-Phase AgZn
Yamada, Yasusada; Noda, Yukio
1988-04-01
The crystal structure of \\zeta-phase AgZn has been reconsidered. It is shown that the static structure of \\zeta-AgZn is expressible in terms of (i) two types of lattice waves (phonon modes) and (ii) one type of probability density wave of Zn atom site-occupation. It is noted that the relevant phonon modes are the soft modes existing commonly in bcc Hume-Rothery alloys. It is pointed out that \\zeta-AgZn shares the common origin of stabilization with 9R martensites and ω-phase.
Spight, Marshall
2008-01-01
Relational lattice is a formal mathematical model for Relational algebra. It reduces the set of six classic relational algebra operators to two: natural join and inner union. We continue to investigate Relational lattice properties with emphasis onto axiomatic definition. New results include additional axioms, equational definition for set difference (more generally anti-join), and case study demonstrating application of the relational lattice theory for query transformations.
kunz, Milan
2006-01-01
Ferrers graphs and tables of partitions are treated as vectors. Matrix operations are used for simple proofs of identities concerning partitions. Interpreting partitions as vectors gives a possibility to generalize partitions on negative numbers. Partitions are then tabulated into lattices and some properties of these lattices are studied. There appears a new identity counting Ferrers graphs packed consecutively into isoscele form. The lattices form the base for tabulating combinatorial ident...
Predicting atomistic properties of a dislocation is a first step toward an understanding of plastic behavior of materials, in particular BCC metals. The core structure and Peierls stress of a screw dislocation in BCC iron have been determined using the first-principles calculations based on the density functional theory with the large-scale supercell containing 231 atoms. For the a0/2[111] screw dislocation of BCC iron, the core structure was found, using 1 × 1 × 4 k-point samplings, to be a symmetric displacement field with no broken symmetry, and the Peierls stress was determined to be 1.1 GPa for the simple shear stress along (-110)<111>. (author)
Lattice degeneracies of fermions
We present a detailed description of the minimal degeneracies of geometric (Kaehler) fermions on all the lattices of maximal symmetries in n = 1, ..., 4 dimensions. We also determine the isolated orbits of the maximal symmetry groups, which are related to the minimal numbers of ''naive'' fermions on the reciprocals of these lattices. It turns out that on the self-reciprocal lattices the minimal numbers of naive fermions are equal to the minimal numbers of degrees of freedom of geometric fermions. The description we give relies on the close connection of the maximal lattice symmetry groups with (affine) Weyl groups of root systems of (semi-) simple Lie algebras. (orig.)
Shindler, A. [Deutsches Elektronen-Synchrotron (DESY), Zeuthen (Germany). John von Neumann-Inst. fuer Computing NIC
2007-07-15
I review the theoretical foundations, properties as well as the simulation results obtained so far of a variant of the Wilson lattice QCD formulation: Wilson twisted mass lattice QCD. Emphasis is put on the discretization errors and on the effects of these discretization errors on the phase structure for Wilson-like fermions in the chiral limit. The possibility to use in lattice simulations different lattice actions for sea and valence quarks to ease the renormalization patterns of phenomenologically relevant local operators, is also discussed. (orig.)
Relative stability of bcc structures in ternary alloys with Ti50Al25Mo25 composition
In reviewing the literature that concerns the CsCl-type intermetallic compound Ti2AlMo (B2), the question arises if this compound is preferably formed with respect to the high-temperature bcc completely disordered structure (A2). Our present work takes an ab initio approach to this subject. The first-principles linear-muffin-tin-orbital method was used to calculate the total energy of a series of bcc structures in the ternary system Ti-Al-Mo. Second, a cluster expansion in the tetrahedron approximation was calculated for each formation energy. Finally, a Connolly-Williams-like inversion scheme was used to obtain the formation energy of both the A2 phase and the partially ordered structure B2. From the calculations, the B2 formation energy came out to be approx. 10 mRy/atom lower than the A2 energy, suggesting a relative greater stability. The relative stability of other possible structures with the same global composition Ti50Al25Mo25 is discussed
Co thin film with metastable bcc structure formed on GaAs(111 substrate
Minakawa Shigeyuki
2014-07-01
Full Text Available Co thin films are prepared on GaAs(111 substrates at temperatures ranging from room temperature to 600 ºC by radio-frequency magnetron sputtering. The growth behavior and the detailed resulting film structure are investigated by in-situ reflection high-energy electron diffraction and X-ray diffraction. In early stages of film growth at temperatures lower than 200 ºC, Co crystals with metastable A2 (bcc structure are formed, where the crystal structure is stabilized through hetero-epitaxial growth. With increasing the film thickness beyond 2 nm, the metastable structure starts to transform into more stable A1 (fcc structure through atomic displacements parallel to the A2{110} close-packed planes. The crystallographic orientation relationship between the A2 and the transformed A1 crystals is A1{111} || A2{110}. When the substrate temperature is higher than 400 ºC, Ga atoms of substrate diffuse into the Co films and a Co-Ga alloy with bcc-based ordered structure of B2 is formed.
Microscopic Origin of Heisenberg and Non-Heisenberg Exchange Interactions in Ferromagnetic bcc Fe.
Kvashnin, Y O; Cardias, R; Szilva, A; Di Marco, I; Katsnelson, M I; Lichtenstein, A I; Nordström, L; Klautau, A B; Eriksson, O
2016-05-27
By means of first principles calculations, we investigate the nature of exchange coupling in ferromagnetic bcc Fe on a microscopic level. Analyzing the basic electronic structure reveals a drastic difference between the 3d orbitals of E_{g} and T_{2g} symmetries. The latter ones define the shape of the Fermi surface, while the former ones form weakly interacting impurity levels. We demonstrate that, as a result of this, in Fe the T_{2g} orbitals participate in exchange interactions, which are only weakly dependent on the configuration of the spin moments and thus can be classified as Heisenberg-like. These couplings are shown to be driven by Fermi surface nesting. In contrast, for the E_{g} states, the Heisenberg picture breaks down since the corresponding contribution to the exchange interactions is shown to strongly depend on the reference state they are extracted from. Our analysis of the nearest-neighbor coupling indicates that the interactions among E_{g} states are mainly proportional to the corresponding hopping integral and thus can be attributed to be of double-exchange origin. By making a comparison to other magnetic transition metals, we put the results of bcc Fe into context and argue that iron has a unique behavior when it comes to magnetic exchange interactions. PMID:27284671
Change in the thermophysical properties of BCC iron during isothermal compression
Magomedov, M. N.
2015-11-01
Equation of state P( V/ V 0, T) and baric dependences of the thermodynamic properties of bcc iron are obtained using the Mie-Lennard-Jones interatomic pair potential and the Einstein model of a crystal without any adjustable parameters. The calculations performed along two isotherms at 300 and 1500 K from P = 0 to 8000 kbar = 800 GPa (i.e., to V/ V 0 = 0.5) show good agreement with the experimental data. Baric graphical dependences are obtained for the following properties: isothermal bulk modulus B T and B'( P), isochoric specific heat C v and C v ' ( P), isobaric specific heat C p , thermal expansion coefficient α p and α p ' ( P), and specific surface energy of (100) face σ and σ'( P). Analytical approximations are obtained for baric dependences B'( P), α p ( P), and σ'( P). It is shown that, at P → ∞, functions B T ( P) and σ( P) for bcc iron change linearly and function α p ' ( P) tends toward zero.
Influence of orientation on the size effect in bcc pillars with different critical temperatures
Research highlights: → Crystallographic orientation has no effect on the stress-strain behavior of bcc micro-and nanopillars. → Size dependence of bcc pillars correlates with the material specific critical temperature. → Dependence on critical temperature shows importance of screw dislocation mobility. → Contribution of screw dislocations is verified by the loading rate dependence of the yield stress and calculated activation volumes. - Abstract: The size effect in body-centered cubic metals is comprehensively investigated through micro/nano-compression tests performed on focused ion beam machined tungsten (W), molybdenum (Mo) and niobium (Nb) pillars, with single slip [2 3 5] and multiple slip [0 0 1] orientations. The results demonstrate that the stress-strain response is unaffected by the number of activated slip systems, indicating that dislocation-dislocation interaction is not a dominant mechanism for the observed diameter dependent yield strength and strain hardening. Furthermore, the limited mobility of screw dislocations, which is different for each material at ambient temperature, acts as an additional strengthening mechanism leading to a material dependent size effect. Nominal values and diameter dependence of the flow stress significantly deviate from studies on face-centered cubic metals. This is demonstrated by the correlation of size dependence with the material specific critical temperature. Activation volumes were found to decrease with decreasing pillar diameter further indicating that the influence of the screw dislocations decreases with smaller pillar diameter.
Molecular dynamics simulation of dislocation-void interactions in BCC Mo
Lee, Hyon-Jee [Department of Nuclear Engineering, University of California, Berkeley, CA (United States)], E-mail: hyon-jee@nuc.Berkeley.EDU; Wirth, Brian D. [Department of Nuclear Engineering, University of California, Berkeley, CA (United States)
2009-04-30
Molecular dynamics (MD) and molecular statics (MS) simulations have been performed to simulate the motion of a screw dislocation and its interaction with voids in irradiated body centered cubic (BCC) Mo. Considering the unique non-planar core structures of the screw dislocation in BCC metals, the behavior of screw dislocation motion as a function of temperature and applied shear stress is first discussed. A transition from smooth to rough motion of the screw dislocation is observed with increasing shear stress, as well as a change of dislocation glide plane from {l_brace}1 1 0{r_brace} to {l_brace}1 1 2{r_brace} with increasing temperature. The interaction of a screw dislocation with nanometer-sized voids observed in both dynamic and static conditions is then reported. The obstacle strength calculated from MS calculations shows a large increase in critical resolved shear stress for void diameter larger than about 3 nm. However, the MD results indicate that the screw dislocation interaction with void occurs via a simple shear mechanism.
Molecular dynamics simulation of dislocation-void interactions in BCC Mo
Lee, H.J. [California Univ., Berkeley, Dept. NE/ERL, CA (United States); Osetksy, Y.N. [Oak Ridge National Laboratory, Computer Sciences and Mathematics Div., TN, AK (United States); Wirth, B. [California Univ., Berkeley, Nuclear Engineering Dept., UC, AK CA (United States)
2007-07-01
Full text of publication follows: The interaction between moving dislocations and nano-meter-sized radiation defect clusters controls the yield strength, ductility and flow localization behavior of structural materials under irradiation. To understand plastic deformation processes in body centered cubic (BCC) Mo, molecular Dynamics (MD) and molecular statics (MS) simulations have been performed. Considering the unique non-planar core structures of BCC screw dislocation, we first report the behavior of screw dislocation motion as a function of temperature and applied shear stress. A transition from smooth to rough motion of the screw dislocation is observed with increasing shear stress. The double kink nucleation and migration occurs in the smooth motion regime while interstitial clusters, prismatic dislocation loops, and vacancies are produced in the rough motion regime. As well, the screw dislocation glide plane is observed to change from {l_brace}110{r_brace} to {l_brace}112{r_brace} with increasing temperature. Then, we introduce the commonly observed radiation obstacles into the system and observe their interaction behavior with screw dislocations in both dynamic and static conditions. The initial results indicate that the screw dislocation interaction with void occurs via a simple shear mechanism, which shears the void by one Burgers vector per each passage. However, the obstacle strength calculated from molecular statics (MS) calculations of the energetics of the interaction shows a large increase in critical resolved shear stress for void diameter larger than about 3 nm. (authors)
Liu, J. B.; Johnson, D. D.
2009-04-01
Using density-functional theory, we calculate the potential-energy surface (PES), minimum-energy pathway (MEP), and transition state (TS) versus hydrostatic pressure σhyd for the reconstructive transformation in Fe from body-centered cubic (bcc) to hexagonal closed-packed (hcp). At fixed σhyd , the PES is described by coupled shear (γ) and shuffle (η) modes and is determined from structurally minimized hcp-bcc energy differences at a set of (η,γ) . We fit the PES using symmetry-adapted polynomials, permitting the MEP to be found analytically. The MEP is continuous and fully explains the transformation and its associated magnetization and volume discontinuity at TS. We show that σhyd (while not able to induce shear) dramatically alters the MEP to drive reconstruction by a shuffle-only mode at ≤30GPa , as observed. Finally, we relate our polynomial-based results to Landau and nudge-elastic-band approaches and show they yield incorrect MEP in general.
Equation of state of bcc-Mo by static volume compression to 410 GPa
Unit cell volumes of Mo and Pt have been measured simultaneously to ≈400 GPa by x-ray powder diffraction using a diamond anvil cell and synchrotron radiation source. The body-centered cubic (bcc) phase of Mo was found to be stable up to 410 GPa. The equation of state (EOS) of bcc-Mo was determined on the basis of Pt pressure scale. A fit of Vinet EOS to the volume compression data gave K0 = 262.3(4.6) GPa, K0′ = 4.55(16) with one atmosphere atomic volume V0 = 31.155(24) A3. The EOS was in good agreement with the previous ultrasonic data within pressure difference of 2.5%–3.3% in the multimegabar range, though the EOS of Mo proposed from a shock compression experiment gave lower pressure by 7.2%–11.3% than the present EOS. The agreement would suggest that the Pt pressure scale provides an accurate pressure value in an ultra-high pressure range
Trap effect of vacancy on hydrogen diffusivity in bcc-Fe
Zhu, Deqiong; Oda, Takuji
2016-02-01
Evaluation of tritium effective diffusivity in structural materials with defects is essential to appropriately estimate the accumulation and leakage of tritium in fusion reactors. In the present work, we focus on the trap effect of vacancy on the diffusivity of hydrogen in bcc-Fe. The hydrogen effective diffusivities in different bcc-Fe systems with varied hydrogen/vacancy ratios (H/V ratios) are evaluated by using molecular statics (MS) and molecular dynamics (MD) simulations, and are compared with that in the system without vacancy. The hydrogen effective diffusivity is obviously decreased due to the vacancy trap effect. The trap effect is more profound in small H/V-ratio systems due to the dependence of V-H binding energy on the number of trapped H atoms. The trapped H atoms hardly contribute to hydrogen diffusivity because of the difficulty in detrapping and small mobility of V-H complex. This fact results in a good correlation between the effective diffusivity of hydrogen and the fraction of H atoms solute in the bulk without being trapped by vacancy.
Counting Hexagonal Lattice Animals
Mohammed, Mohamud
2002-01-01
We describe Maple packages for the automatic generation of generating functions(and series expansions) for counting lattice animals(fixed polyominoes), in the two-dimensional hexagonal lattice, of bounded but arbitrary width. Our Maple packages(complete with source code) are easy-to-use and available from my website.
Epelbaum E.
2010-04-01
Full Text Available We review recent progress on nuclear lattice simulations using chiral eﬀective ﬁeld theory. We discuss lattice results for dilute neutron matter at next-to-leading order, three-body forces at next-to-next-toleading order, isospin-breaking and Coulomb eﬀects, and the binding energy of light nuclei.
Active Optical Lattice Filters
Gary Evans; MacFarlane, Duncan L.; Govind Kannan; Jian Tong; Issa Panahi; Vishnupriya Govindan; L. Roberts Hunt
2005-01-01
Optical lattice filter structures including gains are introduced and analyzed. The photonic realization of the active, adaptive lattice filter is described. The algorithms which map between gains space and filter coefficients space are presented and studied. The sensitivities of filter parameters with respect to gains are derived and calculated. An example which is relevant to adaptive signal processing is also provided.
Flat Band Quastiperiodic Lattices
Bodyfelt, Joshua; Flach, Sergej; Danieli, Carlo
2014-03-01
Translationally invariant lattices with flat bands (FB) in their band structure possess irreducible compact localized flat band states, which can be understood through local rotation to a Fano structure. We present extension of these quasi-1D FB structures under incommensurate lattices, reporting on the FB effects to the Metal-Insulator Transition.
Prathumpai Wai
2010-07-01
Full Text Available Abstract Background Biopolymers have various applications in medicine, food and petroleum industries. The ascomycetous fungus Ophiocordyceps dipterigena BCC 2073 produces an exobiopolymer, a (1→3-β-D-glucan, in low quantity under screening conditions. Optimization of O. dipterigena BCC 2073 exobiopolymer production using experimental designs, a scale-up in 5 liter bioreactor, analysis of molecular weight at different cultivation times, and levels of induction of interleukin-8 synthesis are described in this study. Results In order to improve and certify the productivity of this strain, a sequential approach of 4 steps was followed. The first step was the qualitative selection of the most appropriate carbon and nitrogen sources (general factorial design and the second step was quantitative optimization of 5 physiological factors (fractional factorial design. The best carbon and nitrogen source was glucose and malt extract respectively. From an initial production of 2.53 g·L-1, over 13 g·L-1 could be obtained in flasks under the improved conditions (5-fold increase. The third step was cultivation in a 5 L bioreactor, which produced a specific growth rate, biomass yield, exobiopolymer yield and exobiopolymer production rate of 0.014 h-1, 0.32 g·g-1 glucose, 2.95 g·g biomass-1 (1.31 g·g-1 sugar, and 0.65 g.(L·d-1, respectively. A maximum yield of 41.2 g·L-1 was obtained after 377 h, a dramatic improvement in comparison to the initial production. In the last step, the basic characteristics of the biopolymer were determined. The molecular weight of the polymer was in the range of 6.3 × 105 - 7.7 × 105 Da. The exobiopolymer, at 50 and 100. μg·mL-1, induced synthesis in normal dermal human fibroblasts of 2227 and 3363 pg·mL-1 interleukin-8 respectively. Conclusions High exobiopolymer yield produced by O. dipterigena BCC 2073 after optimization by qualitative and quantitative methods is attractive for various applications. It induced high
Dremov, V. V.; Ionov, G. V.; Sapozhnikov, F. A.; Smirnov, N. A.; Karavaev, A. V.; Vorobyova, M. A.; Ryzhkov, M. V.
2015-09-01
The present work is devoted to classical molecular dynamics investigation into microscopic mechanisms of the bcc-hcp transition in iron. The interatomic potential of EAM type used in the calculations was tested for the capability to reproduce ab initio data on energy evolution along the bcc-hcp transformation path (Burgers deformation + shuffe) and then used in the large-scale MD simulations. The large-scale simulations included constant volume deformation along the Burgers path to study the origin and nature of the plasticity, hydrostatic volume compression of defect free samples above the bcc to hcp transition threshold to observe the formation of new phase embryos, and the volume compression of samples containing screw dislocations to study the effect of the dislocations on the probability of the new phase critical embryo formation. The volume compression demonstrated high level of metastability. The transition starts at pressure much higher than the equilibrium one. Dislocations strongly affect the probability of the critical embryo formation and significantly reduce the onset pressure of transition. The dislocations affect also the resulting structure of the samples upon the transition. The formation of layered structure is typical for the samples containing the dislocations. The results of the simulations were compared with the in-situ experimental data on the mechanism of the bcc-hcp transition in iron.
Research on irradiation of bcc metals and alloys is reported. Data and information are presented in appendixes on low temperature neutron irradiation of Nb, effects of tritium on the yield stress of Nb, multiple dislocation motion, dislocation group motion, dislocation kinetics, and computer simulation of dislocation motion
Courant, E.D.; Garren, A.A.
1985-10-01
A realistic, distributed interaction region (IR) lattice has been designed that includes new components discussed in the June 1985 lattice workshop. Unlike the test lattices, the lattice presented here includes utility straights and the mechanism for crossing the beams in the experimental straights. Moreover, both the phase trombones and the dispersion suppressors contain the same bending as the normal cells. Vertically separated beams and 6 Tesla, 1-in-1 magnets are assumed. Since the cells are 200 meters long, and have 60 degree phase advance, this lattice has been named RLD1, in analogy with the corresponding test lattice, TLD1. The quadrupole gradient is 136 tesla/meter in the cells, and has similar values in other quadrupoles except in those in the IR`s, where the maximum gradient is 245 tesla/meter. RLD1 has distributed IR`s; however, clustered realistic lattices can easily be assembled from the same components, as was recently done in a version that utilizes the same type of experimental and utility straights as those of RLD1.
C and N atoms are the most frequent foreign interstitial atoms (FIAs), and often incorporated into the surface layers of steels to enhance their properties by thermochemical treatments. Al, Si, Ti, V, Cr, Mn, Co, Ni, Cu, Nb and Mo are the most common alloying elements in steels, also can be called foreign substitutional atoms (FSAs). The FIA and FSA interactions play an important role in the diffusion of C and N atoms, and the microstructures and mechanical properties of surface modified layers. Ab initio calculations based on the density functional theory are carried out to investigate FIA interactions with FSA in ferromagnetic bcc iron. The FIA–FSA interactions are analyzed systematically from five aspects, including interaction energies, density of states (DOS), bond populations, electron density difference maps and local magnetic moments
Solute/impurity diffusivities in bcc Fe: A first-principles study
Chinese low activation martensitic steel (CLAM) has been designed with decreased W content and increased Ta content to improve performance. We performed first-principles calculations to investigate the diffusion properties of solute element (Cr, W, Mn, V, Ta) and C diffusion with a nearby solute element inside bcc Fe. The self-diffusion coefficients and solute diffusion coefficients in Fe host were derived using the nine-frequency model. A relatively lower diffusivity was observed for W in paramagnetic state, implying enriched W concentration inside Fe host. The solute atom interacts strongly with C impurity, depending on the interatomic distance. According to our calculations, formation of Ta carbide precipitates is energetically preferred by trapping C impurity around Ta atom. Our theoretical results are helpful for investigating the evolution of microstructure of steels for engineering applications
Kefes, Christoph; Heindl, Emanuel; Vancea, Johann; Back, Christian [Department of Physics, University of Regensburg, 93040 Regensburg (Germany)
2009-07-01
We use the tip of a scanning tunneling microscope to create a nonequilibrium unipolar electron distribution in a metal layer and measure the subsequent perpendicular ballistic hot electron transport through thin single crystalline metallic spin valves by employing ballistic electron emission microscopy (BEEM). By variation of the thickness of one of the ferromagnetic layers we can determine the spin dependent attenuation lengths which reflect the bulk hot electron transport along the[100]-axis of the bcc FeCo-layers. While the minority spin attenuation length is found to be energy independent and about 0.8 nm, the majority spin attenuation length is about 6 times larger within the measured energy interval of 1.3 up to 2 eV above the Fermi level. Consequently, a magnetocurrent effect exceeding 1200 % accompanied by a monotonic bias voltage behavior is observed at room temperature.
Atomistic studies of nucleation of He clusters and bubbles in bcc iron
Yang, L.; Deng, H. Q.; Gao, F.; Heinisch, H. L.; Kurtz, R. J.; Hu, S. Y.; Li, Y. L.; Zu, X. T.
2013-05-01
Atomistic simulations of the nucleation of He clusters and bubbles in bcc iron at 800 K have been carried out using the newly developed Fe-Fe interatomic potential, along with Ackland potential for the Fe-Fe interactions. Microstructure changes were analyzed in detail. We found that a He cluster with four He atoms is able to push out an iron interstitial from the cluster, creating a Frenkel pair. Small He clusters and self-interstitial atom (SIA) can migrate in the matrix, but He-vacancy (He-V) clusters are immobile. Most SIAs form clusters, and only the dislocation loops with a Burgers vector of b = 1/2 appear in the simulations. SIA clusters (or loops) are attached to He-V clusters for He implantation up to 1372 appm, while the He-V cluster-loop complexes with more than one He-V cluster are formed at the He concentration of 2057 appm and larger.
Synergetic effects of Mn and Si in the interaction with point defects in bcc Fe
Bakaev, A., E-mail: abakaev@sckcen.be [SCK-CEN, Nuclear Materials Science Institute, Boeretang 200, Mol B2400 (Belgium); Center for Molecular Modeling, Department of Physics and Astronomy, Ghent University, Technologiepark 903, 9052 Zwijnaarde (Belgium); Department of Experimental Nuclear Physics, Institute of Physics, Nanotechnologies and Telecommunications, St. Petersburg State Polytechnical University, 29 Polytekhnicheskaya Str., 195251 St. Petersburg (Russian Federation); Terentyev, D. [SCK-CEN, Nuclear Materials Science Institute, Boeretang 200, Mol B2400 (Belgium); He, X. [China Institute of Atomic Energy, PO Box 275-51, 102413 Beijing (China); Van Neck, D. [Center for Molecular Modeling, Department of Physics and Astronomy, Ghent University, Technologiepark 903, 9052 Zwijnaarde (Belgium)
2014-12-15
The interaction of Mn, Si and Cr with a vacancy and self-interstitial defects in BCC Fe has been analyzed using ab initio calculations. While the interaction of the considered solute clusters with a single vacancy is linearly additive, there is a considerable synergetic effect in the case of self-interstitial atoms, found to bind strongly with Mn–Si pairs. The latter therefore act as deep trapping configurations for self-interstitials. At the same time, the presence of the point defects nearby weakly attractive Mn–Si pairs significantly enhances the solute–solute binding. The revealed effects are rationalized on the basis of charge density and local magnetic moment distributions.
Magnetic properties and atomic ordering of BCC Heusler alloy Fe2MnGa ribbons
Xin, Yuepeng; Ma, Yuexing; Luo, Hongzhi; Meng, Fanbin; Liu, Heyan
2016-05-01
The electronic structure, atomic disorder and magnetic properties of the Heusler alloy Fe2MnGa have been investigated experimentally and theoretically. BCC Fe2MnGa ribbon samples were prepared. Experimentally, a saturation magnetic moment (3.68 μB at 5 K) much larger than the theoretical value (2.04 μB) has been reported. First-principles calculations indicate that the difference is related to the Fe-Mn disorder between A, B sites, as can also be deduced from the XRD pattern. L21 type Fe2MnGa is a ferrimagnet with antiparallel Fe and Mn spin moments. However, when Fe-Mn disorder occurs, part of Mn moments will be parallel to Fe moments, and the Fe moments also clearly increase simultaneously. All this results in a total moment of 3.74 μB, close to the experimental value.
The effect of hydrogen on the electronic structure of kink in bcc iron
无
2009-01-01
The electronic structures of kinks in the [100](010) and 1/2[111](-110) edge dislocations in bcc iron containing hydrogen are investigated by means of the first-principles DMol method and the discrete variational method.The effects of hydrogen on the kinks are discussed.The results show that hydro-gen forms weak bonding states with its neighboring host atoms,and since hydrogen draws charge from its neighboring host atoms,the interactions between most of the host atoms are weakened com-pared with those of the corresponding atomic pairs in the clean kinks.This indicates that the migration of kink,i.e.the motion of dislocation,is easier in the doping hydrogen kink than in the clean kink,which may be the solid solution softening effect resulting from the impurity hydrogen.
Computer simulation of vacancy and interstitial clusters in bcc and fcc metals
Interstitial clusters in bcc-Fe and fcc-Cu and vacancy clusters in fcc-Cu have been studied by computer simulation using different types of interatomic potentials such as a short-ranged empirical pair potential of Johnson type, short-ranged many-body potentials of Finnis-Sinclair type and long-ranged pair potentials obtained within the generalized pseudopotential theory. The stability of a self interstitial in bcc-Fe was found to be dependent on the range of potential but not on the type. Thus, both short-ranged potentials simulated left angle 110 right angle dumb-bell as a stable configuration while in the case of the long-ranged potential the stable configuration is the left angle 111 right angle crowdion. Nevertheless the structure and properties of interstitial clusters were found to be qualitatively the same with all the potentials. Up to 50 interstitials, the most stable clusters were found as perfect dislocation loops with Burgers vector vectorb=1/2 left angle 111 right angle. The stability of interstitial clusters in Cu also does not depend on the potential and for the same sizes the most stable configurations are faulted Frank loops 1/3 left angle 111 right angle {111} and edge loops in the {110} plane. The structure and stability of vacancy clusters in fcc-Cu were found to be dependent mainly on both the range of potential and equilibrium conditions. Thus for long-ranged non-equilibrium pair potentials vacancy clusters in the {111} plane collapsed and formed vacancy loops or stacking fault tetrahedra depending on the shape of the initial vacancy platelet. For the short-ranged equilibrium many-body potential vacancy clusters do not collapse into loops or tetrahedra. The process of vacancy clustering in the cascade region has been studied by molecular dynamics. (orig.)
Vacancy–solute interactions in ferromagnetic and paramagnetic bcc iron: Ab initio calculations
Highlights: ► Interactions of vacancies with 3p, 3d, and 4d impurities in bcc Fe are calculated ab initio. ► Ferromagnetic and disordered paramagnetic states of the Fe matrix are considered. ► Three groups of impurities whose interactions with vacancies strongly depend on magnetic order. ► Compound-formers (Al, Si, P, S), insoluble elements (Cu, Ag), and magnetic elements (Mn, Co, Ni). ► The results are applied to interpret anomalies of vacancy trapping and impurity diffusion. - Abstract: Vacancy–solute interactions play a crucial role in diffusion-controlled processes, such as ordering or decomposition, which occur in alloys under heat treatment or under irradiation. Detailed knowledge of these interactions is important for predicting long-term behavior of nuclear materials (such as reactor steels and nuclear-waste containers) as well as for advancing our general understanding of kinetic processes in alloys. Using first-principles calculations based on the density functional theory and employing the locally self-consistent Green’s function technique, we develop a database of vacancy–solute interactions in dilute alloys of bcc Fe with 3p (Al, Si, P, S), 3d (Sc–Cu), and 4d (Y–Ag) elements. Unrelaxed interactions within the first three coordination shells have been computed in the ferromagnetic state as well as in the paramagnetic (disordered local moment) state of the iron matrix. Magnetism is found to have a strong effect on the vacancy–solute interactions. Implications of the obtained results for interpreting the effects of vacancy trapping and enhanced impurity diffusion are discussed.
Lattice gauge theory is now ten years old. Apart from the theoretical insight the lattice formulation gives, it is very well suited for computer simulations, as its inventor advocated already some five years ago at this school. Since three years this approach has extracted useful information out of lattice gauge theory and spurred many interesting questions. In the first lecture, I will assume there are no experts in the audience and explain some basic facts in quarkless quantumchromodynamics on a lattice (QCD). Then, in the second lecture, we shall review tests for the consistency of the numerical results so far obtained. The third lecture shall deal with a more esoteric subject: that of large N reduced models. The list of references is by no means meant to be exhaustive; for that the reader is referred to ref. 27
Lattice supersymmetric ward identities
SUSY Ward identities for the N=1 SU(2) SUSY Yang-Mills theory are studied on the lattice in a non-perturbative numerical approach. As a result a determination of the subtracted gluino mass is obtained
The architecture and capabilities of the computers currently in use for large-scale lattice QCD calculations are described and compared. Based on this present experience, possible future directions are discussed
The void ordering under irradiation in simple cubic, bcc, fcc and hcp-crystals is considered within the framework of the dislocation model of void lattice formation based upon the absorption of perfect interstitial loops by voids. The ordering criterion is derived taking into account not only perfect loopd but Frank sessile loops and straight dislocations as well. Analytical dependence of void lattice parameters on the concentration of the loop nucleation sites is derived. Impurities are shown to stimulate or prevent void ordering depending on their influence on the loop nature. Finally, a mechanism of loop-punching from submicroscopic overpressurized gas bubbles is considered as a possible source of perfect loops which could induce the swelling saturation and void ordering in fcc metals with low stacking fault energy. 15 refs.; 3 figs
Vector Lattice Vortex Solitons
WANG Jian-Dong; YE Fang-Wei; DONG Liang-Wei; LI Yong-Ping
2005-01-01
@@ Two-dimensional vector vortex solitons in harmonic optical lattices are investigated. The stability properties of such solitons are closely connected to the lattice depth Vo. For small Vo, vector vortex solitons with the total zero-angular momentum are more stable than those with the total nonzero-angular momentum, while for large Vo, this case is inversed. If Vo is large enough, both the types of such solitons are stable.
Full text: We sketch the general concepts of the lattice regularisation in quantum field theory, which enables Monte Carlo simulations and non-perturbative numerical measurements of observables in particle physics. We then address the status of lattice QCD with 2+1 flavours of dynamical quarks, where hadron masses can now be evaluated from the first principles of QCD close to the percent level. (author)
Bietenholz, Wolfgang [Universidad Nacional Autonoma de Mexico (UNAM) (Mexico)
2011-07-01
Full text: We sketch the general concepts of the lattice regularisation in quantum field theory, which enables Monte Carlo simulations and non-perturbative numerical measurements of observables in particle physics. We then address the status of lattice QCD with 2+1 flavours of dynamical quarks, where hadron masses can now be evaluated from the first principles of QCD close to the percent level. (author)
Automated Lattice Perturbation Theory
Monahan, Christopher
2014-11-01
I review recent developments in automated lattice perturbation theory. Starting with an overview of lattice perturbation theory, I focus on the three automation packages currently "on the market": HiPPy/HPsrc, Pastor and PhySyCAl. I highlight some recent applications of these methods, particularly in B physics. In the final section I briefly discuss the related, but distinct, approach of numerical stochastic perturbation theory.
Lattice Operators and Topologies
Eva Cogan
2009-01-01
Working within a complete (not necessarily atomic) Boolean algebra, we use a sublattice to define a topology on that algebra. Our operators generalize complement on a lattice which in turn abstracts the set theoretic operator. Less restricted than those of Banaschewski and Samuel, the operators exhibit some surprising behaviors. We consider properties of such lattices and their interrelations. Many of these properties are abstractions and generalizations of topological spaces. The approach is...
Bergner, Georg
2016-01-01
We discuss the motivations, difficulties and progress in the study of supersymmetric lattice gauge theories focusing in particular on ${\\cal N}=1$ and ${\\cal N}=4$ super Yang-Mills in four dimensions. Brief reviews of the corresponding lattice formalisms are given and current results are presented and discussed. We conclude with a summary of the main aspects of current work and prospects for the future.
Lattice supersymmetry and string phenomenology
Giedt, Joel
2003-01-01
I discuss the usefulness of lattice supersymmetry in relation to string phenomenology. I suggest how lattice results might be incorporated into string phenomenology. I outline difficulties and describe some constructions that contain an exact lattice version of supersymmetry, thereby reducing fine-tuning of the regulator. I mention some problems that occur for these lattices.
Fine lattice stochastic modeling of particle fuels in HTGR fuel elements
There is growing interest worldwide in high temperature gas-cooled reactors (HTGRs) as candidates for next generation reactor systems. Either in a pebble type or in a prismatic type HTGR, coated particle fuel (TRISO fuel) appears to be the most promising fuel candidate to be used. For design and analysis of such a reactor, transport models, in particular, stochastic models that permit the simulation of neutron transport through the stochastic mixture of fuel and moderator materials, are becoming essential and gaining importance. Naturally, the Monte Carlo methods have been used for this situation. However, the methods reported in the literature all have their own deficiencies. In this thesis, we propose a new Monte Carlo method named fine lattice stochastic (FLS) modeling that is distinct from others. This method is based on fine lattice system in which a lattice circumscribes a fuel particle. Once the problem is given, an interface Fortran code gives out the TRISO particle fuel configurations (a set of lattice center points only) for MCNP input. The number of available lattice center points is far larger than the number of fuel particles according to packing fraction of the fuel element. We apply discrete random sampling here to choose a certain number of lattices to fill with fuel particles. In this aspect, FLS modeling allows more realistic fuel particle distributions. In this thesis, only simple cube (SC) structure is used in cubic lattice. However, FLS model can be easily extended to BCC, FCC structures or hexagonal prism type lattice. The criticality calculations for our FLS modeling were first tested on a small cube problem and compared with other models. The results indicate that the new stochastic model is an accurate and efficient approach to analyze TRISO particle fuel configurations. Then the FLS modeling was performed to analyze HTGR fuel elements for both pebble type and prismatic type and the results were also good as expected
Ion irradiation effects on high purity bcc Fe and model FeCr alloys
FeCr binary alloys are a simple representative of the reduced activation ferritic/martensitic (F-M) steels, which are currently the most promising candidates as structural materials for the sodium cooled fast reactors (SFR) and future fusion systems. However, the impact of Cr on the evolution of the irradiated microstructure in these materials is not well understood in these materials. Moreover, particularly for fusion applications, the radiation damage scenario is expected to be complicated further by the presence of large quantities of He produced by the nuclear transmutation (∼ 10 appm He/dpa). Within this context, an elaborate ion irradiation study was performed at 500 C on a wide variety of high purity FeCr alloys (with Cr content ranging from ∼ 3 wt.% to 14 wt.%) and a bcc Fe, to probe in detail the influence of Cr and He on the evolution of microstructure. The irradiations were performed using Fe self-ions, in single beam mode and in dual beam mode (damage by Fe ions and co-implantation of He), to separate ballistic damage effect from the impact of simultaneous He injection. Three different dose ranges were studied: high dose (157 dpa, 17 appm He/dpa for the dual beam case), intermediate dose (45 dpa, 57 appm He/dpa for dual beam case) and in-situ low dose (0.33 dpa, 3030 appm He/dpa for the dual beam case). The experiments were performed at the JANNuS triple beam facility and dual beam in situ irradiation facility at CEA-Saclay and CSNSM, Orsay respectively. The microstructure was principally characterized by conventional TEM, APT and EDS in STEM mode. The main results are as follows: 1) A comparison of the cavity microstructure in high dose irradiated Fe revealed strong swelling reduction by the addition of He. It was achieved by a drastic reduction in cavity sizes and an increased number density. This behaviour was observed all along the damage depth, up to the damage peak. 2) Cavity microstructure was also studied in the dual beam high dose
Digital lattice gauge theories
Zohar, Erez; Reznik, Benni; Cirac, J Ignacio
2016-01-01
We propose a general scheme for a digital construction of lattice gauge theories with dynamical fermions. In this method, the four-body interactions arising in models with $2+1$ dimensions and higher, are obtained stroboscopically, through a sequence of two-body interactions with ancillary degrees of freedom. This yields stronger interactions than the ones obtained through pertubative methods, as typically done in previous proposals, and removes an important bottleneck in the road towards experimental realizations. The scheme applies to generic gauge theories with Lie or finite symmetry groups, both Abelian and non-Abelian. As a concrete example, we present the construction of a digital quantum simulator for a $\\mathbb{Z}_{3}$ lattice gauge theory with dynamical fermionic matter in $2+1$ dimensions, using ultracold atoms in optical lattices, involving three atomic species, representing the matter, gauge and auxiliary degrees of freedom, that are separated in three different layers. By moving the ancilla atoms...
Graphene antidot lattice waveguides
Pedersen, Jesper Goor; Gunst, Tue; Markussen, Troels;
2012-01-01
We introduce graphene antidot lattice waveguides: nanostructured graphene where a region of pristine graphene is sandwiched between regions of graphene antidot lattices. The band gaps in the surrounding antidot lattices enable localized states to emerge in the central waveguide region. We model...... the waveguides via a position-dependent mass term in the Dirac approximation of graphene and arrive at analytical results for the dispersion relation and spinor eigenstates of the localized waveguide modes. To include atomistic details we also use a tight-binding model, which is in excellent agreement...... with the analytical results. The waveguides resemble graphene nanoribbons, but without the particular properties of ribbons that emerge due to the details of the edge. We show that electrons can be guided through kinks without additional resistance and that transport through the waveguides is robust against...
Electronic structure of nanosized bcc Cu precipitates in Fe-Cu alloys studied by positron 2D-ACAR
Based on the finding with the use of the coincidence Doppler measurements that the nanosized Cu precipitates are coherently embedded in the Fe-Cu matrix alloy, taking a bcc structure and acting as efficient positron traps, we measured 2D-ACARs of carefully grown and heat treated single crystals of Fe-Cu. We found that the precipitates have a Fermi surface with 12 necks touching the {110} Bragg planes of the bcc Brillouin zone, which contrasts distinctly with that of the bulk Cu with 8 necks touching the {111} Bragg planes of the fcc Brillouin zone. The 3 dimensional momentum reconstruction of the 2D-ACAR data showed that the Fermi surface cutoff of the Cu precipitates is blurred considerably as compared with that of bulk fcc Cu, suggesting a marked size effect in the momentum distribution. (orig.)
Interactions of solute (3p, 4p, 5p and 6p) with solute, vacancy and divacancy in bcc Fe
You, Yu-Wei; Kong, Xiang-Shan; Wu, Xue-Bang; Liu, Wei [Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, P.O. Box 1129, Hefei 230031 (China); Liu, C.S., E-mail: csliu@issp.ac.cn [Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, P.O. Box 1129, Hefei 230031 (China); Fang, Q.F. [Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, P.O. Box 1129, Hefei 230031 (China); Chen, J.L.; Luo, G.-N. [Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031 (China); Wang, Zhiguang [Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000 (China)
2014-12-15
Solute–vacancy binding energy is a key quantity in understanding solute diffusion kinetics and phase segregation, and may help choice of alloy compositions for future material design. However, the binding energy of solute with vacancy is notoriously difficult to measure and largely unknown in bcc Fe. With first-principles method, we systemically calculate the binding energies of solute (3p, 4p, 5p and 6p alloying solutes are included) with vacancy, divacancy and solute in bcc Fe. The binding energy of Si with vacancy in the present work is in good consistent with experimental value available. All the solutes considered are able to form stable solute–vacancy, solute–divacancy complexes, and the binding strength of solute–divacancy is about two times larger than that of solute–vacancy. Most solutes could not form stable solute–solute complexes except S, Se, In and Tl. The factors controlling the binding energies are analyzed at last.
Effect of hydrogen atoms on the mobility of a screw dislocation in BCC iron has been evaluated using the first-principles calculation. The stable position of a hydrogen atom is found to be near the screw dislocation core and inside the core respectively when the dislocation is at the easy-core or hard-core configuration in BCC iron. The intrinsically unstable hard-core configuration of the screw dislocation is stabilized when a hydrogen atom is trapped inside the core. On the basis of this first-principles result, an elastic string model of a dislocation is developed to predict the kink motion in the presence of a hydrogen atom. It is found that a double-kink formation is facilitated when a hydrogen atom is located near a dislocation line, however, a kink motion is retarded when a hydrogen atom is behind the kink. (author)
The interaction of an interstitial hydrogen atom with a screw dislocation in BCC iron has been determined using the first principles calculation. The calculation was carried out for a pair of screw dislocations using the large-scale supercell containing 231 atoms and 1 x 1 x 4 k-point samplings. It is found that a hydrogen atom at an octahedral site near the screw dislocation attracts the dislocation core under the applied shear stress condition. However, the resultant stress-strain relation in the presence of a hydrogen atom is found to be almost the same with that of no hydrogen case. This indicates clearly that a significant enhancement of dislocation mobility, previously reported in Al, is not observed in BCC iron. (author)
Interactions of solute (3p, 4p, 5p and 6p) with solute, vacancy and divacancy in bcc Fe
Solute–vacancy binding energy is a key quantity in understanding solute diffusion kinetics and phase segregation, and may help choice of alloy compositions for future material design. However, the binding energy of solute with vacancy is notoriously difficult to measure and largely unknown in bcc Fe. With first-principles method, we systemically calculate the binding energies of solute (3p, 4p, 5p and 6p alloying solutes are included) with vacancy, divacancy and solute in bcc Fe. The binding energy of Si with vacancy in the present work is in good consistent with experimental value available. All the solutes considered are able to form stable solute–vacancy, solute–divacancy complexes, and the binding strength of solute–divacancy is about two times larger than that of solute–vacancy. Most solutes could not form stable solute–solute complexes except S, Se, In and Tl. The factors controlling the binding energies are analyzed at last
Observation of quasi-fast diffusion process in sup 3 He- sup 4 He solid solutions near BCC-HCP
Mikhin, N P; Rudavskij, E Y
2001-01-01
By means of pulsed NMR one investigated into diffusion processes i sup 3 He dilute solid solution in sup 4 He at the BCC-HCP phase equilibrium line and in a melting-point curve. The applied techniques of the spin echo enabled to separate contributions made by all co-existing phases. It is determined that alongside with the contributions relevant to the equilibrium phases a secondary diffusion process characterized by anomalously high value of the diffusion coefficient manifests itself. It is shown to be close to the value of diffusion coefficient for liquid helium while diffusion is a spatially restricted one. One assumes that the observed effect may be associated with occurrence of liquid drops in the BCC-HCP transition process
Gérald Franz
2013-11-01
Full Text Available An advanced elastic-plastic self-consistent polycrystalline model, accounting for intragranular microstructure development and evolution, is coupled with a bifurcation-based localization criterion and applied to the numerical investigation of the impact of microstructural patterns on ductility of single-phase steels. The proposed multiscale model, taking into account essential microstructural aspects, such as initial and induced textures, dislocation densities, and softening mechanisms, allows us to emphasize the relationship between intragranular microstructure of B.C.C. steels and their ductility. A qualitative study in terms of forming limit diagrams for various dislocation networks, during monotonic loading tests, is conducted in order to analyze the impact of intragranular substructure parameters on the formability of single-phase B.C.C. steels.
A systematic study on the interfacial energy of O-line interfaces in fcc/bcc systems
Habit planes between face-centered cubic (fcc)/body-centered cubic (bcc) phases usually exhibit irrational orientations, which often agree with the O-line criterion. Previously, energy calculation was made to test whether the habit planes were energetically favorable, but the values of the energy were found very sensitive to the initial atomic configuration in an irrationally orientated interface. In this paper, under the O-line condition, simple selection criteria are proposed to define and remove interfacial interstitials and vacancies in the initial atomic configuration. The criteria are proved to be effective in obtaining robust energy results. Interfacial energies of two types of O-line interfaces in fcc/bcc systems are calculated following the criteria. The observed transformation crystallography of precipitates in Ni–Cr and Cu–Cr systems can be explained consistently as the irrational habit plane in each system is associated with the lowest energy O-line interface. (paper)
Catterall, Simon; Kaplan, David B.; Unsal, Mithat
2009-03-31
We provide an introduction to recent lattice formulations of supersymmetric theories which are invariant under one or more real supersymmetries at nonzero lattice spacing. These include the especially interesting case of N = 4 SYM in four dimensions. We discuss approaches based both on twisted supersymmetry and orbifold-deconstruction techniques and show their equivalence in the case of gauge theories. The presence of an exact supersymmetry reduces and in some cases eliminates the need for fine tuning to achieve a continuum limit invariant under the full supersymmetry of the target theory. We discuss open problems.
Computational study of atomic mobility for the bcc phase of the U-Pu-Zr ternary system
Li Weibang [State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi' an 710072 (China); Hu Rui, E-mail: lijsh2009@gmail.co [State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi' an 710072 (China); Cui, Y.-W. [Department of Materials Science and Engineering, Ohio State University, Columbus, OH 43210 (United States); Madrid Institute for Advanced Studies in Materials, IMDEA Materials, C/Profesor Aranguren s/n, 28040 Madrid (Spain); Zhong Hong; Chang Hui; Li Jinshan; Zhou Lian [State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi' an 710072 (China)
2010-12-31
Experimental diffusion data in literature has been evaluated to assess the atomic mobility for the bcc phase in the U-Pu-Zr system by means of the DICTRA-type (Diffusion Controlled TRAnsformation) phenomenological treatment. The developed mobility database has been validated by comprehensive comparisons made between the experimental and calculated diffusion coefficients, as well as other interesting details resulting from interdiffusion, e.g. the concentration profile and the diffusion path of diffusion couples.
Computational study of atomic mobility for the bcc phase of the U-Pu-Zr ternary system
Li, Weibang; Hu, Rui; Cui, Y.-W.; Zhong, Hong; Chang, Hui; Li, Jinshan; Zhou, Lian
2010-12-01
Experimental diffusion data in literature has been evaluated to assess the atomic mobility for the bcc phase in the U-Pu-Zr system by means of the DICTRA-type (Diffusion Controlled TRAnsformation) phenomenological treatment. The developed mobility database has been validated by comprehensive comparisons made between the experimental and calculated diffusion coefficients, as well as other interesting details resulting from interdiffusion, e.g. the concentration profile and the diffusion path of diffusion couples.
Computational study of atomic mobility for the bcc phase of the U-Pu-Zr ternary system
Experimental diffusion data in literature has been evaluated to assess the atomic mobility for the bcc phase in the U-Pu-Zr system by means of the DICTRA-type (Diffusion Controlled TRAnsformation) phenomenological treatment. The developed mobility database has been validated by comprehensive comparisons made between the experimental and calculated diffusion coefficients, as well as other interesting details resulting from interdiffusion, e.g. the concentration profile and the diffusion path of diffusion couples.
Mattson, J.E.; Fullerton, E.E.; Sowers, C.H.; Bader, S.D.
1994-04-01
We demonstrate epitaxial growth of the bcc transition metals Nb, Mo, Fe, and Cr via sputtering onto single crystal MgO substrates. The epitaxial growth orientations are (011), (112) and (001) when grown onto MgO (111), (011) and (001), respectively. Further we demonstrate that under appropriate growth conditions, superlattices of these materials (e.g., Fe/Cr, Fe/V and Mo/V) can be grown with the same epitaxial order as the films.
Growth of a brittle crack (001) in 3D bcc iron crystal with a Cu nano-particle
Uhnáková, Alena; Machová, Anna; Hora, Petr; Červená, Olga
2014-01-01
Roč. 83, February (2014), s. 229-234. ISSN 0927-0256 R&D Projects: GA ČR GA101/09/1630 Institutional support: RVO:61388998 Keywords : brittle crack extension * 3D * mode I * bcc iron * Cu nano-particle * molecular dynamics * acoustic emission Subject RIV: JG - Metallurgy Impact factor: 2.131, year: 2014 http://www.sciencedirect.com/science/article/pii/S0927025613006575
Yuxian Fan; Xiaoling Yuan; Jie Qin
2013-01-01
Cultural industries are becoming important drivers for global economic growth. Competitiveness of cultural industries lies in its performance. This paper takes deep research on the cultural industries’ performance of 31 regions in China by the methods of factor analysis and super BCC efficiency model, using the whole statement data of 2010 from cultural industries. As the study shows, there are only 7 provinces which are efficient DMUS in DEA, and inefficacy in scale is one of the most import...
Study of loop-loop and loop-edge dislocation interactions in bcc iron
Recent theoretical calculations and atomistic computer simulations have shown that one-dimensional glissile clusters of self-interstitial atoms (SIAs) play an important role in the evolution of microstructure in metals and alloys under cascade damage conditions. Recently, it has been proposed that the evolution of heterogeneities such as dislocation decoration and rafts has serious impacts on the mechanical properties on neutron-irradiated metals. In the present work, atomic-scale computer modelling (ASCM) has been applied to study the mechanisms for the formation of such microstructure in bcc iron. It is shown that glissile clusters with parallel Burgers vectors interact strongly and can form extended immobile complexes, i.e., rafts. Similar attractive interaction exists between dislocation loops and an edge dislocation. These two mechanisms may be responsible for the formation of extended complexes of dislocation loops below the extra half-plane of edge dislocations. The interaction energies between loops and between an edge dislocation and loops has been calculated as a function of distance using ASCM and the results for long-range interactions are in good agreement with the results of isotropic elasticity calculations
Vacancy properties in 5d bcc transition metals: Ab initio study at finite electron temperature
The self-diffusion constants for the monovacancy mechanism in the 5d transition-metals with bcc structure (β-Hf, Ta and W) are investigated by first-principles pseudopotential calculations within the framework of the Local Density Functional Theory. The formation and migration energies, calculated for relaxed configurations using supercells containing 27 and 54 atomic sites, are in quite good agreement with experimental data in Ta and W, with a discrepancy lower than 10%. Preliminary results in β-Hf using smaller supercells suggest very large and relaxation energies. The effects of finite electron-temperature is shown to be quite important, and very different from one element to the other: the electron contribution to the activation entropy is negative in Ta and positive in W, reaching respectively -2 kB and 2 kB at the melting temperature. Using simple estimates for the attempt frequencies and the vibrational formation entropies, the calculated self-diffusion coefficient is in exceptional agreement with experiments in W, and clearly reproduces an accelerated diffusivity in Ta
Elastic constants of bcc shape-memory binary alloys: Effect of the configurational ordering
Castán, T.; Planes, A.
1988-10-01
The relationship between the elastic shear modulus C'=1/2(C11-C12) and the atomic order state in a shape-memory binary alloy AxB1-x above its martensitic transition temperature is analyzed. We first present a simple method to evaluate the elastic constants in binary alloys, assuming the atoms interact via a two-body Morse potential. For CuZn and AgZn alloys, the potential parameters corresponding to the different A-A, B-B, and A-B pairs are determined from experimental data of the elastic constant C' for different alloy compositions. We next calculate C' at 0 K as a function of the ordering state. To do this, we use atomic configurations obtained with a Monte Carlo simulation of the Ising model for a bcc binary alloy, at each temperature Ti. We obtain a linear relationship between C' and the short-range-order parameter η. We also show that the deviations from the linear behavior observed when C' is represented against the square of the long-rang-order parameter come mainly from the critical behavior of the system near the order-disorder temperature Tc.
Spin dynamics studies of single Fe and Ni impurity in bcc iron (abstract)
Sabiryanov, R.; Jaswal, S.S. [Center for Materials Research and Analysis, University of Nebraska, Lincoln, Nebraska 68588-0111 (United States); Antropov, V.P.; Harmon, B.N.; Liu, S. [Ames Laboratory and Physics Department of ISU, Ames, Iowa 50011 (United States)
1997-04-01
Spin dynamics based on a quasiclassical approximation is used to study a single Fe or Ni magnetic impurity in ferromagnetic bcc iron. The optimization of the magnetic structure of the impurity includes quantum effects such as spatial spin inhomogeneity and different orbital contributions. The Pauli equation for noncollinear magnetic configuration is solved for each impurity. When rotated from ferromagnetic ordering, Fe and Ni impurities show different behavior. The Fe moment remains almost unchanged through a rotation of 0.6{pi} radians and drops to zero when the rotation is increased to 0.72{pi} radians. This behavior is primarily due to the interplay of more localized e{sub g} states and itinerant t{sub 2g} states. On the other hand, the Ni moment drops continuously with rotation, going to zero at 0.5{pi} radians. The different degrees of localization of Ni and Fe moments are analyzed in terms of different spatial anisotropy of magnetization density. The inhomogeneous spin space configurations are presented and analyzed using corresponding space dependent exchange integrals. {copyright} {ital 1997 American Institute of Physics.}
A new mechanism of loop formation and transformation in bcc iron without dislocation reaction
Chen, J.; Gao, N.; Jung, P.; Sauvage, T.
2013-10-01
Structure and kinetics of dislocation loops in α-Fe is an active field in material science, due to their implications on fundamental understanding as well as application of structural materials in irradiation environments. Recent computer simulations provoke new conceptions, which call for experimental verification. The present investigation reports transmission electron microscopy of small interstitial dislocation loops (2.5-10 nm diameters) in bcc iron, irradiated with 25 MeV α-particles at 573 K up to 0.13 dpa. The observed and ½ loops have habit planes of (1 0 0), and (1 1 0), (1 1 1) and (2 1 1), respectively. Furthermore it is observed that loops also contain ½{2 1 1} and {1 0 0} components which are considered as intermediate stages of transformation of ½ loops to . Based on these observations, a new mechanism of loop formation and transformation by self-interstitial atoms aggregation is proposed, with concurrent molecular dynamic simulations supporting the kinetic feasibility of the proposed process.
Calculation of the surface energy of bcc-metals with the empirical electron theory
We have used the dangling bond analysis method (DBAM) based on the empirical electron theory (EET) to establish a database of surface energy for low index surfaces of the bcc-metals such as V, Cr, Fe, Nb, Mo, Ta, and W. And a brief introduction of the new surface energy models will be presented in this paper. Under the first-order approximation the calculated results are in agreement with experimental and other theoretical values. And the calculated surface energy shows a strong anisotropy. As we predicted, the surface energy of the close-packed (1 1 0) is the lowest one of all index surfaces. It is also found that the dangling bond electron density and the spatial distribution of covalent bonds have a great influence on surface energy of various index surfaces. The new calculation method for the research of surface energy provides a good basis for models of surface science phenomena, and the model may be extended to the surface energy estimation of more metals, alloys, ceramics, and so on, since abundant information about the valence electronic structure (VES) can be generated from EET.
3D dislocation dynamics: stress-strain behavior and hardening mechanisms in fcc and bcc metals
A dislocation dynamics (DD) model for plastic deformation, connecting the macroscopic mechanical properties to basic physical laws governing dislocation mobility and related interaction mechanisms, has been developed. In this model there is a set of critical reactions that determine the overall results of the simulations, such as the stress-strain curve. These reactions are annihilation, formation of jogs, junctions, and dipoles and cross-slip. In this paper, we discuss these reactions and the manner in which they influence the simulated stress-strain behavior of fcc and bcc metals. In particular, we examine the formation (zipping) and strength of dipoles and junctions, and effect of jogs, using the dislocation dynamics model. We show that the strengths (unzipping) of these reactions for various configurations can be determined by direct evaluation of the elastic interactions. Next, we investigate the phenomenon of hardening in metals subjected to cascade damage. The investigated microstructure consists of small dislocation loops decorating the mobile dislocations. Preliminary results reveal that these loops act as hardening agents, trapping the dislocations and resulting in increased yield stress
Dislocation dynamics modelling of brittle-ductile transitions in BCC metals
Tarleton, E.; Roberts, S.; Novokshanov, R. [Oxford Univ., Dept. of Materials (United Kingdom)
2007-07-01
Full text of publication follows: Bend tests on single crystals of BCC metals (Tungsten, Iron and Iron Chromium alloys) show the brittle ductile transition temperature of a pre-cracked specimen under 4 point bending decreases by around 10 K for each order of magnitude decrease in strain rate. At higher temperatures or lower strain rates large numbers of dislocations are produced which are able to shield the crack from the external loading. This increased plasticity in the region of the crack tip can delay or even prevent brittle fracture meaning the specimen is ductile. These experiments have been modelled using a 2D dislocation dynamics code which simulates the nucleation and motion of dislocations around a loaded crack, and their effect of the crack-tip stress intensity factor. At high simulated temperatures or low simulated loading rates, dislocations can shield the crack tip sufficiently to prevent fracture. The model gives excellent agreement between predicted and experimental value of BDT temperatures and the variation with loading rate. However this good agreement occurs only the slip direction is at 70 degrees to the crack plane, rather than the 45 degree angle imposed by the crystallography of the real test specimens. 3D modelling of a crack is currently being performed to see if cross slip can account for an effective slip plane angle close to 70 degrees. (authors)
Supong, Khomsan; Thawai, Chitti; Choowong, Wilunda; Kittiwongwattana, Chokchai; Thanaboripat, Dusanee; Laosinwattana, Chamroon; Koohakan, Prommart; Parinthawong, Nonglak; Pittayakhajonwut, Pattama
2016-05-01
An endophytic actinomycete strain BCC72023 was isolated from rice (Oryza sativa L.) and identified as the genus Streptomyces, based on phenotypic, chemotaxonomic and 16S rRNA gene sequence analyses. The strain showed 99.80% similarity compared with Streptomyces samsunensis M1463(T). Chemical investigation led to the isolation of three macrolides, efomycins M (1), G (2) and oxohygrolidin (3), along with two polyethers, abierixin (4) and 29-O-methylabierixin (5). To our knowledge, this is the first report of efomycin M being isolated from a natural source. The compounds were identified using spectroscopic techniques and comparison with previously published data. All compounds exhibited antimalarial activity against the Plasmodium falciparum, K-1 strain, a multidrug-resistant strain, with IC50 values in a range of 1.40-5.23 μg/ml. In addition, these compounds were evaluated for biological activity against Mycobacterium tuberculosis, Bacillus cereus, Colletotrichum gloeosporioides and Colletotrichum capsici, as well as cytotoxicity against both cancerous (MCF-7, KB, NCI-H187) and non-cancerous (Vero) cells. PMID:26809052
Atomic-scale simulations of material behaviors and tribology properties for BCC metal film
H, D. Aristizabal; P, A. Parra; P, López; E, Restrepo-Parra
2016-01-01
This work has two main purposes: (i) introducing the basic concepts of molecular dynamics analysis to material scientists and engineers, and (ii) providing a better understanding of instrumented indentation measurements, presenting an example of nanoindentation and scratch test simulations. To reach these purposes, three-dimensional molecular dynamics (MD) simulations of nanoindentation and scratch test technique were carried out for generic thin films that present BCC crystalline structures. Structures were oriented in the plane (100) and placed on FCC diamond substrates. A pair wise potential was employed to simulate the interaction between atoms of each layer and a repulsive radial potential was used to represent a spherical tip indenting the sample. Mechanical properties of this generic material were obtained by varying the indentation depth and dissociation energy. The load-unload curves and coefficient of friction were found for each test; on the other hand, dissociation energy was varied showing a better mechanical response for films that present grater dissociation energy. Structural change evolution was observed presenting vacancies and slips as the depth was varied. Project supported by la DirecciónNacional de Investigación of the Universidad Nacional de Colombia, “the Theoretical Study of Physical Properties of Hard Materials for Technological Applications” (Grant No. 20101007903).
Hydrogen storage in TiCr{sub 1.2}(FeV){sub x} BCC solid solutions
Santos, Sydney F. [Institut de Recherche sur l' hydrogene, Universite du Quebec a Trois-Rivieres 3351, Boul. des Forges, Trois-Rivieres, Quebec G9A 5H7 (Canada)], E-mail: sfsantos91@yahoo.com.br; Huot, Jacques [Institut de Recherche sur l' hydrogene, Universite du Quebec a Trois-Rivieres 3351, Boul. des Forges, Trois-Rivieres, Quebec G9A 5H7 (Canada)
2009-03-20
The Ti-V-based BCC solid solutions have been considered attractive candidates for hydrogen storage due to their relatively large hydrogen absorbing capacities near room temperature. In spite of this, improvements of some issues should be achieved to allow the technological applications of these alloys. Higher reversible hydrogen storage capacity, decreasing the hysteresis of PCI curves, and decrease in the cost of the raw materials are needed. In the case of vanadium-rich BCC solid solutions, which usually have large hydrogen storage capacities, the search for raw materials with lower cost is mandatory since pure vanadium is quite expensive. Recently, the substitutions of vanadium in these alloys have been tried and some interesting results were achieved by replacing vanadium by commercial ferrovanadium (FeV) alloy. In the present work, this approach was also adopted and TiCr{sub 1.2}(FeV){sub x} alloy series was investigated. The XRD patterns showed the co-existence of a BCC solid solution and a C14 Laves phase in these alloys. SEM analysis showed the alloys consisted of dendritic microstructure and C14 colonies. The amount of C14 phase increases when the amount of (FeV) decreases in these alloys. Concerning the hydrogen storage, the best results were obtained for the TiCr{sub 1.2}(FeV){sub 0.4} alloy, which achieved 2.79 mass% of hydrogen storage capacity and 1.36 mass% of reversible hydrogen storage capacity.
The recent experimental conclusion by Jacques and Robrock that self-interstitial atoms (SIA) in molybdenum migrate at low temperatures by two-dimensional diffusion on [011] planes has led to an exploration of the spatial effects that might arise if this property was extended to temperatures where voids are found under irradiation. Qualitative analysis supported by rate theory shows that the introduction of two-dimensional interstitial migration allows concentration variations of SIA from plane to plane which, in turn, can lead to the small natural spatial variations in void density and radius parameters being amplified with positive feedback. The practical result of such an effect would be the planar ordering of voids on [011] planes. Although the analysis examines effects on only one set of the six [011] planes, extension of planar ordering onto all six sets will, it is argued, lead to a bcc void lattice. The circumstantial evidence to support the model seems fairly strong. For example the tendency for planar ordering is most efficient for high void densities and low dislocation densities while the long range ordering effects on the close packed planes, prior to perfect lattice formation, is a natural consequence of the model. Furthermore a ratio of void lattice parameter to void radius of the correct experimental order can be obtained. The paper discusses these points in detail together with an extension of the model to cover bubble lattice formation in molybdenum. Other metals are also considered; while there is no difficulty for bcc metals, application of the model to fcc and hcp lattices is more speculative. (orig.)
Optimality and uniqueness of the Leech lattice among lattices
Cohn, Henry; Kumar, Abhinav
2004-01-01
We prove that the Leech lattice is the unique densest lattice in R^24. The proof combines human reasoning with computer verification of the properties of certain explicit polynomials. We furthermore prove that no sphere packing in R^24 can exceed the Leech lattice's density by a factor of more than 1+1.65*10^(-30), and we give a new proof that E_8 is the unique densest lattice in R^8.
Singh, Kevin; Geiger, Zachary; Senaratne, Ruwan; Rajagopal, Shankari; Fujiwara, Kurt; Weld, David; Weld Group Team
2015-05-01
Quasiperiodicity is intimately involved in quantum phenomena from localization to the quantum Hall effect. Recent experimental investigation of quasiperiodic quantum effects in photonic and electronic systems have revealed intriguing connections to topological phenomena. However, such experiments have been limited by the absence of techniques for creating tunable quasiperiodic structures. We propose a new type of quasiperiodic optical lattice, constructed by intersecting a Gaussian beam with a 2D square lattice at an angle with an irrational tangent. The resulting potential, a generalization of the Fibonacci lattice, is a physical realization of the mathematical ``cut-and-project'' construction which underlies all quasiperiodic structures. Calculation of the energies and wavefunctions of atoms loaded into the proposed quasiperiodic lattice demonstrate a fractal energy spectrum and the existence of edge states. We acknowledge support from the ONR (award N00014-14-1-0805), the ARO and the PECASE program (award W911NF-14-1-0154), the AFOSR (award FA9550-12-1-0305), and the Alfred P. Sloan foundation (grant BR2013-110).
Bursa, Francis; Kroyter, Michael
2010-01-01
String field theory is a candidate for a full non-perturbative definition of string theory. We aim to define string field theory on a space-time lattice to investigate its behaviour at the quantum level. Specifically, we look at string field theory in a one dimensional linear dilaton background. We report the first results of our simulations.
Kapoor, Monica
A series of high-strength low-carbon bcc-Cu- & B2-NiAl-precipitation-strengthened ferritic steels with Mn, Cu, Ni and Al were studied. The yield strength of these alloys increases with the amount of alloying elements. A maximum strength of 1600 MPa, with 12.40 at. % elements, is achieved which is about 30 % higher than the strength of previously reports NUCu (Northwestern Copper) alloys. All the alloys studied attain a maximum hardness within 1--2 h of aging at 500°C--550°C. Aging at a lower temperature and solution treating at a higher temperature can increase the hardness of all the alloys. The lower aging temperature is limited to 500°C by the slow precipitation kinetics observed at 400°C. The higher solution treatment temperature is limited to 1050°C by the adverse impact on toughness in dilute alloys. The primary strengthening contribution is due to combined precipitation of bcc Cu and NiAl-type intermetallic precipitates. The composition, structure and morphology evolution of the precipitates from the 1600 MPa alloy was studied using atom probe tomography and transmission electron microscopy, as a function of aging time at 550°C. Near the peak hardness, the equiaxed bcc Cu-alloyed precipitates have substantial amounts of Fe and are coherent with the Fe matrix. On subsequent aging, the Cu-alloyed precipitates are progressively enriched with Cu and elongate to transform to the 9R phase. The number density of the Cu-alloyed and NiAl-type precipitate is similar near peak hardness indicating that NiAl-type precipitates nucleate on Cu-alloyed precipitates. Almost all Cu-alloyed precipitates are enveloped on one side by ordered NiAl-type precipitates after aging from 2 h to 100 h. Cu-alloyed precipitates coarsen slower than NiAl-type precipitates because of three possible reasons: interfacial energy differences between the two types of precipitates, slower diffusion kinetics of Cu through the ordered B2 NiAl envelope around the bcc Cu-alloyed precipitate
Mickelsson, J
1996-01-01
A calculation of the chiral anomaly on a finite lattice without fermion doubling is presented . The lattice gauge field is defined in the spirit of noncommutative geometry. Standard formulas for the continuum anomaly are obtained as a limit.
Dissolving, trapping and detrapping mechanisms of hydrogen in bcc and fcc transition metals
Yu-Wei You
2013-01-01
Full Text Available First-principles calculations are performed to investigate the dissolving, trapping and detrapping of H in six bcc (V, Nb, Ta, Cr, Mo, W and six fcc (Ni, Pd, Pt, Cu, Ag, Au metals. We find that the zero-point vibrations do not change the site-preference order of H at interstitial sites in these metals except Pt. One vacancy could trap a maximum of 4 H atoms in Au and Pt, 6 H atoms in V, Nb, Ta, Cr, Ni, Pd, Cu and Ag, and 12 H atoms in Mo and W. The zero-point vibrations never change the maximum number of H atoms trapped in a single vacancy in these metals. By calculating the formation energy of vacancy-H (Vac-Hn complex, the superabundant vacancy in V, Nb, Ta, Pd and Ni is demonstrated to be much more easily formed than in the other metals, which has been found in many metals including Pd, Ni and Nb experimentally. Besides, we find that it is most energetically favorable to form Vac-H1 complex in Pt, Cu, Ag and Au, Vac-H4 in Cr, Mo and W, and Vac-H6 in V, Nb, Ta, Pd and Ni. At last, we examine the detrapping behaviors of H atoms in a single vacancy and find that with the heating rate of 10 K/min a vacancy could accommodate 4, 5 and 6 H atoms in Cr, Mo and W at room temperature, respectively. The detrapping temperatures of all H atoms in a single vacancy in V, Nb, Ta, Ni, Pd, Cu and Ag are below room temperature.
Development of a new formulation of interferons (HEBERPAG for BCC treatment
Bello-Rivero I
2013-12-01
Full Text Available Purpose: This work is aimed to show briefly, the clinical development of a new pharmaceutical formulation of interferons for the treatment of basal cell carcinoma. Methods: A rationale design of the combination of IFN-α2b and -γ based in their anti-proliferative synergism on several tumors cell lines identified adequate proportions to be combined to obtain the best clinical results. The potential mechanism of antitumoral effect was studied by qPCR mRNA quantification. HEBERPAG (anti-proliferative synergistic combination of co-formulated recombinant interferons-α2b and –γ was used in clinical trials in adult patients with non-melanoma skin cancer. Trials were conducted after approval by the ethics review boards of the institutions participating in trials; and the patients gave their written informed consent to be enrolled in the studies and receive HEBERPAG. Results: HEBERPAG inhibits the proliferation of several tumor cell lines in vitro and in vivo. The combination has improved pharmacodinamic properties. Several clinical trials have demonstrated the efficacy of HEBERPAG in BCC, with excellent cosmetic effect and well tolerable, mild side effects. HEBERPAG was approved by State Control Center for Drug, Medical Equipment and Devises in Cuba, for the treatment of basal cell carcinoma of any subtype, size and localization, and adjuvant to other treatments, surgical or not. After 3-year follow-up, a recurrence rate of 0.03% was detected in treated patients. Conclusions: HEBERPAG is a novel formulation of IFNs, more potent than separated IFNs for the treatment of basal cell carcinoma, with more rapid and prolonged clinical effect and excellent cosmetic effect and safety profile.
Elastic properties of Ti-24Nb-4Zr-8Sn single crystals with bcc crystal structure
Research highlights: → The single crystals of Ti2448 alloy with the bcc crystal structure were prepared. → The elastic moduli and constants were measured by several resonant methods. → The crystal shows significant elastic asymmetry in tension and compression. → The crystal exhibits weak nonlinear elasticity with large elastic strain ∼2.5%. → The crystal has weak atomic interactions against crystal distortion to low symmetry. - Abstract: Single crystals of Ti2448 alloy (Ti-24Nb-4Zr-8Sn in wt.%) were grown successfully using an optical floating-zone furnace. Several kinds of resonant methods gave consistent Young's moduli of 27.1, 56.3 and 88.1 GPa and shear moduli of 34.8, 11.0 and 14.6 GPa for the , and oriented single crystals, and C11, C12 and C44 of 57.2, 36.1 and 35.9 GPa respectively. Uniaxial testing revealed asymmetrical elastic behaviors of the crystals: tension caused elastic softening with a large reversible strain of ∼4% and a stress plateau of ∼250 MPa, whereas compression resulted in gradual elastic stiffening with much smaller reversible strain. The crystals exhibited weak nonlinear elasticity with a large elastic strain of ∼2.5% and a high strength, approaching ∼20% and ∼30% of its ideal shear and ideal tensile strength respectively. The crystals showed linear elasticity with a small elastic strain of ∼1%. These elastic deformation characteristics have been interpreted in terms of weakened atomic interactions against crystal distortion to low crystal symmetry under external applied stresses. These results are consistent with the properties of polycrystalline Ti2448, including high strength, low elastic modulus, large recoverable strain and weak strengthening effect due to grain refinement.
Elastic properties of Ti-24Nb-4Zr-8Sn single crystals with bcc crystal structure
Zhang, Y.W.; Li, S.J.; Obbard, E.G.; Wang, H.; Wang, S.C. [Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016 (China); Hao, Y.L., E-mail: ylhao@imr.ac.cn [Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016 (China); Yang, R. [Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016 (China)
2011-05-15
Research highlights: > The single crystals of Ti2448 alloy with the bcc crystal structure were prepared. > The elastic moduli and constants were measured by several resonant methods. > The < 1 1 0> crystal shows significant elastic asymmetry in tension and compression. > The <1 0 0> crystal exhibits weak nonlinear elasticity with large elastic strain {approx}2.5%. > The crystal has weak atomic interactions against crystal distortion to low symmetry. - Abstract: Single crystals of Ti2448 alloy (Ti-24Nb-4Zr-8Sn in wt.%) were grown successfully using an optical floating-zone furnace. Several kinds of resonant methods gave consistent Young's moduli of 27.1, 56.3 and 88.1 GPa and shear moduli of 34.8, 11.0 and 14.6 GPa for the <1 0 0>, <1 1 0> and <1 1 1> oriented single crystals, and C{sub 11}, C{sub 12} and C{sub 44} of 57.2, 36.1 and 35.9 GPa respectively. Uniaxial testing revealed asymmetrical elastic behaviors of the <1 1 0> crystals: tension caused elastic softening with a large reversible strain of {approx}4% and a stress plateau of {approx}250 MPa, whereas compression resulted in gradual elastic stiffening with much smaller reversible strain. The <1 0 0> crystals exhibited weak nonlinear elasticity with a large elastic strain of {approx}2.5% and a high strength, approaching {approx}20% and {approx}30% of its ideal shear and ideal tensile strength respectively. The <1 1 1> crystals showed linear elasticity with a small elastic strain of {approx}1%. These elastic deformation characteristics have been interpreted in terms of weakened atomic interactions against crystal distortion to low crystal symmetry under external applied stresses. These results are consistent with the properties of polycrystalline Ti2448, including high strength, low elastic modulus, large recoverable strain and weak strengthening effect due to grain refinement.
A Simulation of the Upper-Tropospheric Temperature Pattern in BCC_CSM1.1
ZHOU Bo-Yao; ZHANG Li
2012-01-01
A simulation of the upper-tropospheric temperature (UTT) by the Beijing Climate Center Climate System Model version 1.1 (BCC_CSM1.1) model is evaluated through a comparison with NCEP/NCAR reanalysis data. It is shown that this model has the ability to simulate the climate pattern of the UTT in all four seasons. The spatial correlation on the climatological distribution between the simulation and the observation is 0.92, 0.93, 0.90, and 0.93 for spring, summer, autumn, and winter, respectively. The first leading mode of the UTT in the simulation agrees with that in the observation, except that the simulated second leading mode corresponds to the observed first leading mode in spring. The standard deviation distribution of the simulation is also roughly consistent with the observation, with a pattern coefficient of 0.82, 0.78, 0.82, and 0.82 in spring, summer, autumn, and winter, respectively. The potential UTT change in the second half of the 21st century under the Representative Concentration Pathway 8.5 (RCP8.5) scenario is examined. The prominent change is that the summer UTT will increase over Eurasia and decrease over the North Pacific compared with the present, indicating that the zonal thermal contrast between Asia and the North Pacific will be strengthened within the context of future global warming. The intensity of the interannual variability of the UTT over the Asian-Pacific region is also generally increased. The zonal thermal contrast between Asia and the North Pacific will tend to be enhanced in winter, concurrent with the intensified interannual variability.
Energetic stability of solute–carbon–vacancy complexes in bcc iron
Bakaev, Alexander, E-mail: alexander.bakaev@ugent.be [SCK-CEN, Nuclear Materials Science Institute, Boeretang 200, Mol B2400 (Belgium); Department of Experimental Nuclear Physics K-89, Institute of Physics, Nanotechnology and Telecommunications, St. Petersburg State Polytechnical University, 29 Polytekhnicheskaya str., 195251 St. Petersburg (Russian Federation); Center for Molecular Modeling, Department of Physics and Astronomy, Ghent University, Technologiepark 903, 9052 Zwijnaarde (Belgium); Terentyev, Dmitry [SCK-CEN, Nuclear Materials Science Institute, Boeretang 200, Mol B2400 (Belgium); Zhurkin, Evgeny E. [Department of Experimental Nuclear Physics K-89, Institute of Physics, Nanotechnology and Telecommunications, St. Petersburg State Polytechnical University, 29 Polytekhnicheskaya str., 195251 St. Petersburg (Russian Federation); Van Neck, Dimitri [Center for Molecular Modeling, Department of Physics and Astronomy, Ghent University, Technologiepark 903, 9052 Zwijnaarde (Belgium)
2015-06-01
The strong binding between a vacancy and carbon in bcc iron plays an important role in the evolution of radiation-induced microstructure. Our previous ab initio study points to the fact that the vacancy–carbon (V–C) pair can serve as a nucleus for the solute-rich clusters. Here, we continue the ab initio study by considering the interaction of mixed solute clusters (Mn, Ni and Si) with the V–C pair, and the interaction of typical alloying elements of Fe-based steels (i.e., Mn, Ni, Cu, Si, Cr and P) with di-carbon–vacancy pair (V–C{sub 2}). We have identified the sequence of growth of Ni, Si and Mn solute-rich clusters nucleating on the V–C pair. The mixed-solute–V–C configurations are found to be less stable clusters than pure-solute–V–C clusters with the energy difference up to 0.22 eV per four atoms. The V–C{sub 2} pair is found to be as strong nucleation site for the solute-rich clusters as the V–C pair. Only Si solute atom stands out from the trend showing a weaker affinity to the V–C{sub 2} complex by 0.09 eV compared to the attraction to the V–C pair. The overall results point to the importance of taking into account the existence of both V–C and V–C{sub 2} complexes in studying the formation of solute-rich clusters in Fe-based steels for nuclear applications.
Plasticity and Failure in Nanocrystalline BCC Metals via Molecular Dynamics Simulation
Rudd, R E
2010-09-29
Advances in the ability to generate extremely high pressures in dynamic experiments such as at the National Ignition Facility has motivated the need for special materials optimized for those conditions as well as ways to probe the response of these materials as they are deformed. We need to develop a much deeper understanding of the behavior of materials subjected to high pressure, especially the effect of rate at the extremely high rates encountered in those experiments. Here we use large-scale molecular dynamics (MD) simulations of the high-rate deformation of nanocrystalline tantalum at pressures less than 100 GPa to investigate the processes associated with plastic deformation for strains up to 100%. We focus on 3D polycrystalline systems with typical grain sizes of 10-20 nm. We also study a rapidly quenched liquid (amorphous solid) tantalum. We apply a constant volume (isochoric), constant temperature (isothermal) shear deformation over a range of strain rates, and compute the resulting stress-strain curves to large strains for both uniaxial and biaxial compression. We study the rate dependence and identify plastic deformation mechanisms. The identification of the mechanisms is facilitated through a novel technique that computes the local grain orientation, returning it as a quaternion for each atom. This analysis technique is robust and fast, and has been used to compute the orientations on the fly during our parallel MD simulations on supercomputers. We find both dislocation and twinning processes are important, and they interact in the weak strain hardening in these extremely fine-grained microstructures. We also present some results on void growth in nanocrystalline BCC metals under tension.
Hydrostatic pressure derivatives of the single-crystal elastic moduli, dC/sub ij//dP, have been measured ultrasonically for b.c.c. Nb--Mo and Ta--W solid solutions. The composition dependence of various electronic properties of these alloys is known to be reasonably well approximated by a rigid-electron-band filling model where e/a, the electron per atom ratio, is the primary parameter. The results indicate that the elastic moduli and their pressure derivatives may also be calculated in such a model. In particular, the dC/sub ij//dP show relatively sharp increases at e/a compositions of 5.4 for Nb--Mo and 5.7 for Ta--W. Both compositions correspond to changes in Fermi surface topology, as deduced from existing band calculations and the rigid band assumption. The results are discussed in the light of related electronic properties and possible geophysical applications. A comparison is also made between ultrasonic results and X-ray diffraction data for Nb. Using diamond-anvil pressure cell, compression of Nb was determined by X-ray diffraction up to 55 kbar in a liquid medium under purely hydrostatic conditions, and up to 175 kbar in a solid medium under nonhydrostatic conditions. The data obtained under hydrostatic conditions agree well with the ultrasonic equation of state and shock wave data, whereas the nonhydrostatic results tend to imply either a higher bulk modulus K/sub s/ or a higher (par. deltaK/sub s//par. deltaP)/sub T/
Borsanyi, Sz; Kampert, K H; Katz, S D; Kawanai, T; Kovacs, T G; Mages, S W; Pasztor, A; Pittler, F; Redondo, J; Ringwald, A; Szabo, K K
2016-01-01
We present a full result for the equation of state (EoS) in 2+1+1 (up/down, strange and charm quarks are present) flavour lattice QCD. We extend this analysis and give the equation of state in 2+1+1+1 flavour QCD. In order to describe the evolution of the universe from temperatures several hundreds of GeV to several tens of MeV we also include the known effects of the electroweak theory and give the effective degree of freedoms. As another application of lattice QCD we calculate the topological susceptibility (chi) up to the few GeV temperature region. These two results, EoS and chi, can be used to predict the dark matter axion's mass in the post-inflation scenario and/or give the relationship between the axion's mass and the universal axionic angle, which acts as a initial condition of our universe.
A set of eight test lattices for the SSC have been devised for such purposes as the investigation of the dependences of chromatic properties and dynamic aperture on the type, field, physical aperture and errors of the magnets, on the sextupole correction scheme, on the tunes and on the cell phase advances. They are distinguished from realistic lattices in that certain features of the latter are missing - most notably the crossing magnets that bring the two counter-rotating proton beams into collision at the interaction points, and the utility insertions, which are the sites for the injection, beam abort, and radiofrequency systems. Furthermore the placement of magnets in the cells is simplified. 7 refs., 9 figs., 2 tabs
Lattices of dielectric resonators
Trubin, Alexander
2016-01-01
This book provides the analytical theory of complex systems composed of a large number of high-Q dielectric resonators. Spherical and cylindrical dielectric resonators with inferior and also whispering gallery oscillations allocated in various lattices are considered. A new approach to S-matrix parameter calculations based on perturbation theory of Maxwell equations, developed for a number of high-Q dielectric bodies, is introduced. All physical relationships are obtained in analytical form and are suitable for further computations. Essential attention is given to a new unified formalism of the description of scattering processes. The general scattering task for coupled eigen oscillations of the whole system of dielectric resonators is described. The equations for the expansion coefficients are explained in an applicable way. The temporal Green functions for the dielectric resonator are presented. The scattering process of short pulses in dielectric filter structures, dielectric antennas and lattices of d...
Hsu, Hsiao-Ping; Nadler, Walder; Grassberger, Peter
2005-07-01
The scaling behavior of randomly branched polymers in a good solvent is studied in two to nine dimensions, modeled by lattice animals on simple hypercubic lattices. For the simulations, we use a biased sequential sampling algorithm with re-sampling, similar to the pruned-enriched Rosenbluth method (PERM) used extensively for linear polymers. We obtain high statistics of animals with up to several thousand sites in all dimension 2⩽d⩽9. The partition sum (number of different animals) and gyration radii are estimated. In all dimensions we verify the Parisi-Sourlas prediction, and we verify all exactly known critical exponents in dimensions 2, 3, 4, and ⩾8. In addition, we present the hitherto most precise estimates for growth constants in d⩾3. For clusters with one site attached to an attractive surface, we verify the superuniversality of the cross-over exponent at the adsorption transition predicted by Janssen and Lyssy.
Jipsen, Peter
1992-01-01
The study of lattice varieties is a field that has experienced rapid growth in the last 30 years, but many of the interesting and deep results discovered in that period have so far only appeared in research papers. The aim of this monograph is to present the main results about modular and nonmodular varieties, equational bases and the amalgamation property in a uniform way. The first chapter covers preliminaries that make the material accessible to anyone who has had an introductory course in universal algebra. Each subsequent chapter begins with a short historical introduction which sites the original references and then presents the results with complete proofs (in nearly all cases). Numerous diagrams illustrate the beauty of lattice theory and aid in the visualization of many proofs. An extensive index and bibliography also make the monograph a useful reference work.
This review concentrates on progress in lattice QCD during the last two years and, particularly, its impact on phenomenology. The two main technical developments have been successful implementations of lattice actions with exact chiral symmetry, and results from simulations with two light dynamical flavours which provide quantitative estimates of quenching effects for some quantities. Results are presented for the hadron spectrum, quark masses, heavy-quark decays and structure functions. Theoretical progress is encouraging renewed attempts to compute non-leptonic kaon decays. Although computing power continues to be a limitation, projects are underway to build multi-teraflops machines over the next three years, which will be around ten times more cost-effective than those of today. (author)
The goal of the lectures on lattice QCD (LQCD) is to provide an overview of both the technical issues and the progress made so far in obtaining phenomenologically useful numbers. The lectures consist of three parts. The author's charter is to provide an introduction to LQCD and outline the scope of LQCD calculations. In the second set of lectures, Guido Martinelli will discuss the progress they have made so far in obtaining results, and their impact on Standard Model phenomenology. Finally, Martin Luescher will discuss the topical subjects of chiral symmetry, improved formulation of lattice QCD, and the impact these improvements will have on the quality of results expected from the next generation of simulations
Gupta, R.
1998-12-31
The goal of the lectures on lattice QCD (LQCD) is to provide an overview of both the technical issues and the progress made so far in obtaining phenomenologically useful numbers. The lectures consist of three parts. The author`s charter is to provide an introduction to LQCD and outline the scope of LQCD calculations. In the second set of lectures, Guido Martinelli will discuss the progress they have made so far in obtaining results, and their impact on Standard Model phenomenology. Finally, Martin Luescher will discuss the topical subjects of chiral symmetry, improved formulation of lattice QCD, and the impact these improvements will have on the quality of results expected from the next generation of simulations.
The panel was attended by prominent physicists from most of the well-known laboratories in the field of light-water lattices, who exchanged the latest information on the status of work in their countries and discussed both the theoretical and the experimental aspects of the subjects. The supporting papers covered most problems, including criticality, resonance absorption, thermal utilization, spectrum calculations and the physics of plutonium bearing systems. Refs, figs and tabs
One of the major recent developments in particle theory has been the use of very high performance computers to obtain approximate numerical solutions of quantum field theories by formulating them on a finite space-time lattice. The great virtue of this new technique is that it avoids the straitjacket of perturbation theory and can thus attack new, but very fundamental problems, such as the calculation of hadron masses in quark-gluon field theory (quantum chromodynamics - QCD)
Digital lattice gauge theories
Zohar, Erez(Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Str. 1, D-85748, Garching, Germany); Farace, Alessandro; Reznik, Benni; Cirac, J Ignacio
2016-01-01
We propose a general scheme for a digital construction of lattice gauge theories with dynamical fermions. In this method, the four-body interactions arising in models with $2+1$ dimensions and higher, are obtained stroboscopically, through a sequence of two-body interactions with ancillary degrees of freedom. This yields stronger interactions than the ones obtained through pertubative methods, as typically done in previous proposals, and removes an important bottleneck in the road towards exp...
Lattice Vibrations in Chlorobenzenes:
Reynolds, P. A.; Kjems, Jørgen; White, J. W.
1974-01-01
Lattice vibrational dispersion curves for the ``intermolecular'' modes in the triclinic, one molecule per unit cell β phase of p‐C6D4Cl2 and p‐C6H4Cl2 have been obtained by inelastic neutron scattering. The deuterated sample was investigated at 295 and at 90°K and a linear extrapolation to 0°K was...
Homomorphisms on Lattices of Measures
Norris Sookoo
2009-01-01
Full Text Available Problem statement: Homomorphisms on lattices of measures defined on the quotient spaces of the integers were considered. These measures were defined in terms of Sharma-Kaushik partitions. The homomorphisms were studied in terms of their relationship with the underlying Sharma-Kaushik partitions. Approach: We defined certain mappings between lattices of Sharma-Kaushik partitions and showed that they are homomorphisms. These homomorphisms were mirrored in homorphisms between related lattices of measures. Results: We obtained the structure of certain homomorphisms of measures. Conclusion: Further information about homomorphisms between lattices of measures of the type considered here can be obtained by investigating the underlying lattices of Sharma-Kaushik partitions.
Adamatzky, Andrew
2015-01-01
The book gives a comprehensive overview of the state-of-the-art research and engineering in theory and application of Lattice Automata in design and control of autonomous Robots. Automata and robots share the same notional meaning. Automata (originated from the latinization of the Greek word “αυτόματον”) as self-operating autonomous machines invented from ancient years can be easily considered the first steps of robotic-like efforts. Automata are mathematical models of Robots and also they are integral parts of robotic control systems. A Lattice Automaton is a regular array or a collective of finite state machines, or automata. The Automata update their states by the same rules depending on states of their immediate neighbours. In the context of this book, Lattice Automata are used in developing modular reconfigurable robotic systems, path planning and map exploration for robots, as robot controllers, synchronisation of robot collectives, robot vision, parallel robotic actuators. All chapters are...
We present a unified framework to describe lattice gauge theories by means of tensor networks: this framework is efficient as it exploits the high local symmetry content native to these systems by describing only the gauge invariant subspace. Compared to a standard tensor network description, the gauge invariant model allows one to increase real and imaginary time evolution up to a factor that is square of the dimension of the link variable. The gauge invariant tensor network description is based on the quantum link formulation, a compact and intuitive formulation for gauge theories on the lattice, which is alternative to and can be combined with the global symmetric tensor network description. We present some paradigmatic examples that show how this architecture might be used to describe the physics of condensed matter and high-energy physics systems. Finally, we present a cellular automata analysis which estimates the gauge invariant Hilbert space dimension as a function of the number of lattice sites that might guide the search for effective simplified models of complex theories. (paper)
Dielectric lattice gauge theory
Dielectric lattice gauge theory models are introduced. They involve variables PHI(b)epsilong that are attached to the links b = (x+esub(μ),x) of the lattice and take their values in the linear space g which consists of real linear combinations of matrices in the gauge group G. The polar decomposition PHI(b)=U(b)osub(μ)(x) specifies an ordinary lattice gauge field U(b) and a kind of dielectric field epsilonsub(ij)proportionalosub(i)osub(j)sup(*)deltasub(ij). A gauge invariant positive semidefinite kinetic term for the PHI-field is found, and it is shown how to incorporate Wilson fermions in a way which preserves Osterwalder Schrader positivity. Theories with G = SU(2) and without matter fields are studied in some detail. It is proved that confinement holds, in the sense that Wilson loop expectation values show an area law decay, if the Euclidean action has certain qualitative features which imply that PHI = 0 (i.e. dielectric field identical 0) is the unique maximum of the action. (orig.)
Toward lattice fractional vector calculus
Tarasov, Vasily E.
2014-09-01
An analog of fractional vector calculus for physical lattice models is suggested. We use an approach based on the models of three-dimensional lattices with long-range inter-particle interactions. The lattice analogs of fractional partial derivatives are represented by kernels of lattice long-range interactions, where the Fourier series transformations of these kernels have a power-law form with respect to wave vector components. In the continuum limit, these lattice partial derivatives give derivatives of non-integer order with respect to coordinates. In the three-dimensional description of the non-local continuum, the fractional differential operators have the form of fractional partial derivatives of the Riesz type. As examples of the applications of the suggested lattice fractional vector calculus, we give lattice models with long-range interactions for the fractional Maxwell equations of non-local continuous media and for the fractional generalization of the Mindlin and Aifantis continuum models of gradient elasticity.
A Mechanical Lattice Aid for Crystallography Teaching.
Amezcua-Lopez, J.; Cordero-Borboa, A. E.
1988-01-01
Introduces a 3-dimensional mechanical lattice with adjustable telescoping mechanisms. Discusses the crystalline state, the 14 Bravais lattices, operational principles of the mechanical lattice, construction methods, and demonstrations in classroom. Provides lattice diagrams, schemes of the lattice, and various pictures of the lattice. (YP)
Density-functional study of bcc U–Mo, Np–Mo, Pu–Mo, and Am–Mo alloys
Density-functional theory, previously used to describe phase equilibria in the γ-U–Mo alloys [A. Landa, P. Söderlind, P.E.A. Turchi, J. Nucl. Mater. 414 (2011) 132], is extended to study ground-state properties of the bcc-based (γ) X–Mo (X = Np, Pu, and Am) solid solutions. We discuss how the heat of formation correlates with the charge transfer between the alloy components, and how magnetism influences the deviation from Vegard’s law for the equilibrium atomic volume
Choosing weights in optimal solutions for DEA-BCC models by means of a N-dimensional smooth frontier
Flávia Badini Nacif
2009-12-01
Full Text Available The DEA (Data Envelopment Analysis smoothed frontier was introduced to solve the problem of multiple optimal solutions in the extreme efficient DMUs (Decision Making Units, which hinders the knowledge of the substitution rates (tradeoffs. It consists of changing the original frontier (piecewise linear for a smoothed one, being as close as possible to the original one, and having continuous partial derivates at every point. First, a solution was developed only for the BCC (Banker, Charnes and Cooper model with either a single input or a single output. Then, it was generalized for the N-dimensional BCC model with simultaneous multiplicity of inputs and outputs, but limited by the fact that the polynomial of the output needs to be a linear one. The present article presents a general model, which not only expunges the limitations of the previous models but also includes them.A suavização da fronteira DEA (Data Envelopment Analysis - Análise Envoltória de Dados surgiu como uma solução do problema das múltiplas soluções ótimas nas DMUs (Decision Making Units - Unidades Tomadoras de Decisão extremo-eficientes, o que impossibilita o conhecimento das razões de substituição (tradeoffs. Ela consiste na substituição da fronteira original (linear por partes por outra suavizada, de modo que esta fronteira suavizada seja próxima da original, e que tenha derivadas contínuas em todos os pontos. Inicialmente foi desenvolvida solução apenas para o caso do modelo BCC (Banker, Charnes e Cooper com apenas um input, ou apenas um output. Em seguida obteve-se uma generalização da solução para o caso BCC N-dimensional com multiplicidade simultânea dos inputs e dos outputs, porém com a limitação da linearidade do polinômio dos outputs. O presente artigo vem apresentar um modelo geral, que elimina as limitações dos modelos anteriores, e também os engloba.
A new parameterization for ice cloud optical properties used in BCC-RAD and its radiative impact
A new parameterization of the solar and infrared optical properties of ice clouds that considers the multiple habits of ice particles was developed on the basis of a prescribed dataset. First, the fitting formulae of the bulk extinction coefficient, single-scatter albedo, asymmetry factor, and δ-function forward-peak factor at the given 65 wavelengths as a function of effective radius were created for common scenarios, which consider a greater number of wavelengths and are more accurate than those used previously. Then, the band-averaged volume extinction and absorption coefficients, asymmetry factor and forward-peak factor of ice cloud were derived for the BCC-RAD (Beijing Climate Center radiative transfer model) using a parameter reference table. Finally, the newly developed and the original schemes in the BCC-RAD and the commonly used Fu Scheme of ice cloud were all applied to the BCC-RAD. Their influences on radiation calculations were compared using the mid-latitude summer atmospheric profile with ice clouds under no-aerosol conditions, and produced a maximum difference of approximately 30.0 W/m2 for the radiative flux, and 4.0 K/d for the heating rate. Additionally, a sensitivity test was performed to investigate the impact of the ice crystal density on radiation calculations using the three schemes. The results showed that the maximum difference was 68.1 W/m2 for the shortwave downward radiative flux (for the case of perpendicular solar insolation), and 4.2 K/d for the longwave heating rate, indicating that the ice crystal density exerts a significant effect on radiation calculations for a cloudy atmosphere. - Highlights: • A new parameterization of the radiative properties of ice cloud was obtained. • More accurate fitting formulae of them were created for common scenarios. • The band-averaged of them were derived for our radiation model of BCC-RAD. • We found that there exist large differences of results among different ice schemes. • We found
Tougou, Kouichi; Shikata, Akihito; Kawase, Uchu; Onitsuka, Takashi; Fukumoto, Ken-ichi
2015-10-01
To investigate the effect of irradiation hardening of structural materials due to cavity formation in BCC metals for nuclear applications, an in-situ transmission electron microscopy (TEM) observation in tensile test was performed for the helium ion-irradiated specimens of pure molybdenum and pure iron. The obstacle barrier strength, α was calculated from the bow-out dislocation based on line tension model, and the obstacle barrier strengths of cavity in pure molybdenum and pure iron were about 0.5-0.7. The fractions of cross-slip generation of dislocation of screw type due to interaction with the cavities were about 16-18 % for pure molybdenum.
Vibrational properties of vacancy in bcc transition metals using embedded atom method potentials
Vandana Gairola; P D Semalty; P N Ram
2013-06-01
The embedded atom method (EAM) potentials, with the universal form of the embedding function along with the Morse form of pair potential, have been employed to determine the potential parameters for three bcc transition metals: Fe, Mo, and W, by fitting to Cauchy pressure $(C_{12} − C_{44})/2$, shear constants $G_{v} = (C_{11} − C_{12} + 3C_{44})/5$ and 44, cohesive energy and the vacancy formation energy. The obtained potential parameters are used to calculate the phonon dispersion spectra of these metals. Large discrepancies are found between the calculated results of phonon dispersion using the EAM and the experimental phonon dispersion results. Therefore, to overcome this inadequacy of the EAM model, we employ the modified embedded atom method (MEAM) in which a modified term along with the pair potential and embedding function is added in the total energy. The phonon dispersions calculated using potential parameters obtained from the MEAM show good agreement with experimental results compared to those obtained from the EAM. Using the calculated phonons, we evaluate the local density of states of the neighbours of vacancy using the Green’s function method. The local frequency spectrum of first neighbours of vacancy in Mo shows an increase at higher frequencies and a shift towards the lower frequencies whereas in Fe and W, the frequency spectrum shows a small decrease towards higher frequency and small shift towards lower frequency. For the second neighbours of vacancy in all the three metals, the local frequency spectrum is not much different from that of the host atom. The local density of states of the neighbours of the vacancy has been used to calculate the mean square displacements and the formation entropy of vacancy. The calculated mean square displacements of the first neighbours of vacancy are found to be higher than that of the host atom, whereas it is lower for the second neighbours. The calculated results of the formation entropy of the vacancy
Collapsing lattice animals and lattice trees in two dimensions
Hsu, Hsiao-Ping; Grassberger, Peter
2005-01-01
We present high statistics simulations of weighted lattice bond animals and lattice trees on the square lattice, with fugacities for each non-bonded contact and for each bond between two neighbouring monomers. The simulations are performed using a newly developed sequential sampling method with resampling, very similar to the pruned-enriched Rosenbluth method (PERM) used for linear chain polymers. We determine with high precision the line of second order transitions from an extended to a coll...
Sortable elements and Cambrian lattices
Reading, Nathan
2005-01-01
We show that the Coxeter-sortable elements in a finite Coxeter group W are the minimal congruence-class representatives of a lattice congruence of the weak order on W. We identify this congruence as the Cambrian congruence on W, so that the Cambrian lattice is the weak order on Coxeter-sortable elements. These results exhibit W-Catalan combinatorics arising in the context of the lattice theory of the weak order on W.
Rasmussen, S. [Los Alamos National Lab., NM (United States)]|[Santa Fe Institute, NM (United States); Smith, J.R. [Santa Fe Institute, NM (United States)]|[Massachusetts Media Lab., Cambridge, MA (United States). Physics and Media Group
1995-05-01
We present a new style of molecular dynamics and self-assembly simulation, the Lattice Polymer Automaton (LPA). In the LPA all interactions, including electromagnetic forces, are decomposed and communicated via propagating particles, {open_quotes}photons.{close_quotes} The monomer-monomer bondforces, the molecular excluded volume forces, the longer range intermolecular forces, and the polymer-solvent interactions may all be modeled with propagating particles. The LPA approach differs significantly from both of the standard approaches, Monte Carlo lattice methods and Molecular Dynamics simulations. On the one hand, the LPA provides more realism than Monte Carlo methods, because it produces a time series of configurations of a single molecule, rather than a set of causally unrelated samples from a distribution of configurations. The LPA can therefore be used directly to study dynamical properties; one can in fact watch polymers move in real time. On the other hand, the LPA is fully discrete, and therefore much simpler than traditional Molecular Dynamics models, which are continuous and operate on much shorter time scales. Due to this simplicity it is possible to simulate longer real time periods, which should enable the study of molecular self-organization on workstations supercomputers are not needed.