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Articles 1 - 30 of 48
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Modeling Energetics And Noise In Dislocation Patterning, Robb Thomson, M. Koslowski, Richard Alan Lesar
Modeling Energetics And Noise In Dislocation Patterning, Robb Thomson, M. Koslowski, Richard Alan Lesar
Richard Alan Lesar
We explore the underlying physics of dislocation ordering in deforming metals, where we focus on the competing role of energy relaxation and the fluctuations (noise) in the local stress field. We investigate the competition by employing a simple two-dimensional model that exhibits the essential physics, while avoiding extraneous mechanisms that might cloud the issues. We show that noise and energetics are equally important in determining the final state of the system. Quantitative functions for the energetic driving force for ordering and the resistive force owing to the noise are developed that balance one another at the relaxed state. These features …
Scaling Relations For Dislocation Structure And Response, Richard Alan Lesar, M. Koslowski, Robb Thomson, J. M. Rickman
Scaling Relations For Dislocation Structure And Response, Richard Alan Lesar, M. Koslowski, Robb Thomson, J. M. Rickman
Richard Alan Lesar
A major goal in dislocation theory is the development of a coarse-grained method that would enable predictions of dislocation response without resolving the degrees of freedom of all the dislocations. While a number of possible coarse-grained theories have been proposed, all of these need information about structures at scales smaller than the coarse-graining volume. Here we present results from dislocation simulations that yield scaling relations with the hope that they can provide a framework for modeling the sub-scale dislocation structures and dynamics. For example, we show that dislocations have a self-similar (fractal) structure over wide range of stress/strain; dislocations move …
Theoretical Prediction Of New High-Performance Lead-Free Piezoelectrics, Pio Baettig, Charles F. Schelle, Richard Alan Lesar, Umesh V. Waghmare, Nicola A. Spaldin
Theoretical Prediction Of New High-Performance Lead-Free Piezoelectrics, Pio Baettig, Charles F. Schelle, Richard Alan Lesar, Umesh V. Waghmare, Nicola A. Spaldin
Richard Alan Lesar
We predict the occurrence of large ferroelectric polarization and piezoelectricity in the hypothetical perovskite-structure oxides, bismuth aluminate (BiAlO3) and bismuth gallate (BiGaO3), using density functional theory within the local density approximation. We show that BiGaO3 will have a similar structure to PbTiO3, although with much stronger tetragonal distortion and therefore improved ferroelectric properties. Likewise, BiAlO3 shares structural characteristics with antiferrodistortive PbZrO3, but it is also a ferroelectric with large polarization. Therefore, we propose the Bi(Al,Ga)O3 system as a replacement for the widely used piezoelectric material, Pb(Zr,Ti)O3 (PZT), that will avoid the environmental toxicity problems of lead-based compounds. Finally, we show …
Dislocation Structures And The Deformation Of Materials, Marisol Koslowski, Richard Alan Lesar, Robb Thomson
Dislocation Structures And The Deformation Of Materials, Marisol Koslowski, Richard Alan Lesar, Robb Thomson
Richard Alan Lesar
We present results from phase-field simulations of a two-dimensional model of dislocation microstructure development under increasing strain that incorporates the effects of the full, three-dimensional, microstructure in an approximate way. Despite its simplicity, the model yields quantitative predictions of both the deformation properties of face-centered cubic metals as well as key descriptors of the evolving microstructure over a wide range of stress and strain. The present results have important implications for how we interpret and describe the deformation properties of fcc materials.
Avalanches And Scaling In Plastic Deformation, Marisol Koslowski, Richard Alan Lesar, Robb Thomson
Avalanches And Scaling In Plastic Deformation, Marisol Koslowski, Richard Alan Lesar, Robb Thomson
Richard Alan Lesar
Plastic deformation of crystalline materials is a complex nonhomogeneous process characterized by avalanches in the motion of dislocations. We study the evolution of dislocation loops using an analytically solvable phase-field model of dislocations for ductile single crystals during monotonic loading. The distribution of dislocation loop sizes is given by P(A)∼A−σ, with σ=1.8±0.1. The exponent is in agreement with those found in acoustic emission experiments. This model also predicts a range of macroscopic behaviors in agreement with observation, including hardening with monotonic loading, and a maximum in the acoustic emission signal at the onset of yielding.
Modeling Crosshatch Surface Morphology In Growing Mismatched Layers. Part Ii: Periodic Boundary Conditions And Dislocation Groups, A. M. Andrews, Richard Alan Lesar, M. A. Kerner, J. S. Speck, A. E. Romanov, A. L. Kolesnikova, M. Bobeth, W. Pompe
Modeling Crosshatch Surface Morphology In Growing Mismatched Layers. Part Ii: Periodic Boundary Conditions And Dislocation Groups, A. M. Andrews, Richard Alan Lesar, M. A. Kerner, J. S. Speck, A. E. Romanov, A. L. Kolesnikova, M. Bobeth, W. Pompe
Richard Alan Lesar
We present further developments and understanding of the commonly observed crosshatch surface morphology in strain-relaxed heteroepitaxialfilms. We have previously proposed that the crosshatch morphology is directly related with strain relaxation via threading dislocation glide which results in both surface step and misfit dislocation (MD) formation [see Andrews et al., J. Appl. Phys. 91, 1933 (2002)—now referred to as Part I]. In this article, we have used solutions for the stress fields and displacement fields for periodic MD arrays which include the effects of the free surface. These solutions avoid truncation errors associated with finite dislocation arrays that were used in …
Incorporation Of Local Structure In Continuous Dislocation Theory, Richard Alan Lesar, J. M. Rickman
Incorporation Of Local Structure In Continuous Dislocation Theory, Richard Alan Lesar, J. M. Rickman
Richard Alan Lesar
Continuous dislocation theory has not had great practical applicability, owing largely to the dislocation density tensor being an average quantity that includes very limited information about dislocation structures and energetics at length scales smaller than the averaging volume. Here we present a straightforward way to include the effects of underlying structure based on moments of the dislocation distribution and show that the addition of a small number of order parameters enables one to accurately characterize the energetics.
Multipole Representation Of The Elastic Field Of Dislocation Ensembles, Zhiqiang Wang, Nsar Ghoniem, Richard Alan Lesar
Multipole Representation Of The Elastic Field Of Dislocation Ensembles, Zhiqiang Wang, Nsar Ghoniem, Richard Alan Lesar
Richard Alan Lesar
A multipole expansion method is developed to determine the elastic field of dislocation loop ensembles of arbitrary geometric complexity. The method results in reduction of the severe computational requirements in large-scale dislocation dynamics (DD) computer simulations without an artificial cutoff on the interaction range. Order of N, O(N), algorithms for DD simulations is immediately accessible on the basis of the developed procedure. Examples of dislocation interaction with large dislocation arrays representing a tilt boundary and a dislocation wall show that the method results in speeding up the calculation of Peach-Kohler interaction forces by a factor of 100, with an error …
Multipole Expansion Of Dislocation Interactions: Application To Discrete Dislocations, Richard Alan Lesar, J. M. Rickman
Multipole Expansion Of Dislocation Interactions: Application To Discrete Dislocations, Richard Alan Lesar, J. M. Rickman
Richard Alan Lesar
The interaction between dislocations is long ranged and anisotropic, leading to severe computational challenges for large-scale simulations. Here we present a multipolar expansion for the interaction energy between dislocations in three dimensions, based on a straightforward expansion of some familiar energy expressions. An evaluation of the needed line integrals is conveniently performed by numerical integration over parametric representations of the dislocation loops. We show that the multipole expansion converges very quickly, leading to potential savings in computational time for evaluating dislocation interactions.
An Analytic Approach Describing Structural Effects On The Properties Of Molecular Fluids, Richard Alan Lesar
An Analytic Approach Describing Structural Effects On The Properties Of Molecular Fluids, Richard Alan Lesar
Richard Alan Lesar
An analytic approach is used to evaluate how the angular structure of a molecular fluid contributes to its thermodynamic properties. The average interactions (energy, forces, etc.) between molecules are first expressed as integrals over a spherical-harmonic expansion of the pair distribution function of the fluid. Angularly averaged interactions for systems described by site−site representations of the intermolecular potentials are then developed. Specifically, homonuclear diatomic molecules described by two-site potential are considered in this paper. These results are used to examine the role that fluid structure plays in determining the average energy, pressure, and forces on a molecule, using as comparison …
Finite-Temperature Dislocation Interactions, Richard Alan Lesar, J. M. Rickman
Finite-Temperature Dislocation Interactions, Richard Alan Lesar, J. M. Rickman
Richard Alan Lesar
We obtain quantitatively the interaction free energy between two fluctuating dislocation lines that are in contact with a thermal bath. By examining both in-plane and transverse-fluctuation polarizations, we identify attractive and repulsive contributions to the free energy and interpret the results in terms of effective multipolar interactions and the screening of separated lines. We find that the usual form of the dislocation force should be modified to include an additional net attractive force when entropic effects are taken into account.
Multiscale Modeling Of Recrystallization, A. W. Godfrey, E. A. Holm, D. A. Hughes, M. A. Miodownik, Richard Alan Lesar
Multiscale Modeling Of Recrystallization, A. W. Godfrey, E. A. Holm, D. A. Hughes, M. A. Miodownik, Richard Alan Lesar
Richard Alan Lesar
We propose a multi length scale approach to modeling recrystallization which links a dislocation model, a cell growth model and a macroscopic model. Although this methodology and linking framework will be applied to recrystallization, it is also applicable to other types of phase transformations in bulk and layered materials. Critical processes such as the dislocation structure evolution, nucleation, the evolution of crystal orientations into a preferred texture, and grain size evolution all operate at different length scales. In this paper we focus on incorporating experimental measurements of dislocation substructures, misorientation measurements of dislocation boundaries, and dislocation simulations into a mesoscopic …
Short-Range Dislocation Interactions Using Molecular Dynamics: Annihilation Of Screw Dislocations, S. Swaminarayan, Richard Alan Lesar, P. Lomdahl, D. Beazley
Short-Range Dislocation Interactions Using Molecular Dynamics: Annihilation Of Screw Dislocations, S. Swaminarayan, Richard Alan Lesar, P. Lomdahl, D. Beazley
Richard Alan Lesar
We present results of a large-scale atomistic study of the annihilation of oppositely signed screw dislocations in an fcc metal using molecular dynamics (MD) and an Embedded-Atom-Method (EAM) potential for Cu. The mechanisms of the annihilation process are studied in detail. From the simulation results, we determined the interaction energy between the dislocations as a function of separation. These results are compared with predictions from linear elasticity to examine the onset of non-linear-elastic interactions. The applicability of heuristic models for annihilation of dislocations in large-scale dislocation dynamics simulations is discussed in the light of these results.
An Efficient Fast‐Multipole Algorithm Based On An Expansion In The Solid Harmonics, H.Y. Wang, Richard Alan Lesar
An Efficient Fast‐Multipole Algorithm Based On An Expansion In The Solid Harmonics, H.Y. Wang, Richard Alan Lesar
Richard Alan Lesar
We present an efficient variant of the fast‐multipole method for calculating long‐range interactions in three‐dimensional Coulombic systems. Using a multipole expansion based on the solid harmonics instead of the more common spherical harmonics leads to a greater increase in computational efficiency than a recently‐reported fast‐Fourier transform method, with none of the overhead associated with that approach.
Dynamic Simulation Of Crack Propagation With Dislocation Emission And Migration, N. Zacharopoulos, D. J. Rolovitz, Richard Alan Lesar
Dynamic Simulation Of Crack Propagation With Dislocation Emission And Migration, N. Zacharopoulos, D. J. Rolovitz, Richard Alan Lesar
Richard Alan Lesar
We present a simulation procedure for fracture that self-consistently accounts for dislocation emission, dislocation migration and crack growth. We find that the dislocation microstructure in front of the crack tip is highly organized and shows a complex temporal-spatial evolution. The final dislocation microstructure and the number of emitted dislocations immediately proceeding fracture varies rapidly with the loading rate. For high loading rates, fracture occurs at smaller loads with increasing loading rate. However, the load at fracture shows a maximum with respect to loading rates.
Brittle Composites Modeling: Compazisons With Mosi2/Zro2, S.P. Chen, Richard Alan Lesar, A. D. Rollett
Brittle Composites Modeling: Compazisons With Mosi2/Zro2, S.P. Chen, Richard Alan Lesar, A. D. Rollett
Richard Alan Lesar
We have calculated the mechanical properties of brittle composites with spring-network (SN) model. The composites that we studied involve the transformation toughening effects and the accompanying micro-cracking. Our simulation results are consistent with experiments of MoSi2 toughened with ZrO2. By monitoring the stress changes due to the transformation and micro-cracking we are able to separate, for the first time, the contributions from these two competing effects. We also found that the fracture toughness of the composite increases as the modulus, interfacial cohesion of particle increases.
(100) Surface Segregation In Cu-Ni Alloys, H.Y. Wang, R. Najafabadi, D. J. Srolovitz, Richard Alan Lesar
(100) Surface Segregation In Cu-Ni Alloys, H.Y. Wang, R. Najafabadi, D. J. Srolovitz, Richard Alan Lesar
Richard Alan Lesar
Atomistic simulations of segregation to the (100) free surface in Ni-Cu alloys have been performed for a wide range of temperatures and compositions within the solid-solution region of the alloy phase diagram. In addition to the surface-segregation profile, surface structures, free energies, enthalpies, and entropies were determined. These simulations were performed within the framework of the free-energy simulation method, in which an approximate free-energy functional is minimized with respect to atomic coordinates and atomic-site occupation. For all alloy bulk compositions (0.05≤C≤0.95) and temperatures (400≤T≤1000 K) examined, Cu segregates strongly to the surface and Ni segregates to the planes just below …
The Influence Of Molecular Shapes On The Relative Stability Of Solid Phases: Application To N2o, Bogdan Kuchta, R. D. Etters, Richard Alan Lesar
The Influence Of Molecular Shapes On The Relative Stability Of Solid Phases: Application To N2o, Bogdan Kuchta, R. D. Etters, Richard Alan Lesar
Richard Alan Lesar
It has been shown that the shape of molecules, represented by the calculated molecular charge distribution, is a valuable source of information about the nature of the potential between molecules. For solid N2O, calculations based on the Kihara and the isotropic and anisotropic site–site potential models have shown that details of the molecular shape affects the relative stability of cubic, tetragonal, and orthorhombic phases at various pressures. This and details of the utilization of experimental data to characterize the potential show that features of CO2 are also described. Also, a Monte Carlo calculation, using a random variable to simulate the …
Thermodynamic And Structural Properties Of [001] Twist Boundaries In Gold, R. Najafabadi, D. J. Srolovitz, Richard Alan Lesar
Thermodynamic And Structural Properties Of [001] Twist Boundaries In Gold, R. Najafabadi, D. J. Srolovitz, Richard Alan Lesar
Richard Alan Lesar
We have employed the Local Harmonic (LH) model and the Embedded Atom Method (EAM) to examine the structural and thermodynamic properties of a series of twelve [001] twist boundaries in gold for temperatures between 0 K and 700 K. For the majority of the grain boundary misorientations, metastable structures were observed with grain boundary energies that were typically less than 0.1% larger than the stable structures. Four of the twelve grain boundaries underwent first order structural phase transitions as seen by the crossing of the free energy versus temperature curves for the competing structures. Relatively small cusps or inflections in …
Thermodynamics Of Solid And Liquid Eam Metals: A Variational Study, Richard Alan Lesar, R. Najafabadi, D. J.
Thermodynamics Of Solid And Liquid Eam Metals: A Variational Study, Richard Alan Lesar, R. Najafabadi, D. J.
Richard Alan Lesar
We present results of variational calculations of the Helmholtz free energy and the thermodynamic properties of a series of metallic liquids and solids (Ag,Au,Cu,Ni, Pd, Pt) described by embedded‐atom‐method potentials. For the solids, we use a variational procedure based on an Einstein‐model reference state. The free energies of liquids are calculated with an approximate variational method proposed by Ross. At the respective melting points, the present results for the Helmholtz free energy are within about 1% of the results of accurate Monte Carlo (MC) calculations with the same interaction potentials, both for the fluid and the solid. The average error …
Crystal Structures Of N2o To 12 Gpa By X‐Ray Diffraction, R. L. Mills, Bart Olinger, D. T. Cromer, Richard Alan Lesar
Crystal Structures Of N2o To 12 Gpa By X‐Ray Diffraction, R. L. Mills, Bart Olinger, D. T. Cromer, Richard Alan Lesar
Richard Alan Lesar
The structures and phase transitions of N2O were studied by powder x‐ray diffraction in a tungsten–carbide anvil device from about 100 to 300 K and 2 to 12 GPa. Two solid phases, α‐N2O and β‐N2O, were observed. The α pattern is consistent with the known low‐pressure low‐temperature ordered cubic form, space group Pa3, up to 4.8 GPa where transition to a new β solid occurs. From refinements using photographic x‐ray intensities, the β‐N2O structure was determined to be orthorhombic Cmca. There are four molecules in a unit cell with a=4.954 Å, b=4.497 Å, and c=6.201 Å at 5.81 GPa and …
Free Energy Simulation Of Grain Boundary Segregation And Thermodynamics In Ni3−Xal1+X, R. Najafabadi, H.Y. Wang, D. J. Srolovitz, Richard Alan Lesar
Free Energy Simulation Of Grain Boundary Segregation And Thermodynamics In Ni3−Xal1+X, R. Najafabadi, H.Y. Wang, D. J. Srolovitz, Richard Alan Lesar
Richard Alan Lesar
The free energy simulation method is employed to study segregation to Σ5 and Σ13 (001) twist grain boundaries and their free energies in ordered Ni3−xAl1+x. In the temperature range studied (300–900K), it is shown that there is almost no segregation, strong Al segregation, and weak Ni segregation to the grain boundary for the stoichiometric, Al-rich, and Ni-rich bulk compositions respectively. It is also shown that the segregation is limited to a few (002) planes around the grain boundary and its magnitude decreases with increasing temperature. For Al-rich bulk composition, it is demonstrated that segregation at low temperature substantially lowers the …
Finite Temperature Structure And Thermodynamics Of The Au Σ5 (001) Twist Boundary, R. Najafabadi, D. J. Srolovitz, Richard Alan Lesar
Finite Temperature Structure And Thermodynamics Of The Au Σ5 (001) Twist Boundary, R. Najafabadi, D. J. Srolovitz, Richard Alan Lesar
Richard Alan Lesar
The structure and thermodynamic properties of a Σ5 (001) twist boundary in gold are studied as a function of temperature. This study was performed within the framework of the Local Harmonic (LH) model and employed an Embedded Atom Method (EAM) potential for gold. We find that for the Σ5 (001) twist boundary in gold, a distorted CSL structure is stable at low temperatures, but undergoes a phase transformation to a DSC related structure near room temperature. This transformation is shown to be first order. The temperature dependences of the excess grain boundary free energy, enthalpy, entropy, specific heat, and excess …
Calculated Thermodynamic Properties And Phase Transitions Of Solid N2 At Temperatures 0≤T≤300 K And Pressures 0≤P≤100 Gpa, J. Belak, Richard Alan Lesar, R. D. Etters
Calculated Thermodynamic Properties And Phase Transitions Of Solid N2 At Temperatures 0≤T≤300 K And Pressures 0≤P≤100 Gpa, J. Belak, Richard Alan Lesar, R. D. Etters
Richard Alan Lesar
Thermodynamic properties of solid nitrogen are calculated over a variety of isotherms and isobars using a constant pressure Monte Carlo method with deformable, periodic boundary conditions. Vibron frequencies are calculated using a simple perturbation theory. In addition, pressure–volume relations, thermal expansion coefficients, structures, and phase transition pressures and temperatures are determined. In particular, the nature of the orientational disorder in the plastic crystal phases is examined by calculating a variety of orientational order parameters.
Location Of Melting Point At 300 K Of Nitrogen By Monte Carlo Simulation, Evert Jan Meijer, Daan Frenkel, Richard Alan Lesar, Anthony J.C. Ladd
Location Of Melting Point At 300 K Of Nitrogen By Monte Carlo Simulation, Evert Jan Meijer, Daan Frenkel, Richard Alan Lesar, Anthony J.C. Ladd
Richard Alan Lesar
We present an accurate new method to compute absolute free energies of molecular solids by computer simulations. As a first application, we computed the thermodynamicphase transition between the fluid phase and the orientational disordered solid β phase of nitrogen at 300 K, using a well tested pair potential. The computed coexistence pressure and the volume change coincides within the error margins with the experimental values. The coexistence volume differed by 2% from the experimental value. To our knowledge these results constitutes the first numerical calculation of the thermodynamic stability for a model of a realistic molecular solid.
Finite-Temperature Defect Properties From Free-Energy Minimization, Richard Alan Lesar, R. Najafabadi, D. J. Srolovitz
Finite-Temperature Defect Properties From Free-Energy Minimization, Richard Alan Lesar, R. Najafabadi, D. J. Srolovitz
Richard Alan Lesar
We present two simple, but accurate, techniques for calculating the finite-temperature atomic structure and free energy of any solid defect (point or extended). The finite-temperature equilibrium atomic structure and thermodynamic properties of defects in solids are obtained self-consistently by minimizing the rree energy of the solid with respect to the coordinates of the atoms. Application of the two methods to a perfect crystal and one with a vacancy show that both methods yield excellent agreement with Monte Carlo calculations for temperatures up to at least 75% of the melting point.
High-Density Structures And Phase Transition In An Ionic Model Of H 2 O Ice, P. Demontis, M. L. Klein, Richard Alan Lesar
High-Density Structures And Phase Transition In An Ionic Model Of H 2 O Ice, P. Demontis, M. L. Klein, Richard Alan Lesar
Richard Alan Lesar
A self-consistent ionic model of water (protons and O 2- ions) has been used to explore the low-temperature, high-pressure properties of ice. Interionic interactions were calculated with the electron-gas model, and the self-consistent changes in the electronic structure of the oxide ions were modeled with Watson spheres. A phase transition is predicted at about 330 GPa from the observed, low-pressure, symmetric hydrogen-bonded cuprite structure to a fully ordered antifluorite structure. The transition pressure is higher than the 100-GPa estimate obtained from a molecular-dynamics study [Phys. Rev. Lett. 60, 2284 (1988)] that employed more approximate potentials.
Equation Of State Of Dense Helium, Richard Alan Lesar
Equation Of State Of Dense Helium, Richard Alan Lesar
Richard Alan Lesar
Comparison between pressure-volume relations in helium at high pressures indicates that theoretical calculations based on gas-phase pair potentials are not adequate to explain recent x-ray experiments. Regardless of what potential is used, the gas-phase potentials give an equation of state that is much too stiff. It is shown that if the changes in the atomic wave functions induced by the high-pressure crystal environment are taken into account, then agreement between experimental and theoretical pressure-volume curves is greatly improved.
New High-Pressure Phases Of Ice, Pierfranco Demontis, Richard Alan Lesar, Michael L. Klein
New High-Pressure Phases Of Ice, Pierfranco Demontis, Richard Alan Lesar, Michael L. Klein
Richard Alan Lesar
An ionic model has been used in conjunction with classical constant-pressure molecular-dynamics calculations to explore the properties of possible high-pressure phases of ice. Around 100 GPa, the model is found to convert from the symmetric hydrogen-bonded cuprite structure (ice X) to a fully ordered antifluorite structure. On heating, the new phase, ice XI, becomes a fast-ion proton conductor.
Character Of The Α-Β Phase Transition In Solid Oxygen, Richard Alan Lesar, R. D. Etters
Character Of The Α-Β Phase Transition In Solid Oxygen, Richard Alan Lesar, R. D. Etters
Richard Alan Lesar
Properties of the α and β phases of solid O2 are calculated with a deformable-cell Monte Carlo method and the character of the associated phase transition is delineated. We find that the monoclinic α phase is stable at temperatures T≲18 K and in- and out-of-plane antiferromagnetic order is predicted that is in accord with experiment. The magnetic correlations appear to be long ranged. A phase transition into β-O2 occurs at T=17.75±0.2 K. It is first order with a calculated volume change ΔV=0.06±0.10 cm3/mole, and is accompanied by a magnetic transition into a highly dynamic quasihelical state with a correlation length …