Symposium Organizers
Ram Devanathan Pacific Northwest National Laboratory
Maria Jose Caturla Universitat d’Alacant
Alison Kubota Lawrence Livermore National Laboratory
Alain Chartier CEA-Saclay
Simon Phillpot University of Florida
GG1: Advances in Computational Methods
Session Chairs
Monday PM, November 27, 2006
Constitution B (Sheraton)
9:00 AM - **GG1.1
A Quasicontinuum for Complex Crystals.
Ellad Tadmor 1 2
1 Mechanical Engineering, Technion - Israel Institute of Technology, Haifa Israel, 2 Aerospace Engineering and Mechanics, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractThe quasicontinuum (QC) method is a multiscale method coupling atomistic regions with a surrounding continuum modeled within a nonlinear finite element formulation. The constitutive response in the continuum is obtained by application of the Cauchy-Born (CB) rule to the underlying lattice and calculation of the energy and necessary gradients using the same interatomic potentials applied in the atomistic region.Application of QC to complex crystals is straightforward in principle, however a question arises as to which description of the crystal structure should be used with the CB rule. Traditionally, the CB rule has been interpreted as being applied to the simplest periodic structure that reproduces the crystal. However, with this definition the CB rule can fail since some deformations require an increase in the periodic size of the unit cell. The problem is that the minimum required cell size cannot be known a priori. Detection of such period extensions can be critical in many cases where phase transformations occur in the material.To address this issue within QC, a phonon stability analysis is performed locally within each finite element in the continuum region at the end of each load step. This analysis detects the onset of period extension and identifies the new minimal lattice description. To prevent the formation of random degenerate structures and to introduce a lengthscale into the continuum, an estimate for the interfacial energy between elements is added to the total energy. The coupling with the atomistic region and definition of a criterion for mesh refinement must also be considered in the QC setting.Applications of this methodology to one-dimensional test problems that highlight the salient features will be presented. Extension to higher dimensions is straightforward and currently being pursued. Some preliminary results may be shown.
9:30 AM - GG1.2
Uniform Accuracy of the Nonlocal Quasicontinuum Method.
Jerry Yang 1 , Jianfeng Lu 1 , E. Weinan 1
1 Program in Applied and Computational Mathematics, Princeton University, Princeton, New Jersey, United States
Show Abstract9:45 AM - GG1.3
Using Elasticity to Correct for Boundary Effects in Calculations of Stress-Diffusion Coupling Parameters.
Brian Puchala 1 , Michael Falk 1 , Krishna Garikipati 2
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractThe effect of stress on diffusion during semiconductor processing becomes important as device dimensions shrink from microns to nanometers. Simulating these effects require accurate parameterization of the formation and migration volume tensors of the defects that mediate diffusion on the atomistic scale. We investigate the effect of boundary conditions on the accuracy of atomistic calculations of defect formation energies and formation volume tensors. Linear elasticity provides a correction to the effect of the boundaries on the resulting relaxation volume tensor. By a formal proof we show that the correction term is zero for free boundaries and for periodic boundary conditions with zero mean boundary stress. This is demonstrated in the far field for periodic and free boundary conditions for an isotropic (vacancy) and an anisotropic (<110> intersitial) defect in Stillinger-Weber silicon. For periodic boundary conditions, formation volume tensor components converge to within 5% in a 216 atom simulation cell. For free boundary conditions, slow convergence of elastic constants results in slow convergence of formation volumes. Most significantly, this provides a new method to calculate the formation volume from constant volume calculations. This removes the need for relaxing boundaries, allowing for simpler and more efficient algorithms. We apply this method to the <110> interstitial in Stillinger-Weber silicon.
10:00 AM - GG1.4
Multipolar Expansion Methods for Coarse Grained Force Fields.
Sheng Chao 1
1 Institute of Applied Mechanics, National Taiwan University , Taipei Taiwan
Show AbstractCurrent large scale atomistic simulations remain too computationally demanding to be generally applicable to industrial and bioengineering materials. It is desirable to develop multiscale modeling algorithms to perform efficient and informative mesoscopic simulations. Here we present a multipolar expansion method to construct coarse grained force fields (CGFF) for polymer nanostructures and nanocomposites. This model can effectively capture the stereochemical response to anisotropic long-range interactions and can be systematically improved upon adding higher order terms. The coarse-graining procedure forms the basis to perform a hierarchy of multiscale simulations starting with the quantum chemistry calculations to coarse grained molecular dynamics, hopefully toward continuum modeling. We have applied this procedure to molecular clusters such as alkane, benzene, and fullerene. For liquid alkane, molecular dynamics simulations using the CGFF can reproduce the pair distribution functions using atomistic force fields. Molecular mechanics simulations using the CGFF can well reproduce the energetics of benzene clusters from quantum chemistry electronic structure calculations. Subtle anisotropy in the interaction potentials of the fullerene dimer using the Brenner force field can also be well represented by the model. It is promising this procedure can be standardized and further extended.
10:15 AM - GG1.5
An Alternative Way to the Minimum: Inertial Relaxation for Fast Structural Optimization.
Erik Bitzek 1 , Pekka Koskinen 2 , Franz Gaehler
3 , Michael Moseler 2 , Peter Gumbsch 1 2
1 izbs, University of Karlsruhe, Karlsruhe Germany, 2 , Fraunhofer Institut Werkstoffmechanik IWM, Freiburg Germany, 3 Institut fuer Theoretische und Angewandte Physik, University of Stuttgart
, Stuttgart Germany
Show Abstract10:30 AM - GG1:Methods
BREAK
GG2: Fatigue, Fracture and Crack Propagation
Session Chairs
Monday PM, November 27, 2006
Constitution B (Sheraton)
11:00 AM - GG2.1
Fracture of Nanocrystalline Materials at the Atomistic Level: The Most Brittle Grain Size.
Diana Farkas 1
1 Materials Science, Virginia Tech, Blacksburg, Virginia, United States
Show Abstract11:15 AM - GG2.2
Multiscale Modeling of Dislocation/Grain Boundary Interactions
Michael Dewald 1 , William Curtin 1
1 Division of Engineering, Brown University, Providence, Rhode Island, United States
Show AbstractThe interaction of dislocations with grain boundaries (GBs) determines a number of important aspects of the mechanical performance of materials, including strengthening and fatigue resistance. Dislocations and GBs are both non-crystalline defects, and so their fundamental interactions must be studied theoretically at the atomistic level rather than the continuum level. However, dislocation pile-ups in the lattice or in the GB can exert significant additional stresses that could influence the local dislocation/GB interactions. Modeling methods for this problem must thus be multiscale in nature, both to remove the effect of artificial boundaries that usually arise in fully atomistic-scale models, such as molecular dynamics, and to handle the large-scale pile-up behavior. In this talk, we present a multiscale method, the coupled atomistic/discrete-dislocation method (CADD), to study dislocation/grain-boundary deformation. The CADD method couples a continuum region containing discrete dislocations to a fully atomistic region, and dislocations can move seamlessly from one region to another with little or no spurious forces, thus accurately approximating an extremely large atomistic model. The CADD method is applied here to investigate the interaction of screw dislocations with Σ3, Σ11, and Σ9 symmetric tilt boundaries in Al. The low-energy Σ3 and Σ11 boundaries absorb lattice dislocations and generate extrinsic grain boundary dislocations (GBDs). As multiple screw dislocations impinge on the GB, the created GBDs pile-up along the GB, providing a backstress that requires increasing applied load to push the lattice dislocations into the GB. GB migration accompanies each absorbed dislocation, and the reversibility of the process upon unloading is investigated. In no cases is dislocation transmission observed. In contrast, for the Σ9 grain boundary that is composed from a more complex set of structural units, screw dislocations absorbed at the boundary remain localized without the creation of GBDs, presumably due to the much larger Burger’s vector for possible GBDs. With increasing applied stress, new screw dislocations are then nucleated into the opposite grain, not from the location where the original dislocation intersected the boundary but rather from a different nearby structural unit within the GB. The detailed behavior depends, however, on the location of the original incident dislocations and the extent of the pile-up. Finally, we briefly present results for the interaction of 60o and edge dislocations with Σ3 and Σ11 GBs, where a range of complex reactions are observed, depending on loading, pile-ups, and the order in which the partial dislocations of the full lattice dislocation enter the GB.
11:30 AM - GG2.3
Multi-scale Modeling of Dynamic Fracture of Silicon using ReaxFF Reactive Force Fields.
Markus Buehler 1 , Harvey Tang 2 , Janet Ryu 3 , Adri van Duin 4 , William Goddard 4
1 Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 4 Chemistry, California Institute of Technology, Pasadena, California, United States
Show AbstractAtomistic modeling of materials in which formation and breaking of chemical bonds plays an important role in determining their mechanical behavior has so far been a quite difficult and in some cases an impossible endeavor. This is because most empirical interatomic potentials cannot describe breaking and formation of bonds between various chemical elements correctly. To date, only quantum mechanical (QM) methods, for example those based on density functional theory (DFT) can be used. However, the number of atoms accessible to most QM methods is limited to about 100 atoms and to very short time scales. An accurate treatment of bond breaking and formation at scales relevant to mechanical properties of materials involves thousands of atoms, as for example at crack tips, dislocation cores, and grain boundaries or at other defects. Our new concept of reactive potentials (ReaxFF) can overcome some of the limitations imposed by DFT theories and enables large-scale simulations of thousands of reactive atoms with QM accuracy (Buehler et al., Phys. Rev. Lett., 2006). ReaxFF, originally only developed for hydrocarbons has been extended recently to cover a wide range of materials, including metals, semiconductors and organic chemistry. In this paper, we will focus on applying ReaxFF to model deformation and fracture of silicon. We report a study of dynamic cracking in a silicon single crystal in which the ReaxFF reactive force field is used for several thousand atoms near the crack tip, while more than 100,000 atoms of the model system are described with a simple nonreactive Tersoff force field. ReaxFF is completely derived from quantum mechanical calculations of simple silicon systems without any empirical parameters. Our results reproduce experimental observations of fracture in silicon including differences in crack dynamics for loading in the [110] or [100] orientations and instabilities with increasing crack velocity. We also observe formation of secondary micro-cracks ahead of the moving mother crack for large applied strains. Our hybrid model allows studies of crack initiation processes for (111) crack surfaces, providing fundamental insight into the bond breaking and rearrangement processes during fracture initiation. Further, in quantitative agreement with experiment, we observe discrete velocity traps for the crack speeds. We describe the atomistic mechanisms associated with these phenomena. We further demonstrate the competition of chemical processes such as oxidation of silicon surfaces with brittle crack extension. Our new methods allow, for the first time, a thorough coupling of complex chemistry with larger length and time scales relevant for mechanical properties of materials.
11:45 AM - GG2.4
Atomistic Studies of Dislocation Nucleation at Crack Tips.
Peter Gumbsch 1 2 , Erik Bitzek 1
1 izbs, University of Karlsruhe, Karlsruhe Germany, 2 , Fraunhofer Institut Werkstoffmechanik IWM, Freiburg Germany
Show Abstract12:00 PM - GG2.5
Simulation of Directed Nanocrack Patterns for the Fabrication of Nanowires.
David Salac 1 , Wei Lu 1
1 Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractWhile material fracture is generally considered undesirable, recent experimental work has indicated that cracking can be utilized to create small scale nanowire structures with diameters of 100nm and smaller. Typical nanowire fabrication techniques include the use of a scanning tunneling microscope and electrochemical deposition. Compared to fracture fabricated nanowire structures current fabrication techniques are either extremely slow or require large amounts of post-processing. To become a viable nanowire fabrication technique the cracking of materials must be directed. We propose a computational model to predict propagation paths in large scale crack systems. We utilize the level set method to investigate the creation of nanoscale crack patterns. The level set method allows for large scale simulations of many crack tips while easily accommodating large scale deformations. Unlike traditional methods such as finite elements explicit modeling of the cracks is not needed. We show that the use of multiple materials and etched regions can effectively direct the cracking of a thin film. Using these patterns nanowire structures can be constructed which would be difficult to obtain otherwise.
12:15 PM - GG2.6
Modeling the Structural Evolution of Crack Propagation in Brittle Materials.
Artem Levandovsky 1 , Anna Balazs 1
1 Chemical Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
Show Abstract12:30 PM - GG2.7
DD Simulations of Interaction of Dislocations with Different Shape Cracks During Fatigue.
Ioannis Mastorakos 1 , Hussein Zbib 1
1 School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington, United States
Show Abstract12:45 PM - GG2.8
Multiscale Modeling of Brittle Fracture of Graphene Sheets and Carbon Nanotubes
Sulin Zhang 1 2 , Roopam Khare 2 , Steven Mielke 3 , George Schatz 3 , Ted Belytschko 2
1 Mechanical Engineering, University of Arkansas, Fayetteville, Arkansas, United States, 2 Mechanical Engineering, Northwestern University, Evanston, Illinois, United States, 3 Chemistry, Northwestern University, Evanston, Illinois, United States
Show AbstractA coupling method that bridges quantum mechanical (QM) calculations, molecular mechanics (MM) simulations, and continuum mechanics (CM) was developed, and employed to simulate the fracture of defected graphene sheets and carbon nanotubes (CNTs). Different coupling strategies are used at the QM/MM and MM/CM interfaces. Coupling between MM and CM is based on a domain decomposition scheme in which displacement compatibility conditions are imposed. Coupling between QM and MM is achieved by replacing MM calculations of the potential energy of the defect-containing fragments by pure QM simulations. Using the coupling method, we probe the energetics of brittle fracture involving consecutive bond breaking in CNTs and graphene sheets, where lattice trapping effect plays an important role. The coupling method shows a clear advantage over pure QM calculations for its computational efficiency, while offering a computational accuracy comparable to pure QM calculations.
GG3: Modeling from First Principles
Session Chairs
Monday PM, November 27, 2006
Constitution B (Sheraton)
2:30 PM - GG3.1
Calculating Diffusion Coefficients via a First-Principles Approach.
Manjeera Mantina 1 , Raymundo Arroyave 1 , Yi Wang 1 , Chris Wolverton 2 , Long-Qing Chen 1 , Zi-Kui Liu 1
1 , The Pennsylvania State University, State College, Pennsylvania, United States, 2 , Ford Research and Advanced Engineering, Dearborn, Michigan, United States
Show AbstractWe propose a new first-principles-based procedure to determine diffusion coefficients in metals and dilute alloys. In particular, the formation and migration enthalpies and entropies of vacancies are calculated. We illustrate the method by computing the self-diffusion coefficient of fcc Al and the diffusion coefficients of Mg, Si and Cu in Al individually, through a vacancy mechanism. In the case of the dilute alloys we use the fcc-based five-frequency model to describe diffusion. We obtain results from both the local-density approximation (LDA) and the generalized-gradient approximation (GGA). The proposed method yields self-diffusion and impurity diffusion coefficients that are in good agreement with existing experimental measurements.
2:45 PM - GG3.2
Modeling of Self-assembled Nanostructures using First Principles Ab-initio Calculations.
Tejodher Muppidi 1 , Vidvuds Ozolins 1
1 Material Science and Engineering, University of California Los Angeles, Los Angeles, California, United States
Show Abstract3:00 PM - GG3.3
Multiscale Model for Microstructure Evolution in Multiphase Systems with Elastic Misfit.
Danny Perez 1 , Laurent Lewis 1
1 Département de physique, Université de Montréal, Montréal, Quebec, Canada
Show Abstract3:15 PM - GG3.4
Calculation of Stacking Fault Energies Using DFT-based Nudged Elastic Band Methods and the General Trend of Twinnability in FCC metals.
Zhao-Hui Jin 1 , Scott Dunham 2 1 , Horst Hahn 1
1 Institut für Nanotechnologie, Forschungszentrum Karlsruhe, Karlsruhe Germany, 2 Department of Electrical Engineering, University of Washington, Seattle, Washington, United States
Show AbstractPlastic deformation of crystalline materials is mediated by nucleation and motion of lattice dislocations. Rich scenarios of dislocation nucleation processes have been observed in nanostructured materials. To understand fundamental physical mechanisms of dislocation emission, microtwin formation as well as mechanical twinning, we provide a systematic analysis of stable and unstable stacking-fault energies obtained using nudged elastic band methods in combination with ab initio calculations (VASP). With stacking-fault energies computed for several face-centered cubic (FCC) metals, the twinning tendencies are measured based on Peierls criterion for the onset of dislocation and twin emission at a crack tip [1,2,3]. We found that interactions between stacking faults appear small, not just between stable stacking faults, but also between stable and unstable faults. A general trend of twinning versus dislocation mediated slip is established such that the twinnability can be estimated as long as two fundamental material properties, the stable and unstable energies to generate an intrinsic stacking fault, are known. The ratio between the stable and unstable stacking fault energies suffices to describe much of the behavior expected during deformation of nanocrystalline FCC metals. Our results suggest that two processes can be expected to compete with deformation twinning. For example, for Al, emission of trailing partials leading to full dislocations is generally favored over twin nucleation, but existing twins can be expected to grow readily. For Cu, there is little difference in the barriers associated with twin growth and the emission of independent leading partials, so the latter is likely to dominate the response to deformation.References:[1] Rice, J. R. Dislocation nucleation from a crack tip: an analysis based on the Peierls concept. J. Mech. Phys. Solids 40, 239–271(1992).[2] Tadmor, E. B. & Hai, S. A. Peierls criterion for the onset of deformation twinning at a crack tip. J. Mech. Phys. Solids 51, 765–793 (2003).[3] Tadmor, E. B. & Bernstein, N. A first-principles measure for the twinnability of FCC metals. J. Mech. Phys. Solids 52, 2507–2519 (2004).
3:30 PM - GG3:Ab initio
BREAK
GG4: Linking across scales
Session Chairs
Monday PM, November 27, 2006
Constitution B (Sheraton)
4:30 PM - GG4.1
Connecting the Motion of Atoms to Macroscopic Behavior in Studies of Fluid Flow and Solid Contact
Mark Robbins 1 2 , Xiaobo Nie 1 5 , Binquan Luan 1 , Jin Liu 2 , Jean-Francois Molinari 2 , Sangil Hyun 4 , Noam Bernstein 3 , Shiyi Chen 2
1 Physics & Astronomy, Johns Hopkins University, Baltimore, Maryland, United States, 2 Mechanical Engineering, Johns Hopkins University, Baltimore, Maryland, United States, 5 , National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 4 School of Mechanical Engineering, Kyungpook National University, Daegu Korea (the Republic of), 3 Center for Computational Materials Science, Naval Research Laboratory, Washington, District of Columbia, United States
Show Abstract4:45 PM - GG4.2
Linking First-principles Calculations to Al-Si Thermodynamic Modeling
Tao Wang 1 , Yi Wang 1 , Hui Zhang 1 , Long-Qing Chen 1 , Zi-Kui Liu 1
1 , PSU, University Park, Pennsylvania, United States
Show AbstractThe accurate thermodynamic description can be used to calculate the driving forces of various phase transformations, and then predict the life-time of an alloy. As a very important binary in Aluminum alloys, the Al-Si system has been modeled for several times. However, all those efforts have suffered from inaccurate energetics of solid phases. In this work, we sued for help from first-principles calculations which were performed using VASP based on the pseudo-potentials and a plane wave basis set. The lattice stabilities of fcc-structured Si and diamond-structured Al were calculated at 0K. The random fcc and diamond solid solution phases were mimicked by supercell structures and special quasi-random structures (SQS), and their enthalpies of mixing were predicted by first-principles calculations. The SQS configurations were determined by ATAT software, and a convergence test on the diamond phase shows that the 16-atom SQS is good enough to mimic the random solution in Al-Si system, and the enthalpy of mixing is converged to few hundred Joules per mole atoms. By combining CALPHAD approach with first-principles calculations, we performed a new thermodynamic modeling of Al-Si, and both experimental data and the present first-principles results can be well-reproduced by the new thermodynamic description.
5:00 PM - GG4.3
Vertical and Horizontal Communication across Scales in Materials Simulations: Spall Fracture in Al.
Jaime Marian 1 , Alejandro Mota 2 , Jaroslaw Knap 1 , Michael Ortiz 2
1 , Lawrence Livermore National Laboratory, Livermore, California, United States, 2 Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California, United States
Show Abstract5:15 PM - GG4.4
Extending the Time Scale of Molecular Dynamics Using Coupled Static-Dynamic Atomistic Method
Sergey Medyanik 1 2
1 Mechanical Engineering, Northwestern University, Evanston, Illinois, United States, 2 Mechanical and Materials Engineering, Washington State University, Pullman, Washington, United States
Show AbstractIn recent years, multi-scale simulation methods have been relatively successful in bridging the atomistic simulation and experimental length scales. However, the vast gap between the molecular dynamics (MD) and experimental time scales is still unresolved, which makes adequate comparison between the MD simulation and experimental results hardly possible. In this presentation, a novel approach to extending MD time scale will be outlined. The new method consists in coupling molecular dynamics and molecular statics in one numerical simulation. This approach is especially effective when the simulated process consists of the two distinct phases: the slow phase when kinetic energy is rather small and the fast phase associated with a rapid movement of atoms and high kinetic energy of the system. In this case, the slow phase can be effectively modeled using static energy minimization technique, while the fast phase is continued to be modeled dynamically. The method allows for computational cost savings since the major part of the simulation can be modeled as static, without the need to comply with the critical MD time step limitations. This allows for modeling considerably longer periods of time compared to when the whole duration of the process is modeled dynamically. The fundamental issues in developing this approach include the correlation between the MD time scale and step-like quasi-static procedure as well as finding criteria for switching between the static and dynamic regimes. Application of the method to various mechanical problems, such as atomic-scale stick-slip friction, nano-indentation, and mechanics of carbon nanotubes, will be discussed.
5:30 PM - GG4.5
Time-reversible Born-Oppenheimer Molecular Dynamics.
Anders Niklasson 1 2 , Cj Tymczak 1 , Matt Challacombe 1
1 Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 Materials Science and Engineering, Royal Institute of Technology, Stockholm Sweden
Show AbstractTransferMonday 11/26Transfer GG4.6 (4;45 pm) toGG 4.5 (4:30 pm)Time-reversible Born-Oppenheimer Molecular Dynamics. Anders Niklasson
GG5: Poster Session: Multiscale Modeling I
Session Chairs
Maria Caturla
Alison Kubota
Tuesday AM, November 28, 2006
Exhibition Hall D (Hynes)
9:00 PM - GG5.10
Thermodynamic Investigation of Na-induced Embrittlement in Mg-Li Alloys.
Shengjun Zhang 1 2 , Qingyou Han 2 , Zi-Kui Liu 1
1 Department of Material Science and Engineering, The Pennsylvania State University, State College, Pennsylvania, United States, 2 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show Abstract9:00 PM - GG5.11
Fast Algorithms for Studying Polycrystalline Materials.
Mowei Cheng 1 , James Warren 1
1 , National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Show Abstract9:00 PM - GG5.12
Modeling the Deformation of Hierarchical Fractal Structures.
Monica Soare 1 , Catalin Picu 1
1 , Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractMany materials with heterogeneous multiscale fractal structure are found in nature. Examples include biological tissues and bone, some rock such as sandstones, and aero-gels. In such materials the amount of geometrical detail observed in the microstructure increases from scale to scale in a self-similar manner, they lack characteristic length scales and their Hausdorff dimension is smaller than that of the embedding space. For these reasons, using the classical framework of mechanics is tedious or just not tractable as it leads to extremely complicated boundary value problems. In this work, we develop a re-formulation of the concepts of stress and strain and of the balance equations in a manner appropriate for this type of problems. We address boundary value problems on structures with deterministic as well as stochastic fractal microstructures. The talk will review the formulation, the solution procedure and a number of examples used for verification.
9:00 PM - GG5.13
Void Evolution via Coupled Creep, Diffusion and Electromigration in Confined Nano-interconnects.
Wei Lu 1 , Dongchoul Kim 1
1 Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show Abstract9:00 PM - GG5.14
Exploring the 5D Configurational Space of Grain Boundaries in Aluminum: An ab-initio Based Multiscale Analysis.
Liverios Lymperakis 1 , Jörg Neugebauer 1
1 Computational Materials Design department, Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf Germany
Show AbstractGrain boundaries (GB) play a key role in grain growth and recrystallization, and significantly affect the physical and mechanical properties of materials. Therefore, an important topic in materials design is grain boundary engineering, i.e. optimizing the population of GBs with desirable geometry by suitable thermomechanical treatment. To achieve this a deeper understanding and quantification of the interplay between the GB energies with respect to the misorientation of the two grains (3 dimensional phase space) and the inclination of the boundary plane (2 dimensional phase space) are crucial. In this work we combine first principles density functional theory with modified embedded atom method (MEAM) calculations in order to explore the five dimensional phase space of GBs in aluminum. To handle this problem, we have generalized our implicit boundary multiscale schema (IBMS) which had been originally developed and applied to study isolated dislocations [1]. The advantage of this approach is that allows to treat systems consisting of a few 105 atoms with ab-initio accuracy. In this approach the GBs are modeled by periodic supercells. The supercell contains a pair of boundaries to set the total dislocation dipole to zero. The highly distorted material close to the boundary region is described by ab-initio calculations. MEAM calculations are used to describe the elastically strained and bulk like material between the boundaries. The smaller scale ab-initio and the larger scale MEAM calculations are coupled by utilizing implicit boundary conditions. In a first step we explore the three degrees of freedom required to describe the misorientation of the two grains: we calculate symmetrical tilt GBs having the rotational axis along high symmetry directions of the fcc lattice. Next, we focus the low energy misorientation angles and explore the remaining two degrees of freedom associated with the inclination of the boundary plane. For a first time we are able to provide an explicit and on an atomic level calculation of the GB energies associated with the boundary inclination. Finally, based on our data, we are able to understand and interpret recent experimental results on GB occupation [2].[1] L. Lymperakis et al. Phys. Rev. Lett. 93, 196401 (2004)[2] C.-S. Kim et al. Scripta Materialia 54, 1005 (2006)
9:00 PM - GG5.16
The Effects of Small Misorientations in Modeling of FCC Polycrystal Deformations.
Stephen Kuchnicki 1 , Zisu Zhao 2 , Abhishek Bhattarcharyya 3 , Raul Radovitzky 2 , Alberto Cuitino 1
1 Mechanical and Aerospace Engineering, Rutgers University, Piscataway, New Jersey, United States, 2 Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Materials Science and Engineering, Lehigh University, Bethlehem, Pennsylvania, United States
Show Abstract9:00 PM - GG5.17
Inhomogeneous Plastic Deformation in Ni Bicrystal from Polychromatic Microdiffraction and Finite Element Simulations
Rozaliya Barabash 1 2 , J. Pang 1 , G. Ice 1 , W. Liu 3 , O. Barabash 1 , T. Ohashi 4
1 Materials Science and Technology Div., Oak Ridge National Laboratory, Oak Ridge TN, Tennessee, United States, 2 Center for Materials Processing, University of Tennessee, Knoxville, Tennessee, United States, 3 , Advanced Photon Source, Argonne, Illinois, United States, 4 , Kitami Institute of Technology, Kitami Japan
Show Abstract In this study we report the inhomogeneities of plastic deformation in the natural Ni bicrystal after uniaxial pulling. Focused, polychromatic synchrotron X-ray microbeam together with orientation imaging microscopy, scanning electron microscopy, and finite element simulations were used to characterize the physics of geometrically necessary dislocations formation and collective behavior during plastic deformation. Crystallographic orientation of the grain essentially determined it’s plastic response during deformation. Finite element simulations were used to understand the influence of grain orientation and initial structural inhomogeneities on the geometrically necessary dislocations arrangement and distribution. Strain in both grains increases in the vicinity of the coherent boundary.
9:00 PM - GG5.18
Analysis of Disconnections in Tilt Walls.
Sreekanth Akarapu 1 , Hussein Zbib 1
1 Mechanical and Materials Engineering, Washigton State University, Pullman, Washington, United States
Show Abstract9:00 PM - GG5.20
Atomistic Simulations of Exothermic Explosive Reactions in Nanostructured Multilayer Films.
Shijin Zhao 1 , Timothy Germann 1 , Alejandro Strachan 2
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 , Purdue University, West Lafayette, Indiana, United States
Show Abstract9:00 PM - GG5.21
Atomistic Modeling of Silica Nanosprings.
Lilian Davila 1 2 , Valerie Leppert 1 , Eduardo Bringa 2
1 School of Engineering, UC Merced, Merced, California, United States, 2 Chemistry and Materials Science, Lawrence Livermore National Laboratory, Livermore, California, United States
Show AbstractInorganic nanostructures such as nanobelts, nanosprings, nanorings, nanobows, nanohelices, and nanospirals are important morphologies of great recent interest. We have focused our work on the nature and properties of silica nanosprings. Nanosprings tend to have piezoelectric and electrostatic polarization properties that could make them useful in small-scale sensing and micro-system applications. We have performed large-scale molecular dynamics (MD) simulations to study the nature and mechanical properties of silica nanosprings. The behavior of silica nanosprings is studied at the nanoscale using empirical interatomic potentials developed by Feuston and Garofalini. We have centered our studies on nanostructural changes occurring in the material and the correlation between the medium-range order (~10 nm), through the structure factor, and the characteristic ring distribution of this amorphous material. An analysis of the ring distribution and structure factor reveals the changes occurring in amorphous silica nanosprings in this regime. From this analysis, the elastic modulus of the simulated silica nanosprings is derived. Results are extended to experimentally-relevant size scales through multiscale modeling, and compared with recent experimental findings. This investigation contributes to an understanding of the nanoscale nature of silica nanosprings and their properties, influencing structure-dependent applications, with implications for nanotechnology, materials science, and basic science.
9:00 PM - GG5.22
Effect of Surface Roughness on Remelting and Stresses During Splat Solidification.
Guosheng Ye 1 , Rajesh Khare 2 , Michael Gevelber 1 , Donald Wroblewski 2 , Soumendra Basu 1
1 Manufacturing Engineering, Boston University, Brookline, Massachusetts, United States, 2 Aerospace and Mechanical Engineering, Boston University, Boston, Massachusetts, United States
Show AbstractIn order to understand the microstructural evolution in plasma sprayed coatings, the solidification process was modeled using a 2-D FEM model based on enthalpy formation. Studies of the surface of the coatings showed surface roughnesses across multiple length scales. The model was used to examine the effects of the substrate and splat temperatures and the surface roughness features on the onset of remelting of the underlying surface on which the splat solidifies. The surface roughness was found to promote remelting, indicating that it was an important parameter that determines splat solidification. The temperatures of the splat and substrate were consolidated into one non-dimensional parameter that captured the onset of remelting with a non-dimensional remelting point.A fully coupled thermo-mechanical finite element model was also run for a single splat case, to provide more insight stress buildup during solidification. An important result was that the relative size of the surface roughness features, as compared to the splat thickness, is very important. Very large wavelengths compared to splat thickness lead to smaller stresses, since the solidification and the interface are essentially 1-D. Very small wavelengths compared to splat thickness also leads to reduced stresses, since the solidification front quickly becomes 1-D. Only roughness features on the scale of splat thickness are important in providing locations of maximum stress concentration, which are locations of microcrack formation.
9:00 PM - GG5.23
A Finite Temperature Quasicontinuum Method for Multiscale Analysis of Silicon Nanostructures.
Zhi Tang 2 , Narayana Aluru 2
2 Mechanical Science & Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Show Abstract9:00 PM - GG5.24
Application of Geometrically Necessary Dislocations in the Modeling of Hardening Behavior of Crystals Containing Particles
Reza Shahbazian Yassar 1 , Sinisa Mesarovic 2 , David Field 2 , Mark Horstemeyer 1
1 Center for Advanced Vehicular Systems, Mississippi State University, Starkville, Mississippi, United States, 2 School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington, United States
Show AbstractA new approach to model the hardening behavior of crystals containing elastic particles is presented in this paper. This approach utilizes the concept of virtual work to drive the elasto-plastic constitutive relationships. When a crystal contains hardening particles, extra work is required to accommodate the plastic deformation. To calculate this extra work, total deformation can be divided into two steps, in the first step the particles are assumed to deform both plastically and elastically. In the next step, it is assumed that the presence of geometrically necessary (GN) dislocation loops account for the fictitious plastic deformation of particles. The strain energy between GN dislocation loops represents the required extra work. The calculation of this energy results in a new hardening law which includes the size of particles. The model is remarkably simple and falls within the framework of crystal plasticity. Potential validations with light weight alloys are discussed.
9:00 PM - GG5.26
Modeling the Effect of Oxidation and Etching of Silicon Photonic Crystals.
Makhin Thitsa 1 , Haider Ali 1 , Feng Wu 1 , Kurt Peters 1 , Sacharia Albin 1
1 Department of Electrical and Computer Engineering, Old Dominion University, Norfolk, Virginia, United States
Show Abstract9:00 PM - GG5.27
A Thermodynamic-Kinetic Study of Self-Interstitial Aggregation in Ion-Irradiated Silicon using Large-Scale Molecular Dynamics
Sumeet Kapur 1 , Talid Sinno 1
1 Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show Abstract9:00 PM - GG5.28
Modeling of Plastic Deformation Molecular Processes of an high-oriented Crystalline Polymer
Ulmas Gafurov 1
1 , Institute of Nuclear Physics, Tashkent Uzbekistan
Show Abstract9:00 PM - GG5.3
Computer Simulation of Fracture in Aerogels.
Brian Good 1
1 Materials and Structures Division, NASA Glenn Research Center, Cleveland, Ohio, United States
Show Abstract9:00 PM - GG5.31
Ultrafast Laser Induced Void Nucleation and Coalescence in Thin Cu Films.
Ben Torralva 1 , George Gilmer 1 , Michael Armstrong 1 , Steven Yalisove 2 , Jonathan Crowhurst 1 , Joseph Zaug 1 , Hector Lorenzana 1
1 Chemistry and Materials Science, Lawrence Livermore National Laboratory, Livermore, California, United States, 2 Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show Abstract9:00 PM - GG5.4
Atomistic Investigation of Stimulated Dislocation Nucleation from Crack Tips.
Erik Bitzek 1 , Peter Gumbsch 1 2
1 izbs, University of Karlsruhe, Karlsruhe Germany, 2 , Fraunhofer Institut Werkstoffmechanik IWM, Freiburg Germany
Show AbstractOne of the great unknowns in the description of semi-brittle fracture is the origin of the dislocations near the crack tip. Preexisting dislocations are known to influence the low temperature fracture toughness and the brittle-to-ductile-transition (BDT). Experiments on single crystalline silicon show that dislocation nucleation can be a highly inhomogeneous process. A single dislocation at the crack front can stimulate the emission of other dislocations in an avalanche-type multiplication process (Scandian et al., phys. stat. sol. (a) 171, 67, 1999). The detailed mechanisms of such stimulated dislocation multiplication are still largely unknown and the subject of this paper.
Stimulated dislocation nucleation is studied by three dimensional atomistic simulation of a dislocation encounter with an [001](110) mode I crack in nickel. Several different types of dislocations and relative orientations are studied since the events during the interaction with the crack of course depend on the character and glide plane of the dislocation. Frequently observed processes include the stimulated emission of dislocation loops and cross slip of the incoming dislocation. The simulation results are analyzed with respect to the driving forces on the dislocations caused by the crack tip stress field.
9:00 PM - GG5.5
Undissociated Screw Dislocation in Group IV Semiconductors: Glide or Shuffle-set?
Cai-Zhuang Wang 1 , Ju Li 2 , Kai-Ming Ho 1 , Sidney Yip 3
1 Ames Laboratory and Department of Physics, Iowa State University, Ames, Iowa, United States, 2 Department of Materials Science and Engineering, Ohio State University, Columbus, Ohio, United States, 3 Department of Nuclear Science and Engineering and Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show Abstract9:00 PM - GG5.6
Dislocation Dynamics Simulations of High Strain Rate Deformation of FCC Cu.
Zhiqiang Wang 1 , Richard LeSar 1
1 , Los Alamos National Lab, Los Alamos, New Mexico, United States
Show Abstract9:00 PM - GG5.7
The Effects of Various Segregated Solutes on the Embrittlement of bcc Fe Grain Boundaries by the First-Principles Calculation.
Masatake Yamaguchi 1 , Yutaka Nishiyama 2 , Motoyuki Shiga 1 , Hideo Kaburaki 1
1 Center for Computational Science and e-systems, Japan Atomic Energy Agency, Tokai, Ibaraki, Japan, 2 Department of Reactor Safety Research, Japan Atomic Energy Agency, Tokai, Ibaraki, Japan
Show Abstract9:00 PM - GG5.8
First-Principles Calculations of the Adhesive and Mechanical Properties of Alumina/Ni and Alumina/Cu Interfaces and the Development of Interfacial Interatomic Potentials.
Masanori Kohyama 1 , Siqi Shi 1 , Shingo Tanaka 1 , Rui Yang 2 , Sergey Dmitriev 3 , Nobuhiro Yoshikawa 3
1 Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology, Ikeda, Osaka Japan, 2 Department of Computer Science, Australian National University, Canberra, Australian Capital Territory, Australia, 3 Institute of Industrial Science, The University of Tokyo, Meguro-ku, Tokyo, Japan
Show AbstractAlumina/metal interfaces are of great importance in various applications such as thermal barrier coatings, electronic packaging, composites and so on. It is of great importance to develop the multiscale modeling scheme for such systems. However, oxide/metal interfaces have complicated bonding nature, which cannot be modeled by simple functional forms. And for alumina/metal systems, recent studies have indicated that the interface stoichiometry greatly dominates the adhesive properties. Namely, the interface between an O-terminated (O-rich) surface and a metal and that between an Al-terminated (stoichiometric) surface and a metal have quite different characters to each other. Thus first of all, we have to understand the adhesive and mechanical properties of alumina/metal interfaces, depending on the interface stoichiometry and configurations, by first-principles calculations before the development of multiscale schemes. In this paper, first, we perform ab initio calculations of Al2O3(0001)/Ni(111) interfaces. We deal with configurations with different rigid-body translations (RBT) for each interface stoichiometry, and clarify overall features of the energies, configurations, and bonding nature of the Al2O3(0001)/Ni(111) system, which are compared with our recent results of the Al2O3(0001)/Cu(111) system [1,2]. We have observed strong Ni-O interactions with both ionic and covalent characters at the O-terminated interfaces, in contrast to relatively weak electrostatic and Ni-Al covalent interactions at the Al-terminated interfaces. Second, we apply ab initio tensile tests [3] to the Al2O3/Ni interfaces, which can clarify the intrinsic tensile strength and the process of deformation and fracture according to the behavior of valence electrons. Results are also compared with our recent results of the Al2O3/Cu system [1,2]. We have observed that the interface stoichiometry, RBT and metallic species have significant effects on the strength and mechanical behavior. Third, we discuss how to construct effective interfacial interatomic potentials for large-scale simulations through ab initio database of the configurations, adhesive energies and tensile tests of alumina/metal interfaces. We have constructed interfacial interatomic potentials of the Al2O3/Ni system for classical molecular-dynamics including the effects of interface stoichiometry, following recent potential development for the Al2O3/Cu system [4].This study was supported by New Energy and Industrial Development Organization (NEDO) as Nano-Coating Project.[1] R. Yang, S. Tanaka and M. Kohyama, Phil. Mag. Lett. 84, 425 (2004); Phil. Mag. 85, 2961 (2005).[2] S. Tanaka et al., Mater. Trans. 45, 1973 (2004); Phil. Mag. (2006) in press.[3] M. Kohyama, Phys. Rev. B 65, 184107 (2002); G.-H. Lu et al., ibid. 69, 134106 (2004).[4] S.V. Dmitriev et al., Acta Mater. 52, 1959 (2004); Comp. Mater. Sci. 36, 281 (2006).
9:00 PM - GG5.9
Simulation of Dislocation Relaxation in FCC Metals.
Yoshiaki Kogure 1 , Kei Sakieda 1 , Toshio Kosugi 1
1 Environmental Science, Teikyo University of Science and Technology, Uenohara, Yamanashi, Japan
Show AbstractConfiguration and Dynamics of dislocations in copper and aluminum crystals have been studied by means of the molecular dynamics simulation and the elasticity calculation. The embedded atom method (EAM) potential is used express the atomic interaction in the molecular dynamics simulation. It is seen that the dislocation is split to partials, and the width of the splitting of a dislocation in copper is larger than that in aluminum. The Peierls potentials for edge and screw dislocations in those materials are determined by monitoring the motion of dislocation position and the change of the kinetic energy. As the total mechanical energy is conserved during the molecular dynamics simulation, the kinetic energy shows a small dip when a dislocation pass the top of Peierls potential. The magnitude of the dip corresponds to the height of the Peierls potential. By using the determined value of the Peierls potential the activation energy for the mechanical relaxation by kink-pair formation mechanism is calculated based on the elasticity model. The results are compared with the results of internal friction measurements on aluminum. Four relaxation peaks have been observed in aluminum, and two peaks observed at low-temperatures, 10 K and 35 K, are well developed in high-purity specimens. The dislocation configurations and the relaxation mechanisms responsible to these peaks are discussed.
9:00 PM - GG5: Poster1
GG5.2 Transferred to GG12.9/EE8.9
Show Abstract
Symposium Organizers
Ram Devanathan Pacific Northwest National Laboratory
Maria Jose Caturla Universitat d’Alacant
Alison Kubota Lawrence Livermore National Laboratory
Alain Chartier CEA-Saclay
Simon Phillpot University of Florida
GG6: Hybrid Methods
Session Chairs
Tuesday AM, November 28, 2006
Constitution B (Sheraton)
9:00 AM - **GG6.1
Embedding Quantum-Mechanics in an Interatomic Potential Simulation Using Local Energies.
Noam Bernstein 1 , Gabor Csanyi 2
1 Center for Computational Materials Science, Naval Research Lab, Washington, District of Columbia, United States, 2 Cavendish Laboratory, University of Cambridge, Cambridge United Kingdom
Show AbstractAtomistic simulations that use quantum-mechanical total-energy modelsprovide high accuracy and reliability at the price of computationalexpense. Classical approximations such as interatomic potentials aremuch faster, but less transferable. We couple the two approachesconcurrently, to describe part of the system quantum-mechanicallyand part with interatomic potentials, using a weighted sum of atomicenergies. With tight-binding as a model quantum-mechanical method weshow that a localized quantum-mechanical atomic energy can be defined,and that this energy is physically meaningful. The localizationmakes it possible to compute an atomic energy by considering onlya cluster of nearby atoms, and in an insulator the energy convergesquickly as a function of the cluster radius. The atomic energy istherefore independent of atomic displacements far from the atom.The analogous definition for the classical method is straightforward.These definitions enable us to compute a well defined total energyfor the hybrid system with small and controllable errors caused by theboundaries of the QM region. We show that we can efficiently computethe derivatives of the QM contributions to the energy, despite the lackof a Hellmann-Feynman theorem. We can therefore define forces thatare derivatives of the total energy, enabling an energy conservingsimulation for a fixed QM region. We test the method on severalsystems, including a vacancy and oxygen interstitials in silicon.We discuss convergence with respect to various method parameters,and the effects of moving the QM region during the dynamics.
9:30 AM - GG6.2
Interface Conditions for Coupled Atomistic and Continuum Models of Solids for Dynamics Problems at Finite Temperature.
Jerry Yang 1 , Weinan E 1 , Xiantao Li 2
1 Program in Applied and Computational Mathematics, Princeton University, Princeton, New Jersey, United States, 2 Math Department, Penn State University, University Park, Pennsylvania, United States
Show Abstract10:00 AM - GG6.4
Coarse Molecular-Dynamics Determination of the Onset ofStructural Transitions in Condensed Matter
Miguel Amat 1 , Ioannis Kevrekidis 2 , Dimitrios Maroudas 1
1 Department of Chemical Engineering, University of Massachusetts, Amherst, Massachusetts, United States, 2 Department of Chemical Engineering and Program in Applied and Computational Mathematics, Princeton University, Princeton, New Jersey, United States
Show AbstractAccurate determination of the onset of structural transitions in complex physical systems is of crucial importance in materials science. As direct access to such physical responses is typically difficult to attain experimentally, computational techniques such as molecular dynamics (MD) have become powerful tools for probing the underlying atomic-scale dynamics and determining the transition onset; however, computing through MD the evolution of all of the atomic coordinates over coarse time scales poses a severe limitation to the method. In the coarse molecular-dynamics (CMD) method, coarse-grained information is estimated on-the-fly from many short and appropriately initialized independent replica MD simulations. This information can then be used to identify transition points in the physical behavior of the complex systems under consideration. The method is based on the description of the evolution of the probability density, P(ψ,t), as approximated by the Fokker-Planck equation, where ψ(t) is an appropriate coarse-grained observable that describes the state of the physical system.In this presentation, we demonstrate the capability of CMD to determine the onset of structural transitions in condensed matter focusing on the thermodynamic melting of a crystalline silicon model, as well as on stress-induced structural transitions (e.g., bcc-to-hcp) under hydrostatic or uniaxial loading. Specifically, we reconstruct the underlying effective free energy landscape for the melting transition and calculate the effective free energy difference between the molten and solid states as a function of temperature; in conjunction with a phase coexistence criterion, this leads to an efficient and accurate determination of the melting transition onset. In addition, we obtain the effective free energy landscape underlying phase transformations in crystals under specified mechanical loading and its relation to the inherent stability of the corresponding crystalline phases, which is used to determine the onset of the phase transformations. We demonstrate that the CMD approach is quite general and may be helpful in determining other important types of structural-transition onsets in condensed matter, including order-to-disorder (e.g., solid-state amorphization) and disorder-to-order (e.g., crystallization) transitions; selecting appropriate coarse-grained variables is crucial to the success of this approach.
10:15 AM - GG6.5
Atomistic Calculation of Free Energy.
Shenyang Hu 1 , Michael Baskes 1 , Lawrence Pratt 1 , Steven Valone 1
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show Abstract10:30 AM - GG6:Hybrid
BREAK
GG7/JJ2: Joint Session: Radiation Damage
Session Chairs
Maria Caturla
Sergei Dudarev
Tuesday PM, November 28, 2006
Constitution B (Sheraton)
11:00 AM - GG7.1/JJ2.1
Modeling Radiation Damage Evolution in Fe-Cr Alloys.
Brian Wirth 1 , Hyon-Jee Lee 1 , Jae-Hyoek Shim 2 , Kevin Wong 1
1 Nuclear Engineering, University of California, Berkeley, Berkeley, California, United States, 2 , Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show Abstract11:15 AM - GG7.2/JJ2.2
Multiscale Approach for Understanding Advanced Materials.
Wolfgang Hoffelner 1 , Maria Samaras 1 , Manuel Pouchon 1 , Jia Chao Chen 1 , Maximo Victoria 1 , Annick Froideval 1 , Botond Bako 1 , Roberto Iglesias 1
1 , Paul Scherrer Institiute, Villigen Switzerland
Show Abstract11:30 AM - GG7.3/JJ2.3
From Interstitial Clusters to Interstitial Loops in Iron: A Multiscale Approach Based on First Principles.
Mihai-Cosmin Marinica 1 , Francois Willaime 1
1 SRMP, CEA/Saclay, Gif-sur-Yvette France
Show AbstractInterstitial-type defects formed by the clustering of self-interstitials produced under irradiation have rather peculiar properties in alpha-iron. Very small interstitial clusters or loops are formed of <110> dumbbells, whereas larger clusters have either a <111> or a <100> orientation. This contrasts with other BCC metals where they are predominantly <111>. The competition between these different orientations raises the question of their relative stabilities as function of size and temperature, and of the transformation mechanism from <111> to <100> orientations observed experimentally. Their mobilities are also a key issue in the interpretation of experiments. We have addressed these questions by combining first principles and empirical potential approaches. We have determined the stability and mobility of small self-interstitial defects by ab-initio calculations performed on cells containing up to 250 atoms using the SIESTA code. We then used these results to fit and validate a new semi-empirical potential for iron. This potential allows to access dynamical properties and larger sizes. (i) For small clusters, we performed a systematic search of possible configurations, including non-parallel ones, and of migration/rotation mechanisms. (ii) Using lattice-dynamics we studied the vibrational properties of small clusters: low frequency modes have been evidenced in <111>-type defects; the associated large vibrational entropy is shown to have a significant effect on the relative stabilities at finite temperature. (iii) In connection with experiments we have investigated the properties of loops with up to 1000 defects: the relative stabilities as function of Burgers vector, their migration energies, and the loop-loop and loop-surface interactions. We discuss the dependence on the potential of these results.
11:45 AM - GG7.4/JJ2.4
Role of Trapping Impurities on He Desorption and Clustering in Irradiated a-Fe.
Christophe Ortiz 1 , María José Caturla 1 , François Willaime 2 , Chu Chun Fu 2
1 Departamento de Física Aplicada, University of Alicante, E-03690 Alicante, Alicante, Spain, 2 Service de Recherches Métallurgiques, CEA/Saclay, F-91191 Gif-sur-Yvette France
Show AbstractIt is well-known that impurities affect the migration of intrinsic point defects in metals. For instance, carbon is a common impurity in Fe that significantly retards diffusion of vacancies. Under fusion irradiation conditions, high levels of He are produced by transmutation reactions. This element strongly interacts with vacancies produced during irradiation and agglomerate into stable He-vacancy clusters that can deteriorate the mechanical properties of the material. A physically-based model accounting for the interactions between He, point defects (interstitials and vacancies) and trapping impurities is therefore necessary to understand and predict damage evolution in Fe.We have used a multi-scale approach to predict the evolution of He in the presence of impurities in irradiated Fe. Density Functional Theory (DFT) calculations were performed to investigate the migration mechanisms and to determine the activation energies of the different atomistic processes. The influence of impurities - such as carbon - on the binding energies of small He-vacancy clusters was also studied. Using the information obtained by DFT a physically-based model was developed and implemented in a kinetic Monte Carlo (kMC) code to follow the evolution of He in Fe. In addition, a model based on the rate theory (RT) was developed in order to achieve larger simulation times and volumes. Results obtained with this model, which is based on a mean field approximation are compared to those obtained with kMC. Using this multiscale approach, the simulation results are used to interpret the different stages of thermal He desorption experiments and to determine the predominant migration mechanism. The influence of impurities which affect the diffusion of point defects or modify the binding energies of He-vacancy type clusters is also studied.
12:00 PM - **GG7.5/JJ2.5
Multiscale Modelling of bcc-Fe Based Alloys for Nuclear Applications.
Lorenzo Malerba 1
1 RMO, SCK-CEN, Mol Belgium
Show AbstractUnderstanding the basic mechanisms that determine microstructure changesin neutron irradiated steels is vital for a safe lifetime management of existing nuclear reactors and a safe design of future nuclear options. Low-alloyed ferritic steels containing Cu, Ni, Mn and Si as principal solute atoms are used as structural materials for current reactor vessels, while high-Cr ferritic-martensitic steels will be used in future nuclear options. The microstructural evolution under irradiation in alloys is decided by the interplay between defect formation and thermodynamic driving forces, together determining the appearance of phase transformations (precipitation, segregation, ...) and favouring or delaying the nucleation and growth of point-defect clusters, their diffusion and their mutual recombination or removal at sinks. A reliable description of the production, evolution and accumulation of radiation damage must therefore start from the atomic level and requires being able to describe multicomponent systems for timescales ranging from few picoseconds to years. This goal demands firstly the fabrication of interatomic potentials for alloys that must be both consistent with the thermodynamic properties of the system and capable of reproducing correctly the characteristic solute-point defect interactions, versus ab initio or experimental data. Secondly the performance of extensive molecular dynamics (MD) simulations, to grasp the main mechanisms of defect production, diffusion, mutual interaction, and interaction with solute atoms and impurities. Thirdly, the development of simulation tools capable of describing the microstructure evolution beyond the timeframe and lengthscale of MD, while reproducing as much as possible the atomic-level origin of the mechanisms governing the evolution of the system, including phase changes.In this presentation the results of recent efforts made in this direction in the case of Fe-Cu and Fe-Cr alloys, as basic model alloys for the description of steels of technological relevance, are highlighted. In particular, advanced techniques to fit interatomic potentials consistent with thermodynamics are proposed and the results of their application to the mentioned alloys are presented. The results of the use of advanced potentials to study the effect of high concentrations of solute atoms on self-interstitial cluster mobility and their correlation to changes in macroscopic properties, such as swelling, are summarised. And the development of advanced methods, based on the use of artificial intelligence, to improve both the physical reliability and the computational efficiency of kinetic Monte Carlo codes for the study of point-defect clustering and phase changes beyond the scale of MD, is reported. These recent progresses bear the promise of being able, in the near future, of producing reliable tools for the description of the microstructure evolution of realistic model alloys under irradiation.
12:30 PM - GG7.6/JJ2.6
Ab Initio and Kinetic Rate Theory Modeling of 316SS with Oversized Solute Additions on Radiation-Induced Segregation.
Micah Hackett 1 , Gary Was 1
1 Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractDeleterious effects of radiation in nuclear reactor systems cause material degradation and the potential for component failure. Radiation damage is fundamentally due to freely migrating point defects produced in collision cascades. A reduction in the freely migrating point defect population should, then, reduce radiation damage and increase component lifetime. The addition of oversized solute atoms such as Zr or Hf to 316SS, a common structural material in reactors, is expected to reduce point defect population through a trapping mechanism that enhances recombination. The mechanism, however, requires a strong binding energy between the oversized solute atom and vacancies in order for the mechanism to significantly reduce the defect population. Experimental measurements of this binding energy are unavailable, but can be determined with atomistic calculations. Ab initio methods are used here to determine binding energies and atomic volumes of either Hf or Zr oversized solutes with vacancies in a face-centered cubic Fe matrix. The binding energies are then used to parameterize a kinetic rate-theory model, which is used here to calculate radiation-induced segregation (RIS). The calculated values of RIS are then compared to experimental measurements to benchmark the calculations and offer insight into the proposed point defect trapping mechanism.
12:45 PM - GG7.7/JJ2.7
Behaviour of Irradiated hcp Zirconium Coupling Molecular Dynamics and Monte Carlo Simulations.
Cristina Arevalo 1 , Maria Caturla 1 , José Perlado 2
1 Instituto de Fusión Nuclear, UPM, madrid Spain, 2 Física Aplicada, Universidad de Alicante, Alicante Spain
Show AbstractRadiation damage in hexagonal-close-packed (hcp) metals is different from face-centred cubic (fcc) or body-centred (bcc) metals. The experimental study of point defect clustering in hcp metals is dominated by a consideration of the geometry of the hcp lattice and lattice parameters ratio (c/a). Because of this crystallographic anisotropy, defect anisotropic diffusion is expected, that is jump distances and jump rates depend on jump directions. We have studied irradiation of hcp α-Zirconium under different conditions with a kinetic Monte Carlo model. The initial cascade damage from Molecular Dynamics simulations produced by recoils from 10keV to 25 keV energies at 600K which is the operation temperature of the reactor have been followed for times of hours. The evolution of the microstructure under irradiation conditions of dose rate of 10-6 dpa/s and 600K has been studied until a final dose of 0.5 dpa. Using these calculations as the starting point we have compared them studying the influence of several parameters as dose rate, simulation box, grain size, bias and mobility of interstitials in the accumulation results.
GG8: Defects and Diffusion
Session Chairs
Tuesday PM, November 28, 2006
Constitution B (Sheraton)
2:30 PM - **GG8.1
Modeling of Atomistic Processes in Semiconductors:from Defect Signatures to a Hierarchy of Annealing Mechanisms
Michel Bockstedte 1 2
1 Fisica de Materiales, Universidad de Pais Vasco, San Sebastian Spain, 2 Lst. Theor. Festkörperphysik, Universität Erlangen-Nürnberg, Erlangen Germany
Show AbstractPoint defects in semiconductors are pivotal for the materials application in electronic devices. Beneficial substitutional impurities enable as dopants the p-type or n-type conduction. Well-defined grown-in or implanted dopant profiles are the corner stones of device engineering. Un-intentional impurity-related deep defect centers or intrinsic defects counteract these efforts. They compensate dopants and as defects mediating the dopant diffusion degrade dopant profiles. Albeit unwanted and at the same time unavoidable, they play a vital role in the dopant activation and the annealing of process-induced defects. Unraveling the underlying and fundamental mechanisms of the defect diffusion and kinetics is a focus of both experimental and theoretical research. The modeling of atomistic processes in semiconductors aims to derive a microscopic picture of the activation and diffusion of dopants based on ab initio methods combined with a thermodynamic analysis and tools to investigate rate processes. Commencing with the most fundamental intrinsic defects and typical dopants, the modeling investigates thenature of the defects and their complexes. The identification of experimental defect centers by means of calculated defect signatures, such as hyperfine tensors and local vibrational modes, establishes an important link between the predictions of the modeling and their verification in experiments. The analysis of the diffusion mechanisms of intrinsic defects forms the corner stone of the understanding of the vacancy and interstitial mediated dopant diffusion and the annealing kinetics including the recombination of Frenkel pairs and the formation of thermally stable vacancy or interstitial aggregates. Via calculated energy barriers a hierarchy of annealing mechanisms can be established that enables the interpretation of annealing experiments. Effects of the charge state of the key defects determined e.g. by the doping conditions are central to the modeling and have important bearing on the interpretation of experiments.The methods utilized in the modeling will be outlined and applied to the compound semiconductor silicon carbide. In SiC, owing to the compound nature of the material, two different kind of vacancies and interstitials exists that proof to possess distinct physical properties with profound consequences for the defect kinetics and dopant diffusion. The identification of the silicon and carbon vacancies, the divacancy and the carbon vacancy-antisite complex that evolve from the metastability of the silicon vacancy provide the basis for the discussion of the annealing hierarchy of vacancies and interstitials. The theoretically predicted carbon aggregation is manifested by the identification of the tri-carbon antisite. The different role played by silicon and carbon interstitials in the borondiffusion is highlighted.
3:00 PM - GG8.2
Microscopic versus Macroscopic Activation Barriers for Diffusion in Solid Solutions.
Ramanathan Krishnamurthy 3 , David Srolovitz 1 , Mikhail Mendelev 2
3 Technology & Solutions Division, Caterpillar Inc., Peoria, Illinois, United States, 1 Department of Mechanical Engineering and Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton, New Jersey, United States, 2 , Ames Laboratory, Ames, Iowa, United States
Show Abstract3:15 PM - GG8.3
Oxygen Vacancies and Interstitials in Si-HfO2-Si Stacks: Formation and Diffusion.
Chunguang Tang 1 , Rampi Ramprasad 1
1 Department of Chemical, Materials and Biomolecular Engineering, university of connecticut, Storrs, Connecticut, United States
Show AbstractAn important issue in microelectronic devices is the gate dielectric degradation with time, which may be due to defect diffusion through the bulk dielectric and across interfaces. Because of the importance of HfO2 as the potential substitute of SiO2, it is of great interest to explore the diffusion of various defects in HfO2 and across Si-HfO2 interfaces. In this contribution, building on prior insights concerning isolated point defects and perfect Si-HfO2 interfaces, we present a multi-scale study of the diffusion of oxygen vacancies and interstitials in a Si-HfO2-Si stack with abrupt oxygen-terminated interfaces . Our strategy is to perform first principles calculations of oxygen vacancy and interstitial formation energies in bulk HfO2 and at the Si-HfO2 interface, and the transition barriers for defect diffusion between adjacent minimum energy sites. A Monte Carlo method is then employed to model the long range diffusion of oxygen defects in bulk HfO2 and oxygen atoms across the interface. Our results indicate that the oxygen vacancies, initially uniformly distributed in bulk HfO2, tend to gather in the interfacial area, and that oxygen atoms from HfO2 diffuse into Si, resulting in the formation of Hf-Si bonds and a SiOx layer.
3:30 PM - GG8.4
Crowdions in Bcc Transition Metals: From Electronic Structure to Mechanical Motion of Defects.
Duc Nguyen-Manh 1 , S. Dudarev 1
1 Theory and Modelling, EURATOM/UKAEA Fusion Association, Abingdon United Kingdom
Show Abstract3:45 PM - GG8.5
Simulation of Self-healing Oxide Coatings.
Ivan Lazic 1 , Barend Thijsse 1
1 Materials Science and Engineering, Delft University of Technology, Delft Netherlands
Show Abstract4:00 PM - GG8:Diffusion
BREAK
GG9: Microstructure Evolution
Session Chairs
Tuesday PM, November 28, 2006
Constitution B (Sheraton)
4:30 PM - **GG9.1
Multiscale Simulation of Crystal Growth from Aqueous Solution.
Julian Gale 1 , Stefano Piana 1 , Franca Jones 1
1 Nanochemistry Research Institute, Curtin University of Technology, Perth, Western Australia, Australia
Show Abstract5:00 PM - GG9.2
The Linkage of Molecular Dynamics to Phase Field Modeling of Solidification.
James Belak 1 , James Glosli 1 , Patrice Turchi 1 , Mehul Patel 1 , Fred Streitz 1
1 Physics, Lawrence Livermore National Laboratory, Livermore, California, United States
Show AbstractPhase-field modeling has proven an extremely powerful tool for qualitative understanding of solidification and grain coarsening [1]. Quantitative understanding, however, requires a tighter coupling to the underlying atomistic dynamics. In the case of pressure-driven solidification, large parallel computers have enabled MD simulations of sufficient length and time to observe the formation of realistic microstructure [2]. Here, we use these MD simulations to extract information necessary to validate our phase-field models. We calculate the coarse-grained phase-field order parameter from the local atomic coordinates and use the molecular dynamics to drive the evolution of the phase field. The relevant interfacial mobility and energy follows directly from the atomistic dynamics.[1] C.E. Krill III and L.-Q. Chen, “Computer simulation of 3-D grain growth using a phase field model,” Acta Mater., 50, 3057-3073 (2002).[2] F. H. Streitz, J. N. Glosli, and M. V. Patel, “Beyond Finite-Size Scaling in Solidification Simulations,” Phys. Rev. Lett. 96, 225701 (2006).*This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.
5:15 PM - GG9.3
Phase-field Modeling of Void Evolution under Elastic-plastic Deformation.
Shenyang Hu 1 , Yulan Li 1 2 , Michael Baskes 1
1 MST, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 Dept. Mater. Sci. & Eng.MST, Pennsylvania State University, State College, Pennsylvania, United States
Show Abstract5:30 PM - GG9.4
Phase-field Modeling of Surfactant Self-assembly.
Ming Tang 1 , W. Craig Carter 1
1 Dept. of Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractA phase-field model of surfactant-water binary systems was formulated to study surfactant self-assembly processes. The model explicitly incorporates the spontaneous curvature of surfactants, an important parameter that determines the stability of different self-assembled structures. In addition to the interfacial width, a second characteristic length scale, i.e., the surfactant micelle size, is introduced into the model by including negative gradient coefficients and high-order gradient terms in the free energy functional. We show that as a first-order approximation, our model is equivalent to Helfrich's classical formulation of elastic membrane energy. The model is used to study the kinetic pathways of surfactant phase transitions and the effects of external defects such as liquid-air interface and patterned substrates on surfactant self-assembly.
Symposium Organizers
Ram Devanathan Pacific Northwest National Laboratory
Maria Jose Caturla Universitat d’Alacant
Alison Kubota Lawrence Livermore National Laboratory
Alain Chartier CEA-Saclay
Simon Phillpot University of Florida
GG10: Soft Condensed Matter
Session Chairs
Wednesday AM, November 29, 2006
Constitution B (Sheraton)
9:00 AM - **GG10.1
Systematic Coarse-graining and Multiscale Modeling of Soft Matter.
Gregory Voth 1
1 Dept. of Chemistry, and Center for Biophysical Modeling and Simulation, University of Utah, Salt Lake City, Utah, United States
Show AbstractA multiscale theoretical and computational methodology will be presented for describing liquid state, biomolecular, and nanoparticle systems across multiple length- and time-scales. The approach provides an interface between atomistic molecular simulations, mesoscale dynamics, and continuum mechanics. The underlying methodology couples atomistic-level simulations with mesoscale simulations which, in turn, can be bridged to continuum-level modeling where necessary. A new and systematic multiscale coarse-graining strategy for linking the atomistic-scale interactions to the mesoscale will be the primary focus of the presentation. Applications of the overall methodology will be given.
9:30 AM - GG10.2
Coarse-Grained Modeling of the Mechanical Properties of Entangled Polymer Systems
Brian Pasquini 1 , Fernando Escobedo 1 , Yong Joo 1
1 Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York, United States
Show AbstractA coarse-grained simulation approach is used to help understand entangled polymer interactions during tensile strain experiments. The mechanical response of entangled polymer melts is found by deforming a network of entanglement points while calculating the changes to the minimum energy configuration. The entanglement points serve as tetra-functional end points of polymer segments. One method from literature (Smith and Termonia) formulates the free energy of an entanglement network based exclusively on neighboring entanglement points and uses temporary bonds which break as the simulation progresses. Another method (Terzis, Theodorou and Stroeks) treats the coarse-grained units as delocalized polymer density clouds, and formulates an expression for the free energy based on local polymer density. Both methods have been studied and compared to a simple test system, and each give rise to a tensile strain response that exhibits a region of strain softening followed by strain hardening due to the finite extensibility of the chain segments. The method involving polymer density clouds allows for the inherent relaxation of the entanglement network to the correct polymer density but involves numerically integrating an expression for free energy, which is computationally time consuming. The parameters of spacial discretization are studied to evaluate the necessary precision to accurately approximate these integrals. The density cloud method is used to simulate repetitive strain experiments of entanglement networks for validation of numerical stability and simulation size.The coarse-grained nature of this model allows for the possibility to simulate microstructure effects on mechanical properties in materials such as spider silk.
9:45 AM - GG10.3
Multi-scale Simulation of the Formation and Fracture of Highly Cross-linked Network Polymers.
Chandrashekar Shankar 1 , John Kieffer 1
1 Material Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show Abstract10:00 AM - GG10.4
A Method for the Determination of Anisotropic Macromolecular Configuration Properties from Atomistic Oligomeric Simulations.
Frederick Bernardin 1 , Gregory Rutledge 1
1 Chemical Engineering, MIT, Cambridge, Massachusetts, United States
Show Abstract10:15 AM - GG10.5
First-principles Based Hierarchical Multi-scale Modeling of Tropocollagen Molecules.
Markus Buehler 1
1 Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractNew Presentation Time and Paper NumberGG10.59:15 - 9;30 amFirst-principles Based Hierarchical Multi-scale Modeling of Tropocollagen Molecules. Markus J. Buehler
10:30 AM - GG10.6
Explaining the Broadband Absorbance of Melanin.
Jennifer Riesz 1 , Ben Powell 1 , Ross McKenzie 1 , Paul Meredith 1
1 Physics, University of Queensland, Brisbane, Queensland, Australia
Show AbstractNew Presentation Time and Paper NumberGG10.69:30 - 9:45 amExplaining the Broadband Absorbance of Melanin. Jennifer Jean Riesz
10:45 AM - GG10: Polymers
BREAK
GG11: Dislocation Dynamics
Session Chairs
Wednesday PM, November 29, 2006
Constitution B (Sheraton)
11:30 AM - **GG11.1
Multiscale Modeling of Dynamic Strain Aging in Al-Mg Alloys.
William Curtin 1
1 , Brown University, Providence, Rhode Island, United States
Show Abstract12:00 PM - GG11.2
Dislocation Patterns: Short- vs. Long-range Stresses.
Ladislas Kubin 1 , Diego Gomez-Garcia 2 , Devincre Benoit 1
1 CNRS-ONERA, LEM, Chatillon Cedex France, 2 Departamento de Física de la Materia Condensada, Universidad de Sevilla, Sevilla Spain
Show Abstract12:15 PM - GG11.3
Parallel Discrete Dislocation Dynamics: Stress Distribution in Polycrystalline Metallic Film.
Jochen Senger 1 , Werner Augustin 1 , Daniel Weygand 1 , Oliver Kraft 1 2 , Vincent Heuveline 3 , Peter Gumbsch 1 4
1 IZBS, University of Karlsruhe (TH), Karlsruhe Germany, 2 IMF II, Forschungszentrum Karlsruhe, Karlsruhe Germany, 3 IAM II / RZ, University of Karlsruhe (TH), Karlsruhe Germany, 4 IWM, Fraunhofer Institut für Werkstoffmechanik, Freiburg Germany
Show Abstract12:30 PM - GG11.4
Strain Hardening Simulations Using Dislocation Dynamics.
Tom Arsenlis 1 , Vasily Bulatov 1 , Meijie Tang 1 , Moono Rhee 1 , Wei Cai 2 , Tomas Oppelstrup 1 , Gregg Hommes 1
1 , Lawrence Livermore National Laboratory, Livermore, California, United States, 2 Department of Mechanical Engineering, Stanford University, Palo Alto, California, United States
Show AbstractWith the current power of parallel computing architectures, and the development of dislocation dynamics methods capable of using them, simulation of strain hardening are becoming feasible. Dislocation dynamics in this setting can be used as an ab initio plasticity simulation tool capable of validating or invalidating phenomenological continuum plasticity models often used to simulate the behavior of single and poly- crystalline materials. Using the ParaDiS code developed at LLNL, the origins of strength and strain hardening are investigated for body-centered cubic (bcc) crystals, and a new topological element of dislocation microstructure, named the multi-junction, is discovered and observed to play an important role in the dislocation density evolution and subsequent hardening of bcc crystals. The multi-junctions result from ternary dislocation reactions and their existence has been confirmed in TEM investigations. Once formed, multi-junctions are nearly indestructible and act as regenerative sources of dislocation multiplication. Phenomenological continuum plasticity models have included binary reactions but have not considered the importance of ternary reactions to date. These dislocation dynamics simulations that these continuum plasticity models must be extended to accurately capture the physical processes occurring at micron length scales.
12:45 PM - GG11.5
The Copper Hugoniot Curve: Molecular Dynamics Simulations and Comparison to Experiments.
Diana Yi 1 , Alison Kubota 1 , David Reisman 1 , Wilhelm Wolfer 1
1 , Lawrence Livermore National Lab, Livermore, California, United States
Show AbstractGG12/EE8: Joint Session: Multiscale Approaches in Modeling Size Effects in Deformation
Session Chairs
William Curtin
Lyle Levine
Wednesday PM, November 29, 2006
Constitution B (Sheraton)
2:45 PM - GG12.2/EE8.2
Dislocation Density Based Modelling of Plastic Flow and Size-effects in Single Crystals.
Thomas Hochrainer 1 , Peter Gumbsch 1 2
1 Institut fuer Zuverlaessigkeit von Bauteilen und Systemen, izbs, Universitaet Karlsruhe (TH), Karlsruhe Germany, 2 , Fraunhofer Institut fuer Werkstoffmechanik, IWM, Freiburg Germany
Show AbstractWe recently proposed an advanced tensorial dislocation density measure which makes the distinction between geometrically necessary and statistically stored dislocations dispensable. Within this object all dislocations are in equal measure represented as line like objects which evolve accordingly. Based on this measure we show at simple example problems how line-tension effects, assumptions on Taylor hardening and predefined dislocation sources and sinks influence the onset of plastic flow as well as the hardening behaviour of single crystals in a single slip configuration solely from the physical properties and microstructural state of the crystal. Theresults are compared to experimental data found in the literature. We furthermore discuss the different size-effects which are inherent to or may meaningfully be included into the approach.
3:00 PM - **GG12.3/EE8.3
Three-dimensional Boundary Element- Dislocation Dynamics Modeling of Plastic Flow in Small Volumes.
Nasr Ghoniem 1 , Jaafar El-Awady 1
1 Mech & Aerospace, UCLA, Los Angeles, California, United States
Show Abstract3:30 PM - GG12.4/EE8.4
Discrete Dislocation Dynamics Simulation fo Plasticity in Small Systems Using ParaDiS.
Meijie Tang 1 , Guanshui Xu 2 , Gregg Hommes 1
1 , Lawrence Livermore National Lab. , Livermore , California, United States, 2 Mechanical Engineering, UC Riverside, Riverside, California, United States
Show Abstract4:15 PM - **GG12.5/EE8.5
PMFDM: A Tool for Modeling Size Effects and Dislocation Microstructure in Mesoscale Plasticity.
Amit Acharya 1 , Anish Roy 1 , Saurabh Puri 1 , Ankur Sepaha 1
1 , Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
Show Abstract4:45 PM - GG12.6/EE8.6
Size Effects in the Creep of Ice Single Crystals in Torsion: Experiments and Modeling.
Vincent Taupin 1 , Satya Varadhan 2 , Juliette Chevy 3 , Claude Fressengeas 1 , Armand Beaudoin 2 , Paul Duval 3 , Maurine Montagnat 3
1 Laboratoire de Physique et Mecanique des Materiaux, Université Paul Verlaine - Metz / CNRS, Metz France, 2 Department of Mechanical and Industrial Engineering, University of Illinois at Urbana Champaign, Urbana, Illinois, United States, 3 Laboratoire de Glaciologie et Geophysique de l'Environnement, CNRS, Saint Martin d'Heres France
Show Abstract5:00 PM - GG12.7/EE8.7
Hierarchical Modeling of Failure Mechanisms and Grain-Boundary Effects in Nanocrystalline Aggregates
Jibin Shi 1 , Mohammed Zikry 1 , A. Rajendran 1 , Donald Brenner 2 , Tarek Hatem 1
1 Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Materials Science, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractNew computational methodologies have been developed to predict dominant material behavior and mechanisms at scales ranging from the nano to the macro. Physically based scaling relations have been developed to characterize mechanisms and grain-boundary effects. These scaling relations have been used to link molecular dynamic and microstructural modeling to delineate the interrelated effects of grain boundary orientation and structure, dislocation transmission, absorption, and blockage through GBs, such that dominant failure mechanisms can be accurately identified and predicted from initiation to unstable growth. Results are presented for a broad class of CSL and random boundaries, and the effects of these boundaries on crack propagation and void coalescence are investigated at the different physical scales.
5:15 PM - GG12.8/EE8.8
Dislocation Nucleation During Nanoindentation of Aluminum.
Richard Wagner 1 , Li Ma 2 , Francesca Tavazza 2 , Lyle Levine 2
1 Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 2 Metallurgy Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Show Abstract5:30 PM - GG12.9/EE8.9
Molecular Dynamics Simulations of Grain Boundary Sliding: The Effect of Stress and Boundary Mis-Orientation.
Yue Qi 1 , Paul Krajewski 1
1 Materials & Processes Lab, GM R&D Center, Warren, Michigan, United States
Show AbstractMolecular dynamics simulations were used to study the effect of applied force and grain-boundary misorientation on grain-boundary sliding in aluminum at 750K. Two grains were oriented with their <110> axes parallel to their boundary plane, and one grain was rotated around its <110> axis to various misorientation angles. For any given misorientation, increasing the applied force leads to three sliding behaviors: no sliding, constant velocity sliding and a parabolic sliding over time. The last behavior is associated with disordering of atoms along the grain boundary. For the second sliding behavior, the constant sliding velocity varied linearly with the applied stress. However, a linear fit of this relationship did not intersect the stress axis at the origin, implying that a threshold stress for sliding exists. This threshold stress was found to decrease with increasing grain boundary energy. Including the threshold stress in the continuum polycrystal plasticity model, improved the prediction of the stress-v-strain rate behavior comparing with the experimental measurements.
GG13: Poster Session: Multiscale Modeling II
Session Chairs
Alain Chartier
Ram Devanathan
Simon Phillpot
Thursday AM, November 30, 2006
Exhibition Hall D (Hynes)
9:00 PM - GG13.1
Cooling Mechanism and Structural Change of Local Regions With a Different Cooling Rate of Excimer Laser Annealed Si.
Byoung-Min Lee 1 , Baek Seok Seong 1 , Hong Koo Baik 2 , Shinji Munetoh 3 , Teruaki Motooka 3
1 Neutron Physics Department, Korea Atomic Energy Research Institute, Daejeon Korea (the Republic of), 2 Department of Metallurgical Engineering, Yonsei University, Seoul Korea (the Republic of), 3 Department of Materials Science and Engineering, Kyushu University, Fukuoka Japan
Show AbstractTo investigate the cooling mechanism and the local structural changes of excimer-laser annealed Si, the thermal conductivity and the dynamical properties of Si during the natural cooling processes have been measured by using molecular dynamics (MD) simulations. The initial c/l interface system of Si was obtained by immersing the MD cells in a thermal bath with the temperature at 1000 K and 1500 K for Z≦35 Å and 3800 K for Z>35 Å, respectively. The natural cooling processes were reproduced by controlling the temperature of bottom 10 Å of the MD cell with a length of about 50 nm in the direction of heat flow. The amorphous-to-liquid transition near the interface in the temperature range of 1600 K ~ 1700 K was expected with the results of the local diffusion coefficients calculated by integrating velocity autocorrelation functions, where the average was taken over approximately 600 atoms found at t0=0 in each slice with thickness of 5 Å along the <001> direction by keeping track of them for 0.25 ps. It is confirmed that the structure of interface region affects the cooling rate of overall system. The thermal conductivity of c-Si in the Tersoff interatomic potential was measured to be 58 W/mK and 35.7 W/mK at 1000 K and 1500 K, respectively. The change of structural and dynamical properties of l- and a-Si at the different local regions has also been investigated. A detailed analysis of two-body and three-body correlation functions has been performed, and the effect of a different cooling rate at local regions during the natural cooling processes will be discussed.
9:00 PM - GG13.10
Quantum Conductance of Metallic Carbon Nanotubes with Di-vacancy Under Stretching.
Xu Zhang 1 , Chong-yu Wang 1 , Wenhui Duan 1
1 Department of Physics, Tsinghua University, Beijing China
Show Abstract9:00 PM - GG13.12
The Quenching of Carrier Generation in Semiconductors.
James Lavine 1 , William McColgin 1
1 , Eastman Kodak Company, Rochester, New York, United States
Show AbstractThe problem addressed is how effectively carriers in a small volume of a semiconductor quench the generation of additional carriers. The traditional model for generation and recombination assisted by a deep level in the semiconductor energy gap is due to Shockley and Read [1] and Hall [2]. Generation by the defect that introduces the deep level is dampened when the carrier concentration exceeds the intrinsic carrier concentration. Only a few carriers suffice to produce such an equivalent carrier concentration when the volume of interest is of the order of tens of cubic micrometers or less. The Shockley-Read-Hall model then predicts fewer electrons (carriers) versus time than the present experimental data, which covers generation rates of 500 to 40,000 electrons/second. This leads to the need to treat quenching as a more local physical phenomenon. A model is required to connect the time-dependent carrier density at the defect with the generation of additional carriers. The first model considers whether a previously generated electron is in a specified volume around the defect and whether it then blocks the generation of the next electron. This approach uses two parameters and produces plots of the number of electrons generated versus time that resemble the present data for both weak and strong generators. The next step is to show how these parameters are predicted by a finer scale calculation that is based on electron wave functions and perturbation theory. The progress in this direction will be presented. 1. W. Shockley and W. T. Read, Phys. Rev. 87, 835 (1952).2. R. N. Hall, Phys. Rev. 87, 387 (1952).
9:00 PM - GG13.13
Dynamics of Interstitials and Clustering in Two-dimensional Systems.
De Nyago Tafen 1 , Laurent Lewis 1
1 Département de physique, Université de Montréal, Montréal (Québec), Quebec, Canada
Show AbstractThe dynamics of interstitial defects in an hexagonal matrix, with particular emphasis on the temperature and concentration dependence of clustering, is investigated within a kinetic Monte Carlo approach. We show that the defect concentration plays a vital role in the formation of clusters and on dynamical quantities such as diffusion constants of mobile interstitials. We find that increasing the interstitial concentration causes the defect diffusion coefficients to decrease, and leads to a greater number of clusters and a greater clustering rate. In addition, the temperature dependence of the clustering rate exhibits an Arrhenius-type behaviour. Finally, simple scaling laws are determined as a function of interstitial concentration. The lifetime and diffusion coefficient of monomers and dimers, and by extrapolation clusters of size s (s≥2), scale as a function of defect concentration.
9:00 PM - GG13.14
CALPHAD Modeling of the Boron-Carbon System.
James Saal 1
1 Materials Science & Engineering, The Pennsylvania State University, State College, Pennsylvania, United States
Show Abstract9:00 PM - GG13.15
Simulations of Multi-atom Vacancies in Diamond.
Miklos Kertesz 1 , Istvan Laszlo 2 , Yury Gogotsi 3
1 Department of Chemistry, Georgetown University, Washington, District of Columbia, United States, 2 Department of Theoretical Physics, Budapest Technical University, Budapest Hungary, 3 Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania, United States
Show Abstract9:00 PM - GG13.16
Two-region Model for Etching and Deposition of SiO2 and Si in Fluorocarbon Plasmas.
Yeon Ho Im 1 , Hyun-Wook Ra 2 , Sang Hoon Kim 1 , Yoon Bong Hahn 1
1 School of Chemical Engineering and Technology and Nanomaterials Processing Reseaech Centre, Chonbuk National University, Jeonju, Jeonbuk, Korea (the Republic of), 2 Semiconductor Physics Research Center, Chonbuk National University, Jeonju, Jeonbuk, Korea (the Republic of)
Show Abstract9:00 PM - GG13.17
Analytic Bond Order Potentials for the System Fe, Pt and FePt.
Michael Mueller 1 , Paul Erhart 1 , Karsten Albe 1
1 Institute of Materials Science, TU Darmstadt, Darmstadt Germany
Show AbstractDue to their outstanding magnetic properties, FePt alloys are a candidate system for future high density recording applications. However, a prerequisite for the high coercivity of FePt alloys is a chemical ordering of the composing elements. The mechanisms that give rise to or that impede the development of chemical ordering under a given set of processing conditions are still largely unknown, the prominent example is the difficulty of producing nanometer sized FePt particles with internal order.In this context, we present an analytic FePt bond order potential for application in atomistic simulation techniques that can provide valuable insights into the microscopic mechanisms of ordering in FePt nanoparticles. In the development of the potential, care has been taken to also provide an accurate description of the boundary phases Fe and Pt. Important features of the elemental potentials are for example the reproduction of the bcc to fcc phase transition in Fe and the high stacking-fault energy of fcc Pt. The interplay of chemical order with magnetic ordering poses even more complications for the parametrization of the mixed FePt potential, where all electronic degrees of freedom are neglected. We demonstrate that it is possible to bridge the gap between the real system and the coarse grained description of the analytic potential approach by employing an effective Hamiltonian. The presented potential is able to reproduce the bulk ordering transitions of the L10 FePt, as well as the L12 Fe3Pt and FePt3 phases, simultaneously. We show that the properties of our potential compare favorable with already existing potentials.
9:00 PM - GG13.18
Phase-field Modeling of Liquid-phase Sintering – Rigid Body Motion and Wetting.
Klara Gronhagen 1 , Walter Villanueva 2 , John Agren 1 , Gustav Amberg 2
1 Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm Sweden, 2 Department of Mechanics, Royal Institute of Technology, Stockholm Sweden
Show Abstract9:00 PM - GG13.19
First-principles Phonon Investigation of Phase Stability in α- and β-rhombohedral Boron.
Shunli Shang 1 , Yi Wang 1 , Raymundo Arroyave 1 , Zi-Kui Liu 1
1 Department of Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania, United States
Show Abstract9:00 PM - GG13.2
Molecular Dynamics Simulations of Hydration, Dissolution and Nucleation Processes at the α-quartz Surface in Liquid Water.
Zhimei Du 1 2 , Nora de Leeuw 1 2
1 Department of Chemistry , University Collegeg London, London United Kingdom, 2 School of Crystallography, Birkbeck Collegeg, University of London, London United Kingdom
Show Abstract9:00 PM - GG13.20
New Silicon MEAM Potentials for Simulations of Sputter Erosion of Si(001).
Maria Timonova 1 3 4 , Byeong-Joo Lee 2 , Barend Thijsse 1 3 4
1 Materials Science, TUDelft, Delft Netherlands, 3 , Netherlands Institute for Metals Research, Delft Netherlands, 4 , Stichting voor Fundamenteel Onderzoek der Materie (FOM), Utrecht Netherlands, 2 Materials Science and Engineering, Pohang University of Science and Technology, Pohang, Pohang 790-784, Korea (the Republic of)
Show Abstract9:00 PM - GG13.21
Multi-scale Models of Magnetization Process in Composite Nano-structures.
Oleg Mryasov 2 , Oksana Chubykalo-Fesenko 1 , Felipe Garcia Sanchez 1 , Roy Chantrell 3
2 , Seagate Research, Pittsburgh, Pennsylvania, United States, 1 , Material Science Institute of Madrid, Madrid Spain, 3 , University of York, York United Kingdom
Show Abstract9:00 PM - GG13.22
Calculation of Pre-exponential Factors for Surface Self-diffusion Within the Transition State Theory.
LingTi Kong 1 , Laurent Lewis 1
1 Departement de Physique et Regroupement Quebecois sur les Materiaux de Pointe, Universite de Montreal, C.P. 6128, Succursale Centre-Ville, Montreal, Quebec, Canada
Show AbstractThe pre-exponential factors for self-diffusion via hopping and/or exchange on the (001), (110) and (111) surfaces of Cu, Ag and Ni are examined within transition state theory and the harmonic approximation. It is found that the present approach is able to predict prefactors within the same precision as those obtained from molecular dynamics simulations, where in principle no approximation is included, thus establishing the reliability of the method employed. It also reveals that the calculated prefactors have a weak temperature dependence above room temperature, and that within transition state theory, the Vineyard method provides a rather accurate description of them. However, the prefactors obtained by including only the local thermodynamical properties, i.e. neglecting the contributions from the substrate, are found to be inaccurate.
9:00 PM - GG13.3
Mechanisms of Diffusion and Structural Reconstruction of Tilt Grain Boundaries.
Mikhail Starostenkov 1 , Roman Rakitin 1 , Genady Poletaev 1
1 , ASTU, Barnaul Russian Federation
Show Abstract9:00 PM - GG13.4
Coalescence Process of Platinum Nanoparticles Examined by Phase-field Simulation.
Shunsuke Yamakawa 1 , Shi-aki Hyodo 1
1 , Toyota Central R&D Labs., Inc., Nagakute, Aichi, Japan
Show AbstractOne of the obstacles hindering the industrial widespread application of nanoparticles is the morphology change due to the coalescence process. In particular, the degradation of catalyst particles arising from the coarsening or sintering under operating conditions is regarded as a serious problem. Therefore, there is the significant interest in the basic formation process of inter-particle bonds by atomic motion. The phase-field method [1,2] has recently attracted increasing attention as a possible approach for the simulation of the solid-state sintering. The objective of this study is to estimate the validation of the phase-field approach for the coalescence process of platinum nanoparticles. The bulk chemical free energy was based on the regular solution approximation of the metal-vacancy complexes. The microstructure evolution of nanoparticles was described by the temporal evolution of the field variables related to the concentration of vacancies and the long-range crystallographic ordering. The resultant formulation was close to the one reported in Ref. [2]. The accuracy of this model was discussed by application to the sintering process of two spherical platinum nanoparticles. For the two particles with a 10nm diameter at a temperature of 1800K, the particles changed within 10 ns into the quasi-stationary shape related to a surface/grain-boundary energy ratio. The initial neck growth rate observed in this simulation was consistent with the theoretical predictions assuming a surface diffusion mechanism. These results showed that the phase-field method utilizing the metal-vacancy regular solution model provided a reasonable microstructure evolution for the platinum nanoparticles. Furthermore, we have examined the sintering simulation for the random spatial distribution of irregular shaped particles. The simulation clarified that the coarsening was sensitive to the spatial inter-particle correlation. A part of this work was supported by a grant from Core Research for Evolutional Science and Technology (CREST) by the Japan Science and Technology Agency (JST), Japan. [1] Y. U. Wang, Acta Mater. 54, 953(2006). [2] K. Asp and J. Ågren, Acta Mater. 54, 1241(2006).
9:00 PM - GG13.5
Aggregatization of Interstitial Atoms.
Mikhail Starostenkov 1 , Mikhail Aksenov 1 , Gennady Poletaev 1
1 , ASTU, Barnaul Russian Federation
Show Abstract9:00 PM - GG13.6
A Procedure of Determining Parameters to Expand Applicability of Modified Embedded Atom Method to Non-bulk Systems.
Kento Tokumaru 1 , Kunio Takahashi 1 , Yingchen Yin 1 , Shigeki Saito 2
1 Department of International Development Engineering, Tokyo Institute of Technology, Tokyo Japan, 2 Department of Mechanical and Aerospace Engineering, Tokyo Institute of Technology, Tokyo Japan
Show Abstract9:00 PM - GG13.9
Growth Study of Nanocrystalline Ni3Al Alloy and Ni Using Molecular Dynamics.
Z. Zhang 1 , Xiang-Yang Liu 1 , X. Meng 1 , J. Wang 2 , H. Huang 2
1 Department of Materials Science and Engineering, Nanjing University, Nanjing 210093 China, 2 Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Show Abstract9:00 PM - GG13:Poster 2
GG13.24 Transferred to GG3.2
Show Abstract9:00 PM - GG13:Poster 2
GG13.25 Transferred to GG4.2
Show Abstract9:00 PM - GG13:Poster 2
GG13.26 Transferred to GG3.1
Show Abstract9:00 PM - GG13:Poster 2
GG13.28 Transferred to GG14.5/FF9.5
Show Abstract9:00 PM - GG13:Poster 2
GG13.11 Transferred to GG15.4
Show Abstract
Symposium Organizers
Ram Devanathan Pacific Northwest National Laboratory
Maria Jose Caturla Universitat d’Alacant
Alison Kubota Lawrence Livermore National Laboratory
Alain Chartier CEA-Saclay
Simon Phillpot University of Florida
GG14/FF9: Joint Session: Modeling Composite Materials
Session Chairs
Peter Anderson
Ram Devanathan
Thursday AM, November 30, 2006
Back Bay C (Sheraton)
9:30 AM - GG14.1/FF9.1
Atomistic Study of Structure and Failure of fcc/bcc Heterophase Boundaries
Adham Hashibon 1 2 , Peter Gumbsch 1 2 , Yuri Mishin 3 , Christian Elsässer 1
1 , Fraunhofer IWM
, Freiburg Germany, 2 , IZBS, University of Karlsruhe, Karlsruhe Germany, 3 , George Mason University, Fairfax, Virginia, United States
Show AbstractHeterophase interfaces between fcc and bcc metals are present in microelectronic devices and composite materials. The functional properties of these systems hinges strongly on the micro and nano-mechanics of the interfaces. The Copper-Tantalum system is a technologically important fcc/bcc interface system, since Ta may be used as a diffusion barrier to keep the Cu from interacting with the Si-chip. The structure and mechanical properties of the Copper-Tantalum interfaces are investigated by atomistic methods employing a specifically developed Cu-Ta interatomic potential which is based on a generalization of the embedded atom method by the addition of angular-dependent interactions. These angular forces are believed to be important in correctly describing the structure of dislocation cores and of interfaces. Interfaces considered include those with the two well known low energy orientation relationships for fcc/bcc interfaces, namely the Kurdjumov-Sachs (KS) and Nishiyama-Wasserman (NW) orientations, for which the closed packed planes and directions are parallel to each other. In addition, interfaces are formed by depositing liquid Cu on Ta free surfaces. It is found that for the equilibrium interface structure, the first layer of Cu has a distinct structure from the rest of the fcc Cu lattice. This layer facilitates the transition from the bcc to the fcc crystal structures across the interface. Results for the properties of this specific layer, its role in interface adhesion, the vacancy formation energy profile, pore nucleation and for dislocations at the interface will be presented.
9:45 AM - GG14.2/FF9.2
Plasticity mechanisms of Cu/Nb nanofilaments: a Molecular Dynamics study
Ludovic Thilly 1 , Peter Derlet 2 , Helena Van Swygenhoven 2
1 Lab. Metallurgie Physique, University of Poitiers, Futuroscope France, 2 NUM ASQ, Paul Scherrer Institut, Villigen Switzerland
Show AbstractUltra high strength Cu/Nb nanofilamentary conductors are processed by severe plastic deformation for the windings of coils producing non-destructive high magnetic fields with long pulse duration. They are composed of a <111> textured Cu matrix embedding <110> oriented Nb nanofilaments. Plasticity of these structures is characterized by a single dislocation regime in the nanometer scaled Cu and whisker-like behavior in the Nb nanofibers. Because of the severe plastic deformation process, the Cu/Nb interfaces are semi-coherent with misfit dislocations every 8 or 9 atomic planes in the Cu phase. These interfaces are assumed to act as dislocation barriers.To study the role of the interfaces in the plasticity mechanisms of the Cu/Nb nanocomposites with respect to dislocation nucleation, propagation and absorption/transmission, molecular dynamics simulations using embedded atom method were conducted at 300K. A Cu/Nb nanostructure composed of a single crystalline Nb <110> nanofiber embedded in single crystalline <111> Cu matrix (fiber size: 25nm, fiber spacing: 8nm) is simulated. The role of interface structure and interfacial misfit dislocations on dislocation nucleation and propagation is studied during tensile loading of the Cu/Nb nanostructure along the Nb nanofilament axis at constant strain rate. The stress distribution is computed in both phases, accounting for the different elastic-plastic regimes. The results are discussed in terms of the atomic level structure of the interfaces.
10:00 AM - **GG14.3/FF9.3
On the Role of Interfaces in Providing Strength and Radiation Damage Resistance in Nanolayered Composites.
Richard Hoagland 1 , M. Demkowicz 1 , A. Misra 1
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractThe theoretical strength is nearly achieved in layered metallic composites when the layer thicknesses are reduced to about 5 nm or less. In cases where the constituents have the same phase and are coherent or even semicoherent, high strengths are attributable to coherency stresses. However, coherency stresses cannot be important in incoherent systems when the phases of the components are dissimilar. Nevertheless, composites with constituents separated by incoherent interfaces also achieve very high strength levels. We describe the results of experiments and atomistic simulations that reveal some of the features of fcc/bcc composite materials that are important to strength. Another important discovery is that composite nanolayered systems with incoherent interfaces, such as Cu/Nb, possess an unusual resistance to radiation damage. TEM studies of He implanted samples with nanometer layer thicknesses reveal delayed onset of bubble formation and a greatly reduced population of defect clusters. This behavior is attributed to accelerated annihilation of Frankel pairs at or in the near vicinity of the interfaces. This research supported by OBES of the U. S. Dept. of Energy.
10:30 AM - GG14.4/FF9.4
Brittle-Ductile Transition in Heterogeneous Metallic Materials
Silvester Noronha 1 , Nasr Ghoniem 1
1 Mechanical & Aerospace Engineering, University of California Los Angeles, Los Angeles, California, United States
Show AbstractLow temperature fracture behavior of multiphase metallic materials is controlled by the microcracks originated in brittle precipitates embedded metallic matrix. Fracture in these materials propagates by the extension of 'critical microcrack' situated in the plastic zone of macrocrack ahead of it. The crack-tip plasticity of both microcrack and macrocrack are simulated as dislocation arrays using as discrete dislocation simulation. The analysis reveals the factors that contribute to the exponential increase in fracture toughness with temperature at the brittle - ductile temperature (BDT). They are found to be: (a) the marginal increase in microscopic fracture stress, (b) the increase in crack-tip blunting with increase in plastic-flow with temperature, and (c) the increase in dislocation mobility with temperature. On applying the model to a set of microcrack distributions it has been found that (1) always it is one of the largest microcracks that lead to the fracture (2) the scatter in fracture toughness measurements is due the scatter in the size of the microcracks not their relative position to the macrocrack.
10:45 AM - GG14.5/FF9.5
Influence of Mineral -polymer Interactions on Molecular Mechanics of Polymer in Composite Bone Biomaterials.
Rahul Bhowmik 1 , Kalpana Katti 1 , Dinesh Katti 1
1 Civil Engineering, North Dakota State University, Fargo, North Dakota, United States
Show Abstract11:00 AM - GG14/FF9
BREAK
GG15: Nanoscale Phenomena
Session Chairs
Thursday PM, November 30, 2006
Constitution B (Sheraton)
11:30 AM - **GG15.1
First Principles Calculations for Nanoscale Capacitors.
Nicola Spaldin 1 , Massimiliano Stengel 1
1 Materials Department, University of California, Santa Barbara, California, United States
Show Abstract12:00 PM - GG15.2
On the Dimensional Reduction of the Structure Problem of Nanoscale Materials.
Dan Negrut 3 , Srujan Rokkam 1 , Mihai Anitescu 4 , Peter Zapol 5 , Anter El-Azab 1 2
3 Mechanical Engineering , University Wisconsin - Madison, Madison, Wisconsin, United States, 1 Mechanical Engineering, Florida State University, Tallahassee, Florida, United States, 4 Mathematics and Computer Science Division, Argonne National Laboratory, Argonne, Illinois, United States, 5 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States, 2 School of Computational Science, Florida State University, Tallahassee, Florida, United States
Show AbstractNanoscale materials often lack structural periodicity, which makes it difficult to compute their electronic and atomic structure. A multiscale approach specifically designed for this task will be presented. This approach features a quasicontinuum scheme for the nuclei positions and a reconstruction scheme for the electron density distribution, both implemented within the framework of the Orbital-Free Density Functional Theory. Both schemes are based on the assumption that the Cauchy-Born hypothesis is valid, which permits the reduced representation of nuclei and the application of perturbation of periodic boundary conditions to establish the reconstruction scheme for the electron density. In this work, the validity of the Cauchy-Born hypothesis in the case of nano particles is tested first using molecular dynamics simulations to define the nano particle size limits above which its crystalline identity is maintained and hence the quasicontinuum approximation is valid. It is found that oxide nano particles can turn amorphous or undergo significant disorder at small sizes. Only at several nano meters, the crystalline identity of particles is maintained and hence the Cauchy-Born hypothesis applies. The details of the reconstruction approach for computing the electronic density in large nanostructures are then presented. In particular, the representation of the electron energy functional in terms of a reduced subset of the electronic degrees of freedom is highlighted and the associated mathematical optimization problem is given. Electronic structure computed based on this multi-scale approach are presented, which illustrate the benefits of the method presented here in dealing with the structure problem of atomic systems that are too large to handle using current density functional methods.
12:15 PM - GG15.3
Charge Optimized Many-Body (COMB) Potential for the Silicon-Silica System.
Jianguo Yu 1 , Susan Sinnott 1 , Simon Phillpot 1
1 Materials science and engineering, University of Florida, Gainesville, Florida, United States
Show Abstract12:30 PM - GG15.4
Multiscale Modeling of Frictional Behavior of Highly-Ordered Carbon Nanotube/Ceramic Nanocomposites
Zhenhai Xia 1
1 Department of Mechanical Engineering, University of Akron, Akron, Ohio, United States
Show AbstractMicromechanics model incorporating with molecular dynamics (MD) simulation is developed to simulate the frictional behavior of carbon nanotube (CNT) arrays in ceramic nanocomposites. MD model is used to compute the interaction force and simulate failure mechanisms of individual nanotube at atomic length scale. The force and deformation calculated from MD simulation are passed to the continuum model to simulate the interaction between nanotube arrays and AFM tips. The coefficient of friction is determined at different load levels. The simulation shows that the low friction in the thick-wall CNT systems occurs because the stiffer CNTs are more resistant to collapse under the applied loads. The predictions for the coefficient of friction are consistent with nanoscale tests.
GG16: Nanoparticles
Session Chairs
Thursday PM, November 30, 2006
Constitution B (Sheraton)
2:30 PM - **GG16.1
Atomic Scale Modeling of Functional Nanoparticles.
Karsten Albe 1
1 Inst. f. Materialwissenschaft, TU Darmstadt, Darmstadt Germany
Show Abstract3:00 PM - GG16.2
Multiscale Modeling of Growth and Structure of Silicon Nanoparticles in an Oxide Matrix.
Gyeong Hwang 1 , Decai Yu 1 , Sangheon Lee 1
1 Chemical Engineering, The University of Texas at Austin, Austin, Texas, United States
Show Abstract3:15 PM - GG16.3
Multiscale Modeling of Silicon Nanocrystals Embedded in Amorphous Silicon Dioxide.
Dundar Yilmaz 1 , Ceyhun Bulutay 1
1 Department of Physics, Bilkent University, Ankara Turkey
Show Abstract3:30 PM - GG16.4
Molecular Dynamics Simulation of Nanoparticle Chain Aggregate Sintering.
Takumi Hawa 2 1 , Michael Zachariah 2 3 1
2 Mechanical Engineering, University of Maryland, College Park, Maryland, United States, 1 Process Measurements Division, NIST, Gaithersburg, Maryland, United States, 3 Chemistry, University of Maryland, College Park, Maryland, United States
Show AbstractSintering of silicon nanoparticle chain aggregates are investigated using molecular dynamics (MD) simulations. The chain aggregates consist of up to 80 particles, with primary ranging in size from 2.5 to 7 nm in diameter. We found that sintering of chain aggregates consists of three steps, 1) where reaction between particles to minimize surface defects and development of a column like shape, comprise an induction period. 2) contraction of the column structure, which actually consists of two contraction stages. The first stage is the local contraction occurs only at the ends of the particle chain, and the second stage the global contraction sintering process. The last step (3) is the final and nominal sintering process from an oval to spherical shape. As expected the sintering time increases with increase in the total volume of the chain aggregate or with increase the exposed initial surface area of the chain. A mathematical model was developed to describe the dynamics of sintering of chain aggregates. The model was able to predict the sintering time in excellent agreement with the results obtained from MD simulations. We also studied the chain aggregate with secondary branches to begin to mimic the behavior of fractal aggregates. These involve L-shape and T-shape aggregates. In general, the sintering time changes as much as 30% of that of a straight chain containing the same volume of particles.