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Electronic properties of Ar and Xe under pressure

Description: A simple model for calculating ground- and excited-state properties of molecular and rare-gas crystals is presented. The electrons are considered to be tightly bound to their molecular or atomic sites and the effects of the crystal potential, calculated with local-density functionals, are treated as a perturbation of the molecules or atoms. Results for Ar to 500 kbar show that the ground-state atoms compress as the pressure is increased and that there is a gradual increase in exciton energies. Preliminary results on ground-state Xe to 1.5 Mbar show that, to about 1 Mbar, the electronic distributions of the Xe atoms compress, but beyond that there is a slight expansion.
Date: January 1, 1983
Creator: LeSar, R.
Partner: UNT Libraries Government Documents Department

Structure and properties of simple molecular systems at very high density

Description: The use of computer simulations in the study of molecular systems at very high density is reviewed. Applications to the thermodynamics of dense fluid nitrogen and phase transitions in solid oxygen are presented. The effects of changes in the atomic electronic structure on the equation of state of very dense helium are discussed. 19 refs., 2 figs.
Date: January 1, 1989
Creator: LeSar, R.
Partner: UNT Libraries Government Documents Department

The electronic structure of condensed molecular systems

Description: We have reviewed some of the basic properties of the electronic structure of condensed molecular systems. For the rare-gas solids, we concentrated our discussion on changes in the ground- and excited-state crystal-atomic wave functions as calculated with an approximate theoretical method. Compression of these wave functions leads to a softening of the equation of state at high densities, which seems to account for much of the total many-body effects. This compression is a true many-body effect and cannot be easily decomposable into a sum of 3-body and higher terms. We reviewed the electronic properties of four molecular systems, each manifesting different behavior at high densities. Because of a general lack of theory of the electronic structure of molecular solids, we restricted ourselves to a descriptive account. Solid oxygen, for instance, seems to exhibit the beginnings of covalent bonding between the ..pi..* orbitals on adjacent molecules in its epsilon phase. It was a combination of optical-absorption data and infrared and Raman spectroscopy that led to these conclusions. Iodine is unique in that it becomes metallic as a molecular crystal at pressures easily obtainable experimentally. It is interesting that the x-ray data, which indicates a transition to a monatomic lattice at 21 GPa, and the Moessbauer spectra, which implies that molecular character is retained to 30 GPa, are in such disagreement. The next system discussed, solid acetylene, is a nice example of high-pressure polymerization and study of this system should shed light on the polymerization of more complicated systems. Finally, we briefly discussed the predicted dissociation of solid molecular nitrogen at high pressures. Here, theory has made a prediction and experiment has disproven it. Molecular systems show a diverse range of behavior in electronic structures at high pressures, from metallization to chemistry; theory is lagging. 68 refs., 10 figs.
Date: January 1, 1988
Creator: LeSar, R.A.
Partner: UNT Libraries Government Documents Department

Opportunities in computational mechanics: Advances in parallel computing

Description: In this paper, the authors will discuss recent advances in computing power and the prospects for using these new capabilities for studying plasticity and failure. They will first review the new capabilities made available with parallel computing. They will discuss how these machines perform and how well their architecture might work on materials issues. Finally, they will give some estimates on the size of problems possible using these computers.
Date: February 1, 1999
Creator: Lesar, R.A.
Partner: UNT Libraries Government Documents Department

A fully coupled 2D model of equiaxed eutectic solidification

Description: We propose a model of equiaxed eutectic solidification that couples the macroscopic level of heat diffusion with the microscopic level of nucleation and growth of the eutectic grains. The heat equation with the source term corresponding to the latent heat release due to solidification is calculated numerically by means of an implicit finite difference method. In the time stepping scheme, the evolution of solid fraction is deduced from a stochastic model of nucleation and growth which uses the local temperature (interpolated from the FDM mesh) to determine the local grain density and the local growth rate. The solid-liquid interface of each grain is tracked by using a subdivision of each grain perimeter in a large number of sectors. The state of each sector (i.e. whether it is still in contact with the liquid or already captured by an other grain) and the increase of radius of each grain during one time step allows one to compute the increase of solid fraction. As for deterministic models, the results of the model are the evolution of temperature and of solid fraction at any point of the sample. Moreover the model provides a complete picture of the microstructure, thus not limiting the microstructural information to the average grain density but allowing one to compute any stereological value of interest. We apply the model to the solidification of gray cast iron.
Date: December 31, 1995
Creator: Charbon, Ch. & LeSar, R.
Partner: UNT Libraries Government Documents Department

Industrial application for the Los Alamos Materials Modeling Platform

Description: This is the final report of a one-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). Casting and solidification of molten metals and metal alloys is a critical step in the production of high-quality metal stock and in the fabrication of finished parts. Control of the casting process can be the determining factor in both the quality and cost of the final metal product. Major problems with the quality of cast stock or finished parts can arise because of the difficulty of preventing variations in the alloy content, the generation of porosity or poor surface finish, and the loss of microstructure controlled strength and toughness resulting from the poor understanding and design of the mold filling and solidification processes. In this project, we sought to develop a new set of applications focused on adding the ability to accurately model solidification and grain growth to casting simulations. We implemented these applications within the Los Alamos Materials Modeling Platform, LAMMP, a graphical-based materials, and materials modeling environment being created at the Computational Testbed for Industry.
Date: September 1, 1996
Creator: Lesar, R.; Charbon, C.; Kothe, D.; Wu, D. & Reddy, A.
Partner: UNT Libraries Government Documents Department

Modeling for environmentally conscious manufacturing

Description: This is the final report of a one-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). The goal of this project was to begin development of tools needed for the creation of an integrated simulation tool to help the development of environmentally benign advanced manufacturing. The specific manufacturing process chosen as the first example in this project was casting. The objective was to develop better models for the solidification process and to couple those into heat/fluid flow codes as the first step in the development of an advanced casting-simulation tool. There were a number of accomplishments in this project, each focused on a different aspect of solidification and its connection to manufacturing processes: (a) development of a coupled microstructure/heat flow model for equiaxed eutectic solidification, (b) development of a coupled microstructure/heat flow model for quiescent crystallization of semicrystalline polymers, (c) development of a model of the directed light fabrication process, and (d) direct modeling of dendritic growth.
Date: August 1, 1997
Creator: Charbon, C. & LeSar, R.A.
Partner: UNT Libraries Government Documents Department

Multiscale Modeling of Recrystallization

Description: We propose a multi length scale approach to modeling recrystallization which links a dislocation model, a cell growth model and a macroscopic model. Although this methodology and linking framework will be applied to recrystallization, it is also applicable to other types of phase transformations in bulk and layered materials. Critical processes such as the dislocation structure evolution, nucleation, the evolution of crystal orientations into a preferred texture, and grain size evolution all operate at different length scales. In this paper we focus on incorporating experimental measurements of dislocation substructures, rnisorientation measurements of dislocation boundaries, and dislocation simulations into a mesoscopic model of cell growth. In particular, we show how feeding information from the dislocation model into the cell growth model can create realistic initial microstructure.
Date: December 7, 1998
Creator: Godfrey, A.W.; Holm, E.A.; Hughes, D.A.; Lesar, R. & Miodownik, M.A.
Partner: UNT Libraries Government Documents Department

Integrated approach to advanced machining

Description: The residual stress state induced by machining in a Ti alloy as function of cutting tool sharpness and depth of cut was predicted and measured. Residual stresses were greater for the dull tool than for the sharp tool. XRD was used to measure the residual stress state of the material; these measurements revealed that the hoop stress increased with depth of cut; however the radial stress decreased with depth of cut. An elastic-plastic model provided a possible explanation for this behavior in that, for small depths of cut, the tool makes multiple passes through the damage subsurface layer. This causes both residual stress components to increase, but the radial stress increases by a much greater amount than the hoop stress.
Date: August 1, 1997
Creator: LeSar, R.A.; Bourke, M.A.M.; Rangaswamy, P.; Day, R.D. & Hatch, D.J.
Partner: UNT Libraries Government Documents Department

Free energy simulation of grain boundary segregation and thermodynamics in Ni sub 3-x Al sub 1+x

Description: The free energy simulation method is employed to study segregation to {Sigma}5 and {Sigma}13 (001) twist grain boundaries and their free energies in ordered Ni{sub 3-x}Al{sub 1+x}. In the temperature range studied (300--900K), it is shown that there is almost no segregation, strong Al segregation, and weak Ni segregation to the grain boundary for the stoichiometric, Al-rich, and Ni-rich bulk compositions respectively. It is also shown that the segregation is limited to a few (002) planes around the grain boundary and its magnitude decreases with increasing temperature. For Al-rich bulk composition, it is demonstrated that segregation at low temperature substantially lowers the grain boundary free energy. 8 refs., 7 figs.
Date: January 1, 1990
Creator: Najafabadi, R.; Wang, H.Y.; Srolovitz, D.J. (Michigan Univ., Ann Arbor, MI (USA). Dept. of Materials Science and Engineering) & LeSar, R. (Los Alamos National Lab., NM (USA))
Partner: UNT Libraries Government Documents Department

Modeling of transformation toughening in brittle materials

Description: Results from modeling of transformation toughening in brittle materials using a discrete micromechanical model are presented. The material is represented as a two-dimensional triangular array of nodes connected by elastic springs. Microstructural effects are included by varying the spring parameters for the bulk, grain boundaries, and transforming particles. Using the width of the damage zone and the effective compliance (after the initial creation of the damage zone) as measures of fracture toughness, we find that there is a strong dependence of toughness on the amount, size, and shape of the transforming particles, with the maximum toughness achieved with the higher amounts of the larger particles.
Date: January 24, 1992
Creator: LeSar, R.; Rollett, A.D. (Los Alamos National Lab., NM (United States)) & Srolovitz, D.J. (Michigan Univ., Ann Arbor, MI (United States). Dept. of Materials Science and Engineering)
Partner: UNT Libraries Government Documents Department

Dynamical simulation of dislocation microstructure

Description: We report the development of a computer simulation technique to model the motion of a large number of mutually interacting dislocations under the influence of an applied stress. The dislocation motion is simulated using a method akin to molecular dynamics except that dislocation coordinates are employed instead of atomic coordinates. The formation of dislocation substructure is studied as a function of applied stress and density and the resulting spatial distribution of dislocations is analyzed. 5 refs., 3 figs.
Date: January 1, 1989
Creator: Gulluglo, A.; Srolovitz, D.J.; LeSar, R.; Lomdahl, P.S. (Michigan Univ., Ann Arbor, MI (USA). Dept. of Materials Science and Engineering & Los Alamos National Lab., NM (USA))
Partner: UNT Libraries Government Documents Department