Galen K. Straub

Modeling microstructure evolution in three dimensions with Grain3D and LaGriT

This paper will describe modeling microstructure evolution using a combination of our gradient-weighted moving finite elements code, Grain3D and our 3-D unstructured grid generation and optimization code, LaGriT. Grain boundaries evolve by mean curvature motion, and Grain3D allows for the incorporation of grain boundary orientation dependence modeled as anisotropic mobility and energy. We also describe the process of generating an initial computational grid from images obtained from electron backscatter diffraction. We present the grid optimization operations developed to respond to changes in the physical topology such as the collapse of grains and to maintain uniform computational grid quality. For 3-D columnar microstructures, validation of the method is demonstrated by comparison with experiments. For large systems of fully 3-D microstructures, simulations compare favorably to the parabolic law of normal grain growth.

Linking Experimental Characterization and Computational Modeling of Grain Growth in Al-Foil

Experimental results on grain boundary properties and grain growth obtained using the Electron Backscattered Diffraction (EBSD) technique are compared with the Finite Element simulation results of an Al-foil with a columnar grain structure. The starting microstructure and grain boundary properties are implemented as an input for the three-dimensional grain growth simulation. In the computational model, minimization of the interface energy is the driving force for the grain boundary motion. The computed evolved microstructure is compared with the final experimental microstructure, after annealing at 550°C. Good agreement is observed between the experimentally obtained microstructure and the simulated microstructure. The constitutive description of the grain boundary properties was based on a 1-parameter characterization of the variation in mobility with misorientation angle.

The Thermal Properties of Metallic Sodium Near Melt From Molecular Dynamics Calculation

Molecular dynamics simulations of the thermal properties of metallic sodium were performed in the high temperature anharmonic region near the melting point. The ion-ion interaction potential was derived from pseudopotential theory. From the molecular dynamics results, the anharmonic thermal energy was determined directly without the use of thermodynamic perturbation theory. Comparison of the calculated melting temperature, latent heat of fusion, fluid phase diffusion coefficient, and the atomic distribution function are all in good agreement with experiment.

Comparison of Experimental and Computational Aspects of Grain Growth in Al-Foil

Grain boundary and crystallographic orientation information of an Al-foil with a columnar grain structure is characterized by Electron Backscattered Diffraction (EBSD) technique. The starting microstructure and grain boundary properties are implemented as an input for the three- dimensional grain growth simulation. In the computational model, minimization of the interface energy is the driving force for the grain boundary motion. The computed evolved microstructure is compared with the final experimental microstructure, after annealing at 550 °C. Good agreement is observed between the experimentally obtained microstructure and the simulated microstructure. The constitutive description of the grain boundary properties was based on a 1- parameter characterization of the variation in mobility with misorientation angle.

Dipole interactions and trapping effects of positive muons in fcc and bcc metals

In this paper the results of calculations for a single muon trapped at various lattice sites in an external magnetic field are presented. As the direction of the magnetic field is changed, the results show a large variation of the local field experienced by a muon at the octahedral (o-sites) and tetrahedral (t-sites) symmetry lattice interstitial sites. These results show that a careful measurement of the ..mu../sup +/ depolarization rate for various external magnetic field orientations would indicate a preferential position for the trapped particle. At this time analysis of the experimental data does not present a clear interpretation for the determination of the ..mu../sup +/ site. The calculational procedure is outlined and the results, including an estimate of the effects of lattice distortion on the ..mu../sup +/ depolarization rate when the muon is localized are presented.

EXTRAPOLATION OF THE SHEAR MODULUS TO HIGH COMPRESSIONS AND NEGATIVE PRESSURES

An analytic form for the shear modulus G is presented and its fit to data at different compressions is demonstrated for copper and tungsten. When only the shear modulus at zero pressure and its logarithmic derivative are known, an approximate procedure is used to generate the parameters in the analytic form for G. This procedure is demonstrated, and estimates are provided of G as a function of compression, for Al, Cu, Fe, Mo, W, Ta, and U.

The Overlap of Electron Core States for Very High Compressions

At normal density and for modest compressions, the electronic structure of a metal can be accurately described by treating the conduction electrons and their interactions with the usual methods of band theory. The core electrons remain essentially the same as for an isolated free atom and do not participate in the bonding forces responsible for creating a condensed phase. As the density increases, the core electrons begin to “see” one another as the overlap of the tails of wave functions can no longer be neglected. The electronic structure of the core electrons is responsible for an effective repulsive interaction that eventually becomes free-electron-like at very high compressions.

Molecular dynamics study of sodium using a model pseudopotential

The dynamics of sodium is investigated using the coulomb and Born‐Mayer interaction augmented by a model pseudopotential to represent the electron interactions including screening, exchange, and correlation. The model parameters were previously determined and have been shown to accurately reproduce experimental equation‐of‐state, lattice vibration, and crystal phase properties of sodium in the harmonic limit. In this paper the equation‐of‐state and structural properties are examiend in molecular dynamics calculations. The long range effects of the potential are included. Typically, each particle interacts with about 500 neighbors. The calculated equation of stte of sodium in the hcp, bcc, and liquid structures is discussed.