Molecular dynamics simulation of brittle fracture in silicon

The fracture process involves converting potential energy from a strained body into surface energy, thermal energy, and the energy needed to create lattice defects. In dynamic fracture, energy is also initially converted into kinetic energy. This paper uses molecular dynamics (MD) to simulate brittle frcture in silicon and determine how energy is converted from potential energy (strain energy) into other forms

Stress-induced platelet formation in silicon:a molecular dynamics study

The effect of stress on vacancy cluster configurations in silicon is examined using molecular dynamics. At zero pressure, the shape and stability of the vacancy clusters agrees with previous atomistic results. When stress is applied the orientation of small planar clusters changes to reduce the strain energy. The preferred orientation for the vacancy clusters under stress agrees with the experimentally observed orientations of hydrogen platelets in the high stress regions of hydrogen implanted silicon. These results suggest a theory for hydrogen platelet formation

Interatomic potentials for atomistic simulations of the Ti-Al system

Semi-empirical interatomic potentials have been developed for Al, alpha-Ti, and gamma-TiAl within the embedded atomic method (EAM) by fitting to a large database of experimental as well as ab-initio data. The ab-initio calculations were performed by the linear augmented plane wave (LAPW) method within the density functional theory to obtain the equations of state for a number of crystal structures of the Ti-Al system. Some of the calculated LAPW energies were used for fitting the potentials while others for examining their quality. The potentials correctly predict the equilibrium crystal structures of the phases and accurately reproduce their basic lattice properties. The potentials are applied to calculate the energies of point defects, surfaces, planar faults in the equilibrium structures. Unlike earlier EAM potentials for the Ti-Al system, the proposed potentials provide reasonable description of the lattice thermal expansion, demonstrating their usefulness in the molecular dynamics or Monte Carlo studies at high temperatures. The energy along the tetragonal deformation path (Bain transformation) in gamma-TiAl calculated with the EAM potential is in a fairly good agreement with LAPW calculations. Equilibrium point defect concentrations in gamma-TiAl are studied using the EAM potential. It is found that antisite defects strongly dominate over vacancies at all compositions around stoichiometry, indicating that gamm-TiAl is an antisite disorder compound in agreement with experimental data.Comment: 46 pages, 6 figures (Physical Review B, in press

Iron under Earth's core conditions: Liquid-state thermodynamics and high-pressure melting curve

{\em Ab initio} techniques based on density functional theory in the projector-augmented-wave implementation are used to calculate the free energy and a range of other thermodynamic properties of liquid iron at high pressures and temperatures relevant to the Earth's core. The {\em ab initio} free energy is obtained by using thermodynamic integration to calculate the change of free energy on going from a simple reference system to the {\em ab initio} system, with thermal averages computed by {\em ab initio} molecular dynamics simulation. The reference system consists of the inverse-power pair-potential model used in previous work. The liquid-state free energy is combined with the free energy of hexagonal close packed Fe calculated earlier using identical {\em ab initio} techniques to obtain the melting curve and volume and entropy of melting. Comparisons of the calculated melting properties with experimental measurement and with other recent {\em ab initio} predictions are presented. Experiment-theory comparisons are also presented for the pressures at which the solid and liquid Hugoniot curves cross the melting line, and the sound speed and Gr\"{u}neisen parameter along the Hugoniot. Additional comparisons are made with a commonly used equation of state for high-pressure/high-temperature Fe based on experimental data.Comment: 16 pages including 6 figures and 5 table

Modified embedded atom method calculations of interfaces

The Embedded Atom Method (EAM) is a semi-empirical calculational method developed a decade ago to calculate the properties of metallic systems. By including many-body effects this method has proven to be quite accurate in predicting bulk and surface properties of metals and alloys. Recent modifications have extended this applicability to a large number of elements in the periodic table. For example the modified EAM (MEAM) is able to include the bond-bending forces necessary to explain the elastic properties of semiconductors. This manuscript will briefly review the MEAM and its application to the binary systems discussed below. Two specific examples of interface behavior will be highlighted to show the wide applicability of the method. In the first example a thin overlayer of nickel on silicon will be studied. Note that this example is representative of an important technological class of materials, a metal on a semiconductor. Both the structure of the Ni/Si interface and its mechanical properties will be presented. In the second example the system aluminum on sapphire will be examined. Again the class of materials is quite different, a metal on an ionic material. The calculated structure and energetics of a number of (111) Al layers on the (0001) surface of sapphire will be compared to recent experiments

Properties of a single asperity and the interface between molecular dynamics and continuum mechanics: A commentary

The speakers in this session attempted to bridge the large spatial gap between the atomistic processes occurring at a sliding interface and the continuum description of such processes. This task is indeed formidable. One may ask why should we study such elementary processes at all if what we are really interested in is a global picture of friction. Real surfaces are uneven, impure, and may be covered by nasty things like lubricants specifically placed there to modify frictional behavior. Isn`t the real world of friction too ``dirty`` to be studied by surface science techniques? Indeed, even if we were to understand the interaction of every geometry of single asperity under every environment, how to average this information to produce a model of friction is unknown. Does this mean that we shouldn`t attempt to measure and calculate these simple processes? I think not. Understanding the response of a single asperity is an important essential element which will lead to a thorough predictive understanding of friction. But clearly our work cannot end with the study of single asperities. There are two critical phenomena which have to be added to a single asperity model: first the inclusion of a distribution in both size and location of single asperities and second the role of microstructure evolution. Clearly single asperities do not respond independently from each other. The proximity of two asperities changes both the local stress distribution as well as the contact area. I believe the greatest challenge that faces us is how to assemble the vast amount of single asperity data that we can generate and from it create useful engineering models

Molecular dynamics studies of thin-films of Sn on Cu

Using Molecular Dynamics, the evolution dynamics of Sn on the (111) and (100) surfaces of Cu have been investigated as a function of coverage and temperature. The interaction potentials are described by modified embedded atom method (MEAM) potentials. The calculated diffusion activation energies of Cu in Sn and Sn in Cu agree reasonably well with experimental values. The authors find that the structure of the overlayer depends on the morphology of the substrate and remains stable up to temperatures of the order of 75% of the melting temperature of the substrate at which diffusion of Sn into the substrate and Cu atoms onto the overlayer is observed