Vacancy formation and clustering significantly affect structural properties of transition-metal aluminides. Ab-initio quantum mechanical total-energy calculations using a full-potential linear combination of muffin-tin orbitals (LMTO) technique provide a convenient method of studying relevant characteristics such as changes in density of states, and charge redistribution around defects. Augmented with Hellmann-Feymann forces, LMTO allows calculations of relaxation geometries and relaxation energies. We have performed such calculations for vacancies and antisite substitutional point defects in Fe3Al with DO3 crystallographic structure. There are two limiting factors complicating calculations of defect formation energies directly from ab-inito calculations. The first is that a single defect, due to the lattice periodicity necessitated by the use of ab-inito total energy techniques, cannot be considered as an isolated defect, even in the maximum computable simulation cell. Unlike previous calculations [ I ], which did not find a dependency on the size of the simulation cell, our calculations have shown a significant difference in results for 32- and 16- atom cells. This difference provides information about vacancy clustering since it can be explained by a relatively small attractive interaction energy ~0.2 eV between two vacancies located in adjacent simulation cells and separated by the lattice constant distance (5.52Å) [2]. By comparing the internal energies for two configurations of 30 atom cells (32 atom - 2 vacancies) we were able to estimate that the attractive interaction between two vacancies could reach 1.2 eV. The second complication is the fact that chemical potentials of elements cannot be directly extracted from the total energy calculations for the compound. To deal with this problem, we considered two possible approximations and compared results, which were found to be quite similar for iron vacancies.

A study of vacancy defect migration pathways and energetics in a-quartz is carried out using an empirical simulation methodology that is coupled with the nudged elastic band method. Results from this study indicate that the migration pathway for migration is between adjacent sites. We anticipate the results will guide modifications to empirical potentials for use in the study of point defect formation of more complex systems.

The morphology and microstructure of RF diode sputter deposited materials is a complicated function of many parameters of the reactor operating conditions. Using a combination of computational fluid dynamics (CFD), RF plasma, molecular dynamics (MD) sputter, and direct simulation Monte Carlo (DSMC) transport models, a multiscale approach has been used to analyze the RF diode sputtering of copper. The CFD model predicts the velocity and pressure distribution of the working gas flows in the deposition chamber. The plasma model uses these CFD results to compute ion energies and fluxes at the target and substrate. The MD model of sputtering is used to determine the initial energy distribution of sputtered atoms and reflected neutral working gas atoms and both of their angular distributions. A DSMC transport model then deduces the target atom deposition efficiency, the spatial distribution of the film thickness, the target and reflected neutral atoms energy and impact angle distributions given reactor operating input conditions such as background pressure, temperature, gas type, together with the reactor geometry. These results can then be used in atomistic growth models to begin a systematic evaluation of surface morphology, nanoscale structure, and defects dependences upon the reactor design and its operating conditions.

A combination of transmission electron microscopy (TEM) and image simulations has facilitated a highly quantitative measure of superdislocation dissociations. The experimental observations have been corrected for image shifts within the framework of anisotropic elasticity by comparison with simulated images. This allows experimental quantification of planar fault energies, thus providing a benchmark for first principles and atomistic simulations and fundamental insight towards alloy modeling and design. Such measurements of superdislocation dissociations on the order of 1-15 nm have been recorded for the pseudobinary Ni3Ge-Fe3Ge alloy system. These detailed measurements of fault widths obtained by weak-beam TEM observations of deformation structures will be presented and discussed as a function of alloy composition. The transition from anomalous to normal temperature dependence of yielding behavior in these alloys will also be discussed in terms of observed dislocation structures and planar fault energies calculated using the above measurements.

A theoretical analysis based on continuum elasticity theory and atomistic simulations is presented of the interfacial stability with respect to misfit dislocation formation, the strain fields, and the film surface morphology during layer-by-layer semiconductor heteroepitaxy. The energetics of the transition from a coherent to a semicoherent interface consisting of a misfit dislocation network, the structure of this semicoherent interface, the resulting strain fields and the morphological characteristics of the epitaxial film surfaces are calculated for InAs/GaAs(111)A. Continuum elasticity is found to describe the atomistic simulation results very well. Our theoretical results are discussed in the context of recent experimental data.

Epitaxial growth on the Si(111) surface is studied using first-principles total-energy pseudopotential calculations. The energetics of added Si atoms essentially determines epitaxial growth modes under different growth conditions (surface temperature, Si flux rate, and surface step density). We have determined the surface adatom diffusion barriers and cluster formation energies; we use these microscopic energy parameters to address the possibilities for macroscopic morphological evolution of the surface under different conditions.

First-principle theoretical investigation of the basic defects such as a molecular vacancy, a vacancy dimer, an edge dislocation, and a micro-crack in organic explosive molecular crystals is presented. As an example we considered solid RDX (C3H6N6O6) which is well studied unstable solid. It was established that external hydrostatic pressure changes optical properties of defect-free RDX as well as of the crystal with defects narrowing the band gap. The lattice defects (especially dislocations) are identified with the so-called “hot spots.” The nature of local electronic states introduced in the band gap by the edge dislocation and formed mainly by molecular orbitals of N-NO2 group is analyzed. Favorable conditions for molecular dissociation due to electronic excitation are shown.

The results of an atomistic investigation on the coalescence mechanisms of self-interstitial {311} defects are presented. Formation energies and equilibrium configurations of defect structures are determined by tight-binding molecular dynamics simulation. We focus on the characterization of the lattice strain field around the defect complex: By means of the determination of the atomic stress distribution, we discuss how it may influence the formation mechanisms of the planar {311} structures. We also attempt a correlation between structural features and electronic properties through the analysis of defect-related orbitals occupations and inverse participation ratios.

Constant temperature and pressure molecular dynamics are employed to study bulk Al and the substitutional Al-Li alloy using the embedded atom model. The appropriate embedding energy functionals and pair potentials have been determined using known experimental data of bulk Al and Li, together with the results of density functional theory studies of small Aln-Lim clusters and the ordered Al3Li crystallographic phase. The primary goal is to study the role vacancies play in the early stages of the nucleation and growth of the Al3Li crystallographic phase. To this end we present the preliminary results of a study of vacancy diffusion in Al, and Li diffusion through the Al matrix via a vacancy mechanism. In the present work the results of vacancy diffusion are analysed using a modified inverse Laplace transform, producing a continuous distribution of vacancy hopping times which reproduces the characteristic times for both the single and double vacancy hopping events.

A periodic density functional study of the high-energy π-bonded (100) stacking fault in diamond that can serve as a prototype of a twist grain boundary has been carried out. Information on formation energies, geometries and the electronic structure has been obtained. A single point electronic structure calculation of a ∑5 twist grain boundary based on the geometry taken from a molecular dynamics simulation has also been performed.

The substitution behavior of alloying elements in NiAl is investigated on the basis of electronic structure calculation using the discrete variational Xα cluster method. The method proposed by the authors to predict the substitution behavior is extended to considering lattice relaxation around an alloying atom. A diagram is drawn using the binding energy of clusters when each alloying element occupies either Ni or Al sites as parameters. Two straight lines on the diagram with slope of unity and passing the point of Ni or Al separate the alloying elements into three groups. The elements above the upper line will take Al sites and those below the lower line will take Ni sites regardless of the composition. The substitution behavior of elements in between the two lines will be affected by the alloy composition. The driving force for an element to order on one sublattice is related to its position on the diagram. The effects of temperature, composition and multi-element alloying are discussed. The concepts of site competition and stepped ordering are put forward.

We investigate the atomic-scale structures of high-sigma (∑) tilt grain boundaries in aluminum using lattice statics calculations. In particular, we examine the efficacy of the structural unit model (SUM) in the context of long-period boundaries. Our investigation of both the equilibrium and metastable structures for certain high ∑ boundaries may necessitate a revision of the SUM formalism. We also consider further departures from high symmetry boundaries by considering the structural rearrangements induced by steps on boundaries.

We use the ab initio plane wave pseudopotential method and the density functional theory (DFT) to study the arsenic(As)-vacancy interactions in silicon. The detailed lattice distortions surrounding the As-vacancy defect and the energetics of As-vacancy reaction around the six-fold ring are investigated. We find that the As displaces its neighboring silicon atoms outward while the vacancy attracts its neighboring atoms inward. The binding energy and the formation energy of an As-vacancy pair are 1.21 eV and 2.37 eV, respectively. Once the vacancy and As binds together, the highest migration barrier for the whole complex is 1.19 eV, which is in good agreement with the experimental measurement of 1.07 eV. The calculated activation energy for the vacancy mediated diffusion of the neutral As in silicon is 3.56 eV. The nature of the binding between As and vacancy is explained from the lattice distortions introduced by the As-vacancy complex.

The Korringa-Kohn-Rostoker method with Green's function averaged over the atomic configurations in a complex Ising lattice and a muffin-tin potential was used to calculate the electronic-band structure in lithium containing vacancies and s, p, and d impurities. It is shown that substantial changes in the profile of the Fermi surface do not lead to necking, as was postulated previously, but cause splitting of the electronic states at the face of the Brillouin zone. This is attributed to the reduced symmetry of the crystal lattice with impurity excitation of the electronic-subsystem.

We report simulations on the plastic behaviour of nanocrystalline Ni and Cu with grain sizes in the range of 3-12 nm. We observe a change in deformation mechanism in both materials: at the smallest grain sizes all deformation is accommodated in the grain boundaries, while at higher grain sizes we observe intragrain deformation. Analysis of the atomic configurations shows that stacking faults are produced by the passage of partial dislocations generated and absorbed in opposite grain boundaries. In Cu, we observe the stacking faults at smaller grain sizes than in Ni (8 and 12nm, respectively), which is attributed to the lower stacking fault energy of copper. Dislocations appear on slip systems that are not necessarily those favoured by the Schmid factor. Atomic displacement analysis shows deformation starts at triple points, with grain boundary sliding followed by the creation of intragrain partial dislocations.

Secondary grain boundary dislocations in large-angle grain boundaries (also called dsc-dislocations) can be described as local variations in the density of the primary (or lattice) dislocations that make up the boundary. We present a simple meso-scale simulation in which the interactions of primary dislocations with each other and with the crystal lattice produce secondary dislocations with smaller Burgers vectors and larger spacing. We use the model to explore the interactions of the primary and secondary defects with a free surface terminating the grain boundary, and demonstrate cases in which the primary dislocations dominate the interactions, forcing the secondary dislocations to increase their energy. Other cases are found, for which all of the dislocations can respond in such a way that their energy decreases. The creation of interfacial disclinations is also demonstrated.

A tight-binding molecular dynamics investigation on the structure and energetics of self-interstitial clusters in silicon is presented. We discuss how a small number of self-interstitial atoms give rise to the formation of tedrahedally-shaped clusters, while a larger number of defects exhibit a self-organization mechanism driving the system to form rod-like defects.

We investigated the interaction of point defects (vacancy and self-interstitials) with an intrinsic stacking fault in silicon. The calculations were carried out using ab initio total energy methods. The results show that point defects at an intrinsic stacking fault display a different behavior as compared to the same defect in the crystalline environment. This is evidenced by differences in formation energies and electronic structures. These results suggest that there is migration of point defects from the bulk to a stacking fault. This could affect the dislocation mobility in the crystal.

Obtaining a good atomistic description of diffusion dynamics in materials remains a daunting task due to the time-scale limitations of the molecular dynamics method. We discuss new methods, derived from transition state theory, for accelerating molecular dynamics simulations of these infrequent-event processes. Two of these methods (hyperdynamics and parallel replica dynamics) have been presented previously, and are briefly reviewed here. The third, temperature-accelerated dynamics (TAD), is presented in detail. In TAD, the system temperature is raised to stimulate more rapid escape out of each potential basin, but attempted transitions are filtered to allow only those that would have occurred at the normal temperature. The characteristics of the methods are compared.

The validity and predictive capability of continuum models of fracture rests on basic informations whose origin lies at the atomic scale. Examples of such crucial informations are, e.g., the explicit form of the cohesive law in the Barenblatt model and the shear-displacement relation in the Rice-Peierls-Nabarro model. Modem approaches to incorporate atomic-level information into fracture modelling require to increase the size of atomic-scale models up to millions of atoms and more; or to connect directly atomistic and macroscopic, e.g. finite-elements, models; or to pass information from atomistic to continuum models in the form of constitutive relations. A main drawback of the atomistic methods is the complexity of the simulation results, which can be rather difficult to rationalize in the framework of classical, continuum fracture mechanics. We critically discuss the main issues in the atomistic simulation of fracture problems (and dislocations, to some extent); our objective is to indicate how to set up atomistic simulations which represent well-posed problems also from the point of view of continuum mechanics, so as to ease the connection between atomistic information and macroscopic models of fracture.