Showing papers by "David J. Srolovitz published in 2003"

Development of new interatomic potentials appropriate for crystalline and liquid iron

Abstract: Two procedures were developed to fit interatomic potentials of the embedded-atom method (EAM) form and applied to determine a potential which describes crystalline and liquid iron. While both procedures use perfect crystal and crystal defect data, the first procedure also employs the first-principles forces in a model liquid and the second procedure uses experimental liquid structure factor data. These additional types of information were incorporated to ensure more reasonable descriptions of atomic interactions at small separations than is provided using standard approaches, such as fitting to the universal binding energy relation. The new potentials (provided herein) are, on average, in better agreement with the experimental or first-principles lattice parameter, elastic constants, point-defect energies, bcc–fcc transformation energy, liquid density, liquid structure factor, melting temperature and other properties than other existing EAM iron potentials.

Brittle fracture in polycrystalline microstructures with the extended finite element method

Abstract: A two-dimensional numerical model of microstructural effects in brittle fracture is presented, with an aim towards the understanding of toughening mechanisms in polycrystalline materials such as ceramics. Quasi-static crack propagation is modelled using the extended finite element method (X-FEM) and microstructures are simulated within the framework of the Potts model for grain growth. In the X-FEM, a discontinuous function and the two-dimensional asymptotic crack-tip displacement fields are added to the finite element approximation to account for the crack using the notion of partition of unity. This enables the domain to be modelled by finite elements with no explicit meshing of the crack surfaces. Hence, crack propagation can be simulated without any user-intervention or the need to remesh as the crack advances. The microstructural calculations are carried out on a regular lattice using a kinetic Monte Carlo algorithm for grain growth. We present a novel constrained Delaunay triangulation algorithm with grain boundary smoothing to create a finite element mesh of the microstructure. The fracture properties of the microstructure are characterized by assuming that the critical fracture energy of the grain boundary (Gcgb) is different from that of the grain interior (Gci). Numerical crack propagation simulations for varying toughness ratios Gcgb/Gci are presented, to study the transition from the intergranular to the transgranular mode of crack growth. This study has demonstrated the capability of modelling crack propagation through a material microstructure within a finite element framework, which opens-up exciting possibilities for the fracture analysis of functionally graded material systems. Copyright © 2003 John Wiley & Sons, Ltd.

Interatomic potential for vanadium suitable for radiation damage simulations

Abstract: The ability to predict the behavior of point defects in metals, particularly interstitial defects, is central to accurate modeling of the microstructural evolution in environments with high radiation fluxes. Existing interatomic potentials of embedded atom method type predict disparate stable interstitial defect configurations in vanadium. This is not surprising since accurate first-principles interstitial data were not available when these potentials were fitted. In order to provide the input information required to fit a vanadium potential appropriate for radiation damage studies, we perform a series of first-principles calculations on six different interstitial geometries and vacancies. These calculations identify the 〈111〉 dumbbell as the most stable interstitial with a formation energy of approximately 3.1 eV, at variance with predictions based upon existing potentials. Our potential is of Finnis–Sinclair type and is fitted exactly to the experimental equilibrium lattice parameter, cohesive energy, elastic constants and a calculated unrelaxed vacancy formation energy. Two additional potential parameters were used to obtain the best fit to the set of interstitial formation energies determined from the first-principles calculations. The resulting potential was found to accurately predict both the magnitude and ordering of the formation energies of six interstitial configurations and the unrelaxed vacancy ground state, in addition to accurately describing the migration characteristics of the stable interstitial and vacancy. This vanadium potential is capable of describing the point defect properties appropriate for radiation damage simulations as well as for simulations of more common crystal and simple defect properties.

Origins of growth stresses in amorphous semiconductor thin films.

Abstract: Stress evolution during deposition of amorphous Si and Ge thin films is remarkably similar to that observed for polycrystalline films. Amorphous semiconductors were used as model materials to study the origins of deposition stresses in continuous films, where suppression of both strain relaxation and epitaxial strain inheritance provides considerable simplification. Our data show that bulk compression is established by surface stress, while a subsequent return to tensile stress arises from elastic coalescence processes occurring on the kinetically roughened surface.

Molecular dynamics study of the threshold displacement energy in vanadium

Abstract: The threshold displacement energy (TDE) is calculated for vanadium as a function of temperature and orientation by molecular dynamics simulations. The TDE varies from 13 to 51 eV, depending on orientation and is nearly temperature independent between 100 and 900 K. The lowest TDE is in the 〈100〉 direction. We characterize the defects associated with the displacement simulations and found that they consist of vacancies and 〈111〉-split dumbbells.

Morphological Stability during Electrodeposition I. Steady States and Stability Analysis

Abstract: We develop a formalism for predicting morphology evolution during electrodeposition as a function of the deposition parameters, composition of the electrolyte, and the species being deposited. Our model explicitly couples the electrostatic fields and the metal cations and spectator ions of arbitrary concentrations. We first perform a mixed asymptotics analysis to predict the self-consistent, uniform, steady-state electrostatic, and concentration fields. Morphology evolution is analyzed within the framework of perturbation theory, where we linearize around the uniform, steady-state fields. We find that the surface is unstable at long length scales due to a diffusional instability, in agreement with previous results. Furthermore, we find that both increasing the deposition rate and the spectator ion concentration within the electrolyte at fixed deposition rate increases surface roughness, also in agreement with common experience. We provide an approximate analytical formula for the perturbation growth rates as a function of the spectator ion concentration. The formalism developed here provides a rigorous, self-consistent foundation upon which the effects of additives on surface morphology are analyzed in a companion paper.

Morphological Stability during Electrodeposition II. Additive Effects

Abstract: Common experience shows that electrodeposited (ED) metallic films exhibit rough surfaces unless the electrochemical bath contains small quantities of molecular additives. We develop a formalism for describing the effects of additives on surface morphology evolution, which builds on that in a companion paper for the additive-free case. We demonstrate that the additives suppress the morphological instability that leads to roughening by preferentially accumulating near surface protrusions and blocking growth sites. Our chemically based model shows that additives which readily adsorb onto the surface and have a strong tendency to complex with the metal cations reduce the driving force for the instability and thus enhance leveling. Furthermore, polar additives provide an additional stabilizing effect, in accord with experimental observations. It is also shown that linearly stable growth can be achieved over a wide range of deposition flux at sufficiently large additive bulk concentrations. We predict the ED conditions necessary for growing flat films and demonstrate that these are in good agreement with experimental observations. © 2003 The Electrochemical Society. All rights reserved.

Discrete dislocation simulations of the development of a continuum plastic zone ahead of a stationary Mode III crack

Abstract: We present results from discrete dislocation simulations showing the development of the plastic zone in front of a Mode III crack under constant load. We find that the equilibrated zone is circular, in agreement with continuum mechanics predictions of the elastic-perfectly plastic Mode III crack. The size of the equilibrated zone scales as the square of the applied load (KIII), also in agreement with the continuum results. The zone approaches saturation exponentially, with a time that scales as KIII2/σp3, where σp is the Peierls stress. These results delineate conditions under which the classical, continuum predictions of elastic–plastic fracture mechanics are applicable.

Mechanism map for a misfitting film on a viscous substrate

Abstract: Compressively stressed elastic films bonded to a viscous interlayer that separates the film from the substrate can experience stress relief by one of several mechanisms: buckling, in-plane expansion at free edges, or delamination from free edges. Here, results for edge relaxation and delamination are combined with prior work on buckling to provide a comprehensive map showing how the dominant stress relief and failure mechanisms depend on geometrical and material parameters. The mechanism map is constructed by comparing the characteristic relaxation times for each relaxation mechanism. The map provides the basis for simple engineering design rules to achieve a preferred failure mechanism.