D.J. Srolovitz

Development of new interatomic potentials appropriate for crystalline and liquid iron

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.

Development of an interatomic potential for phosphorus impurities in α-iron

We present the derivation of an interatomic potential for the iron–phosphorus system based primarily on ab initio data. Transferability in this system is extremely problematic, and the potential is intended specifically to address the problem of radiation damage and point defects in iron containing low concentrations of phosphorus atoms. Some preliminary molecular dynamics calculations show that P strongly affects point defect migration.We present the derivation of an interatomic potential for the iron phosphorus system based primarily on {it ab initio} data. Transferrability in this system is extremely problematic, and the potential is intended specifically to address the problem of radiation damage and point defects in iron containing low concentrations of phosphorus atoms. Some preliminary molecular dynamics calculations show that P strongly affects point defect migration.

Surface stress model for intrinsic stresses in thin films

A simple model was presented for intrinsic stress generation in thin films resulting from surface stress effects. This mechanism can explain the origin of compressive stresses often observed during island growth prior to coalescence, as well as intrinsic compressive stresses reported for certain continuous, fully grown films. In some cases, surface stress effects may contribute to a sudden change in the intrinsic stress during island coalescence.

MISORIENTATION DEPENDENCE OF INTRINSIC GRAIN BOUNDARY MOBILITY: SIMULATION AND EXPERIMENT

Abstract Both experimental and atomistic simulation measurements of grain boundary mobility were made as a function of temperature and boundary misorientation using the same geometry that ensures steady-state, curvature-driven boundary migration. Molecular dynamics simulations are performed using Lennard–Jones potentials on a triangular lattice. These simulations represent the first systematic study of the dependence of intrinsic grain boundary mobility on misorientation. The experiments focus on high purity Al, with 〈111〉 tilt boundaries, which are isomorphic to those examined in the simulations. Excellent agreement between simulations and experiments was obtained in almost all aspects of these studies. The boundary velocity is found to be a linear function of the curvature and the mobility is observed to be an Arrhenius function of temperature, as expected. The activation energies for boundary migration varies with misorientation by more than 40% in the simulations and 50% in the experiments. In both the simulations and experiments, the activation energies and the logarithm of the pre-exponential factor in the mobility exhibited very similar variations with misorientation, including the presence of distinct cusps at low Σ misorientations. The activation energy for boundary migration is a logarithmic function of the pre-exponential factor in the mobility, within a small misorientation range around low Σ misorientations.

Molecular dynamics simulation of triple junction migration

We present a molecular dynamics simulation study of the migration of grain boundaries with triple junctions. We have monitored the grain boundary profile, triple junction angles and rate of grain boundary migration with and without triple junctions as a function of grain size, grain misorientation, direction of migration and temperature in a series of configurations designed to ensure steady-state migration. The present results demonstrate that triple junction mobility is finite and can be sufficiently small to limit the rate of grain boundary migration. The drag on grain boundaries due to limited triple junction mobility is important at small grain sizes, low temperature and near high symmetry grain misorientations. This drag limits the rate of grain boundary migration and leads to triple junction angles that differ substantially from their equilibrium value. Simulation data suggest that triple junction drag is much more a factor at low temperature than at high temperature. The triple junction mobility is shown to depend upon the direction of triple junction migration. The present results are in excellent qualitative agreement with experimental observations.

Effect of Fe segregation on the migration of a non-symmetric Σ5 tilt grain boundary in Al

We present an analysis, based upon atomistic simulation data, of the effect of Fe impurities on grain boundary migration in Al. The first step is the development of a new interatomic potential for Fe in Al. This potential provides an accurate description of Al–Fe liquid diffraction data and the bulk diffusivity of Fe in Al. We use this potential to determine the physical parameters in the Cahn–Lucke–Stuwe (CLS) model for the effect of impurities on grain boundary mobility. These include the heat of segregation of Fe to grain boundaries in Al and the diffusivity of Fe in Al. Using the simulation-parameterized CLS model, we predict the grain boundary mobility in Al in the presence of Fe as a function of temperature and Fe concentration. The order of magnitude and the trends in the mobility from the simulations are in agreement with existing experimental results.

Dislocation climb effects on particle bypass mechanisms

We examine the effects of dislocation climb on the mechanisms by which dislocations bypass particles. The analysis is based upon three-dimensional, level-set, dislocation dynamics simulations that include all elastic interactions, dislocation glide, cross-slip and climb, and particles that are either impenetrable or penetrable and either with or without misfit. When the particle is misfitting with respect to the matrix, the dislocation migration is strongly influenced by the elastic fields created by the misfit. An edge dislocation tends to climb towards either the top or bottom of the particle and may remain there if the stress is not too large. A screw dislocation may wrap around the particle several times, creating a helical dislocation structure at small applied stresses. If the stress is increased, these helices break into an array of loops. Without misfit, climb invariably lowers the threshold stress particle bypass. However, in the misfit case, climb can also lead to more stable dislocation structures and, hence, increases the threshold stress. This report emphasizes the detailed bypass and pinning mechanisms and provides insight into the conditions under which these mechanisms operate.

Effects of boundary inclination and boundary type on shear-driven grain boundary migration

A series of molecular dynamics simulations was performed on a bicrystal to which a fixed shear rate was applied parallel to the boundary plane. Under some conditions, grain boundary motion is coupled to the relative tangential motion of the two grains. In order to investigate the generality of this type of coupled shear/boundary motion, simulations were performed for both special (low Σ) and general (non-Σ) [010] tilt boundaries over a wide range of grain boundary inclinations. The data point to the existence of two critical stresses: one for coupled shear/boundary motion and the other for grain boundary sliding. For the non-Σ boundaries, the critical stress for coupled shear/boundary motion is typically smaller than that for sliding; coupled shear/boundary motion occurs for all inclinations. For Σ5 boundaries, for which the critical stress is smaller and depends on boundary inclination, coupled shear/boundary motion occurs for some, but not all inclinations.

Stress distributions in growing oxide films

We present a new continuum model for the growth of an oxide film that examines the generation of self-stresses. The model self-consistently accounts for the thermodynamics and kinetics of the evolution of film thickness, diffusion of all components, oxidation reaction rates and the effects of stresses on these. Numerical solution of the models shows that large compressive stresses and significant stress gradients can be developed across the oxide layer. The signs of the stress and stress gradients are consistent with experimental observations. Following an initial transient, the concentration profiles and stresses settle into a steady-state, in which the concentration profiles and stresses are independent of the relative magnitudes of the oxygen and cation diffusivities. We have developed an approximate, analytical solution for the composition and stress that accurately matches the steady-state results obtained numerically.

A regular solution model for impurity drag on a migrating grain boundary

We analyze the effect of solution thermodynamics on the mobility of grain boundaries in the presence of diffusing impurities within the framework of the impurity drag theory originally proposed by Cahn and Lucke and Stuwe for an ideal solution. The new derivation is performed within the framework of the regular solution model. The effects of non-ideality are largest when the impurities are attracted to the grain boundary and the deviation from ideality is positive. Positive deviations from ideality lead to enhanced impurity drag on the boundary and negative deviations lead to higher boundary mobility. When the impurities are attracted to the boundary, positive (negative) deviations from ideality produces larger (smaller) activation enthalpies for boundary migration (relative to the ideal solution). At large impurity concentrations, the activation enthalpy becomes impurity concentration independent. Decreasing impurity concentrations in ideal solutions and regular solutions with a negative deviation from ideality leads to a decrease in the activation enthalpy for migration.