Grain Boundaries

The present document is a progress report describing the work accomplished to date during the second year of our four-year grant (February 15, 1990--February 14, 1994) to study grain boundaries. The research was focused on the following three major efforts: Study of the atomic structure of grain boundaries by means of x-ray diffraction, transmission electron microscopy and computer modeling; study of short-circuit diffusion along grain boundaries; and development of a Thin-film Deposition/Bonding Apparatus for the manufacture of high purity bicrystals

COMPUTER MODELING OF GRAIN BOUNDARIES IN CUBIC METALS

On rappelle les connaissances actuelles sur la structure des joints de grains dans les cristaux cubiques, modélisée par les techniques informatiques de simulation. On énumère et l'on met en valeur les avantages et les limites des méthodes et des potentiels interatomiques utilisés. Deux approches différentes sont décrites : La première comporte des exemples de structures spéciales de joints isolés qui concordent bien avec certaines observations expérimentales et la seconde illustre une série de structures plus générales de joints, qui réunis, forment un modèle d'unité structurelle (structural unit model) compatible avec la théorie des réseaux de coïncidence, et les observations des dislocations secondaires des joints de grains. De plus on rappellera brièvement les informations relatives aux énergies calculées de joints de grains et l'on discutera leur importance et leur degré de précision. On s'intéressera tout particulièrement aux relations entre l'énergie et la désorientation en termes relatifs au modèle de réseau des sites de coïncidence.The current knowledge of grain boundary structure in cubic crystals modeled by computer simulation techniques is reviewed. The advantages and limitations of the methods and interatomic potentials employed are detailed and emphasized. Two different approaches are described : firstly, examples are given of individual special boundary structures that have matched well with particular experimental observations and, secondly, a series of more general boundary structures are illustrated which together form a structural unit model consistent with coincidence lattice theory and observations of secondary grain boundary dislocations. The information on computed grain boundary energies is also reviewed briefly and their importance and reliability discussed. The energy /misorientation relationship is discussed particularly in terms of the coincidence lattice model

STRUCTURAL UNIT/GRAIN BOUNDARY DISLOCATION MODEL FOR TWIST BOUNDARIES IN CUBIC CRYSTALS

The systematics of [001] twist boundary structure is presented formally in terms of a structural unit/grain boundary dislocation hierarchical model and the earlier model of Sutton is generalized. By comparison with experimental observation and atomistic calculation using pair-potential models the physical significance of the individual members of the hierarchy is determined. Comparison with experiment indicates a strong type primary relaxation for θ ≤ 36.9° and a significant secondary relaxation near Σ5 which must result from "oblique" perturbations in the array of primary GBD's. On the other hand, comparison with available calculated results indicates a strong type primary relaxation at low angles but a progressively weaker relaxation at higher angles. Also, no evidence is found for any significant secondary relaxations when at least one pair potential is employed. However, very recent studies indicate stronger secondary relaxations with other potentials, and this, in future work, should lead to better agreement between calculations and experiment

Ground Boundaries

The present document is a progress report describing the work accomplished on the study of grain boundaries in Ag, Au, Ni, Si, and Ge. Research was focused on the following four major efforts: study of the atomic structure of grain boundaries by means of x-ray diffraction, transmission electron microscopy and computer modeling; grain boundary migration; short-circuit diffusion along grain boundaries; and development of Thin-Film Deposition/Bonding Apparatus for the manufacture of high purity bicrystals. 10 refs., 1 fig

Volume dependence of computed grain boundary energy

Over the past five years there have been numerous studies of grain boundary structure using the method of computer molecular statics which assume pairwise central potentials for the interatomic interaction. Emphasis is usually placed on relative grain boundary energies but these may be inaccurate due to various, but related, approximations and constraints implicity imposed on the calculation-namely central forces, finite model size, fixed border conditions and volume dependent contributions to the energy of the system. It is the purpose of this work to clarify how these particular properties of the model can affect the computed grain boundary energy and demonstrate instances in which the quoted energy has strictly been inaccurate. The implication of these results, especially on how they affect the method of relaxation and the resulting grain boundary structure is discussed

Diffraction Effects from (111) Twist Boundaries in Gold

The structural characteristics of (111) twist boundaries in gold are investigated using a combination of x-ray diffraction and computer modeling techniques. Comparison of the measured scattering effects with those generated from EAM computer models reveals that the (111) boundary displacement field is weak, rotational in form and centered on O' lattice sites. Furthermore, the measured intensities of the strong O' lattice reflections decrease smoothly with increasing boundary angle up to 30{degree}, as calculated from the model. The effect of double positioning on the diffraction pattern and the structural analysis is discussed. 9 refs., 2 figs