Tilt Boundaries In Gold Research Articles

Accommodation of coherency strain by interfacial disconnections at a 90° ⟨110⟩ grain boundary in gold

The structure of interfacial disconnections (i.e. line defects possessing both step and dislocation character) at a 90° d 110 ¢ tilt boundary in gold has been investigated. High-resolution transmission electron microscopy observations identify these defects separating short (40-60 Å) d 111 ¢ - d 112 ¢ terraces along the boundary. Because the ratio of the periodic length in the d 111 ¢ direction to that in the d 112 ¢ direction is irrational (2 1/2 ), the crystallography of this boundary is incommensurate. By measuring the dislocation content of the disconnections, it is shown how the defects accommodate the 5.7% 'misfit' in the directions parallel to the interface and thereby enable coherency to be achieved across the d 111 ¢ - d 112 ¢ terraces. This structural relaxation is markedly different from that at d 100 ¢ - d 110 ¢ grain boundary facets in this same system, for which previous observations have found an incoherent structure with no strain localization.

A dislocation-based description of grain boundary dissociation: application to a 90° 〈110〉 tilt boundary in gold

High resolution transmission electron microscopy (HRTEM) observations and atomistic simulations of {1 1 ̄ 1}/{121} facets in a gold 90° tilt boundary show the presence of a ∼10 Å wide layer with stacking faults distributed one to every three close-packed planes. This interfacial reconstruction, which forms the rhombohedral 9R stacking arrangement, is similar to that found previously for near−Σ=3{112} boundaries in low stacking fault energy metals. We discuss a general approach for partitioning grain boundary orientation into a set of Shockley partial dislocations and then apply this description to the {1 1 ̄ 1}/{121} interface. The analysis explains both the distribution of faults and the geometry of the local plane bending and shows, further, that the 9R stacking occurs in both the Σ=3{112} and {1 1 ̄ 1}/{121} interfaces due to the similar ratios of 30° and 90° Shockleys in both cases. Finally, we discuss limitations of this description, in particular concerning a 1/8[ 1 ̄ 01] relaxation that is predicted by the atomistic simulations.

A Study of the Dependence of the Structure of Σ = 3 Tilt Boundaries in Gold on the Inclination of the Boundary Plane

AbstractThe atomic structure of Σ = 3 tilt boundaries with variously inclined boundary planes has been studied using the many body potentials for gold. A chain unit model, analogous to the structural unit model, describes the relationship between different boundaries. The basic units in this model are units of the (111) and (112) twin boundaries and the model corresponds to atomic level faceting into these boundaries.

Structural multiplicity observed at a Σ = 5/[001] 53·1° tilt boundary in gold

Abstract Using high-resolution transmission electron microscopy the observation of two different metastable structures at a Σ = 5/[001]53·1° (210) symmetrical tilt boundary are reported upon. A structural unit approach is used to describe the differences between these structures. The results are consistent with computer simulation studies where a small difference in minimum energy states was previously reported.

Atomic structure and energy of ∑ = 5 tilt boundaries in gold

The atomic structure of 2 = 5(θ = 36.9 deg) [001] tilt boundaries in gold has been investigated by high-resolution electron microscopy (HREM). Image simulations and experimental conditions for observing grain boundary atomic structure in gold are presented. Two preferred orientations corresponding to symmetric {310} and asymmetric {430}//{100} inclinations have been observed frequently. A single symmetric {210} inclination has also been observed. The atomic structures of these three boundaries are presented. An average ratio of grain boundary to surface energy, γgb/γs, of 0.62 has been measured at 200 °C. Unique atomic structures are observed for {310}, {430}//{100}, and {210} inclinations, and multiplicities in atomic structures have not been detected. Interfacial volume expansion of these interfaces is presented and compared to computational models.

Multiplicity of atomic structure for Σ = 17/[001] symmetrical tilt boundaries in gold

The first direct observations of structural multiplicity in a metal at atomic resolutions are reported upon. Here two different metastable structures of Σ = 17/[001] 28.1° (410) symmetrical tilt boundaries in gold have been observed by high resolution transmission electron microscopy. These boundaries can be described in terms of structural units that are consistent with earlier atomistic computer modelling studies which have found that similar minimum energy states exists in boundaries near the Σ = 17 misorientation.

Dislocation reaction in a Σ = 41 related asymmetrical tilt boundary in gold

An asymmetrical tilt boundary was fabricated in gold by joining a {100} single crystal to a {111} single crystal with a 〈110〉 direction as nearly as possible common to both crystals. The interaction of a lattice dislocation with one of many line defects in the boundary suggests that crystal lattice dislocations have a role in the accommodation of misorientation and strain in this boundary.

High-resolution imaging of Σ=5 tilt boundaries in gold

Until recently most of the information concerning the atomic structure of grain boundaries in metals has been obtained using molecular static and molecular dynamic computational techniques. With the recent developments of intermediate voltage microscope (300-400KV) this situation has changed and grain boundary atomic resolution is now possible for most metals. The purpose of this research is to examine the atomic structure of Σ=5 tilt boundaries in Au by high resolution microscopy and to compare the results to computational models.Thin film Au bicrystals containing Σ=5 (θ=36.5°±0.5) tilt grain boundaries were produced by epitaxial growth on NaCl bicrystalline substrates using a technique described in detail elsewhere. All high resolution images were obtained with a Philips 430 ST microscope using axial illumination and without objective aperture. All image simulations were obtained using the multislice formalism with EMS programs. All four {200} reflections from each crystal were used for the simulations with the following instrumental parameters; accelerating voltage V=300KV, spherical aberration constant Cs=1.1mm, defocus spread Δ=8nm and semi-angle beam divergence α=8mrad.

Structure of an asymmetrical tilt boundary in gold

Balluffi and coworkers developed a technique for making grain boundaries, with chosen misorientations, by welding together appropriately oriented single crystal films. This technique has been used to make gold bicrystals with a misorientation which does not correspond to any short period structure of the kind which is usually thought to have low energy. The procedure followed was to join {100} and {111} single films with <110> directions in each film approximately parallel. With [110](001) and [110](111) exactly parallel an asymmetrical tilt boundary with misorientation 54.73°/[110] is produced. For Σ, the reciprocal fraction of coincidence sites <50, the periodic boundary structures nearest in misorientation are 50.48°/[l10], Σ=11; 55.88°/[110], Σ = 33. Analysis of the present boundaries can therefore give direct evidence relating to the controversial question of the extent of the physical significance of the dislocation model for grain boundaries.Fig. 1 shows a typical region of the interface. Patches of grain boundary are surrounded by regions where the two films are not in contact.

Observations of hierarchical grain-boundary dislocation structures in [001] symmetric tilt boundaries in gold

Abstract Molecular statics calculations using pairwise interatomic potentials by Sutton, Vitek and Wang have predicted the existence of a hierarchy of localized edge-type grain-boundary dislocations (GBDs) in [001] symmetric tilt boundaries in f.c.c. noble metals. Furthermore, the geometry of these GBDs can be well described by the structural unit/GBD model of Sutton and Vitek as extended by Balluffi and Bristowe. In the present work, a search was made for the presence of such a hierarchy in [001] symmetric tilt boundaries in gold by means of transmission electron microscopy. Numerous arrays of various localized GBDs were detected under various conditions of observation in specimens of differing thickness under a variety of imaging conditions. When the present results are combined with results from other investigations, it is concluded that the first four members of the hierarchy have been detected. One GBD array belonging to an appreciably higher-order hierarchical member has been found, and it seems likely that higher-order members (which became increasingly difficult to detect) also exist. The results provide support for both the calculations and the structural unit/GBD model.

Structure Determination of Tilt Boundaries in Gold by High Resolution Electron Microscopy

ABSTRACTA number of tilt grain boundaries prepared from evaporated gold thin films have been investigated by high resolution transmission microscopy. When the grain boundary is parallel to the electron beam and the beam is parallel to a low index rotation axis such as [110] or [001]it is possible to identify atomic positions at the cores of these boundaries as demonstrated here by a Σ = 3 70.5°/[110], (112) growth twin. In many cases it is not possible to make a full atomistic structure determination because the specimen does not have translational periodicity in the beam direction. This may be because the boundary plane is not parallel to the beam or the specimen contains dislocations which have a component of Burgers vector parallel to the beam. Examples are given of various low angle boundary structures in goldwhere there are complications because of the three dimensional nature of their structure.

Diffusion, Migration and the Atomic Structure of some Grain Boundaries

AbstractThe atomic structure of representative tilt boundaries in gold has been determined by high resolution transmission electron microscopy. Characteristic and varying regions of decreased density and coordination have been identified and related to mechanisms of grain boundary diffusion and migration

Characterization of [001] tilt boundaries in gold by high-resolution transmission electron microscopy

Abstract Selected [001] tilt boundaries in bicrystalline thin films of gold, corresponding to angles of misorientation ranging from 5° to 39°, have been examined by high-resolution transmission electron microscopy in order to characterize structure of low-angle, high-angle, and coincidence grain boundaries. Both bright-field and lattice-fringe images reveal that such boundaries are composed of a regular array of primary relaxations, with mean spacing identical to that of the O lattice, and a more widely spaced array of secondary relaxations, which accommodate small deviations from exact coincidence orientations. Results indicate that all tilt boundaries are characterized by a periodic structure, corresponding to primary relaxations between O-Lattice points. Moreover, primary and secondary relaxations occur concurrently near coincidence orientations.

The Simulated and Observed Structure of a Σ=11 Tilt Boundary in Gold and the Vibration of Individual Atoms at the Grain Boundary

The atomic structure of a Σ=11 tilt grain boundary in gold was simulated by the molecular dynamic techninue using the Morse potential. The simulated structure is compared with the lattice image of a Σ=11 boundary in gold observed by high resolution electron microscopy. The local density of states of phonons was calculated by the recursion method for atoms near the boundary plane. It was found that the resonant mode with a large amplitude occurs with atoms at the boundary plane.

X-ray diffraction study of the structure of small-and large-angle [001] tilt boundaries in gold

Abstract The structures of selected small- and large-angle [001] tilt boundaries in gold bicrystals were studied using X-ray diffraction techniques. All the boundaries examined gave rise to extra reflections. It was shown that these reflections could not result from (1) double diffraction, (2) surface diffraction or (3) diffraction of radiation with one-half or one-third of the characteristic CuKα wavelength and, therefore, must come from the grain boundary. The presence of these reflections is direct evidence for the periodic nature of all the grain boundaries studied. The grain boundary reflections were frequently split into pairs which are present on the high- and low-angle side of the tail of the f.c.c. reflection. This splitting was related to the presence of a small twist component superimposed on the tilt boundary. In general the low-angle reflections were observed to be higher intensity than the high-angle reflections. The analysis of these intensity observations indicates that the tilt-boundary structure in the plane of the boundary consists of a large region with a small expansion followed by a small region with a large contraction. The diffraction technique is unique in being able to obtain this type of detailed information on the structure of grain boundaries.

Serration and dissociation of edge dislocations in low angle tilt boundaries in gold

Abstract The fine structure of a variety of low angle tilt boundaries in gold has been studied by high resolution transmission electron microscopy and electron diffraction in thin-film bicrystal specimens. The boundaries were found to consist of arrays of dislocations running on average along the tilt axis as expected. However, in the general case where the tilt axis did not lie in a {111} plane the dislocations were serrated into a “hill and valley” structure which allowed them to dissociate on {111} planes into partial dislocations and patches of stacking faults. This configuration produced a decrease in core energy at the expense of increased line length energy and is expected to be a general feature of dislocations in low angle tilt boundaries in metals with low (and moderately low) stacking fault energies. The importance of this phenomenon in the interpretation of measurements of pipe diffusion along dislocations in low angle tilt boundaries is discussed.

OBSERVATION DE LA STRUCTURE DES JOINTS DE GRAINS DANS DES LAMES MINCES BICRISTALLINES PRODUITES PAR UNE NOUVELLE TECHNIQUE

A new technique for production of thin-film bicrystals containing a grain boundary of any desired orientation is described. These bicrystals are grown epitaxially from the vapor phase on bicrystalline NaCl substrates containing the desired grain boundary orientation and then removed to permit direct observation of boundary structure by transmission electron microscopy. This technique is utilized to study dislocation arrays in symmetric tilt boundaries in gold. Observations are consistent with existing experimental and theoretical results.

Structure of a Coincidence and Near-Coincidence Tilt Boundary in Gold

The structure of tilt grain boundaries formed by a rotation of 60° and 30° about a 〈111〉 axis has been examined by transmission electron microscopy. In specimens containing 30° boundaries, low-angle dislocation arrays with a tilt angle of 2.2° are commonly observed. This is attributed to the presence of a coincidence site lattice relation at 27.8°.

Observation of dislocation arrays in low angle tilt boundaries in gold

AbstractTechniques were developed for studying the structure of low angle tilt boundaries by transmission electron microscopy. Boundaries were observed: 1) at normal incidence in bicrystals prepared by welding single crystal films together face‐to‐face at predetermined misorientations, 2) edge‐on in polycrystals obtained by the annealing of welded thin‐film bicrystals initially containing only twist boundaries. Symmetric (100) boundaries with [001] tilt axes consisted of either: a) arrays of uniformly spaced single edge dislocations running parallel to [001] with Burgers vectors b = a [100]; or b) arrays of uniformly spaced close pairs of dislocations running parallel to [001] with the dislocations in each pair having b = a/2 [110] and b = a/2 [110]. A choice between possibilities a) and b) could not be made on the basis of either the experimental observations or simple energy considerations. Symmetric (110) boundaries with [001] tilt axes consisted, as expected, of arrays of single edge dislocations parallel to [001] with b = a/2 [110]. Dislocation structures in (100) and (110) boundaries which possessed particularly small tilt angles and tilt axes lying along directions other than [001] in the boundary plane were also studied. In many cases, when the tilt axes were in certain favorable ranges of orientation, the dislocations broke up into segments in the boundary planes along directions which allowed them to dissociate on {111} planes into partial dislocations and stacking fault ribbons.

Dislocations in symmetric high angle [001] tilt boundaries in gold

AbstractThe structure of high angle tilt boundaries possessing small tilt deviations (i.e., 1 to 2°) from various special high coincidence site density tilt misorientations was studied by transmission electron microscopy. The latter misorientations corresponded to rotations of θc = 22.6, 28.1, 36.9, 53.1, 61.9, 67.4, and 73.7° around [001]. The grain boundary planes were of the symmetric type, and the tilt deviations from the high coincidence site density misorientations were around various axes lying in the grain boundary planes. The boundaries were examined at normal incidence in bicrystals prepared by welding single crystal thin films together face‐to‐face at predetermined misorientations and also edge‐on in polycrystals obtained by the annealing of welded bicrystals initially containing only twist boundaries. Arrays of parallel dislocations were found in a number of boundaries which appeared to be in good agreement with the structures predicted by use of Bollmann's analysis of the geometry of dislocations in high angle grain boundaries. It is therefore concluded that the structure of such high angle boundaries consists of an array of evenly spaced edge dislocations (or possibly close pairs of dislocations in certain cases) running parallel to the tilt deviation axis embedded in the interface corresponding to the nearby high coincidence site density misorientation. It seems likely that such structures exist over wider ranges of misorientation than were investigated experimentally.