Weak-beam Dark-field Electron Research Articles

Iron nitride, α″-Fe16N2, around interstitial type dislocation loops in neutron-irradiated iron

ABSTRACT Atom probe tomography (APT) and selected area electron diffraction (SAED) by transmission electron microscopy (TEM) were applied to nano-scale precipitates formed in a neutron-irradiated iron (99.99% pure) at high temperatures. In the conventional TEM, we discovered donut-type precipitates on the {100} planes that formed at the dilatational side of interstitial type dislocation loops with the Burgers vector of <100> on the habit planes of {100} in the bcc iron lattice. The precipitates were identified as nitride, bct α″-Fe16N2, by chemical composition and lattice structural analyses using APT and SAED. Image contrasts of the α″-Fe16N2 in a weak beam dark-field electron microscopy study were carefully analysed with the diffraction vector and the sign of deviation parameter from the Bragg condition, and it was concluded to be a selective visualisation of α″-Fe16N2 on one of the {100} planes. These results were in agreement with the crystallographic orientation between the α″-Fe16N2 and matrix iron, thereby bridging a knowledge gap in α″-Fe16N2 formation during neutron irradiation using a sodium-cooled experimental fast reactor, JOYO, or a 14 MeV D-T neutron source, RTNS-II.

Three-Dimensional Atomic Structure of Grain Boundaries Resolved by Atomic-Resolution Electron Tomography

Summary Grain boundaries (GBs) determine the properties of polycrystals, and tailoring the GB structure offers a promising method for the discovery and engineering of new materials. However, GB structures are far from well understood because of their structural complexity and limitations of conventional projection imaging methods. Here, we decipher three-dimensional atomic structure and crystallography of GBs in nanometals using atomic-resolution electron tomography. Unlike conventional descriptions, whereby they are either straight or curved planar planes with one-dimensional translational symmetry, we show that the high-angle GBs completely lose translational symmetry due to undulated curvature related to configurations of structural units. Moreover, we directly visualize kinks and jogs at the single-atom scale in dislocation-type GBs and investigate their mobilities. Our findings bring new insights to the conventional wisdom of GBs and show the importance of developing methods to include the non-planar nature of GBs to statistically evaluate the behavior of GBs in modeling studies.

Three-dimensional visualization of dislocation-precipitate interactions in a Al–4Mg–0.3Sc alloy using weak-beam dark-field electron tomography

Abstract

Tilt generation in step-graded InxGa1−xAs metamorphic pseudosubstrate on a singular GaAs substrate using a low-temperature grown InGaP interlayer

Metamorphic pseudosubstrates of In0.42Ga0.58As were grown by molecular beam epitaxy using step-graded InxGa1−xAs buffer layers grown either directly on a (001) GaAs substrate or on a GaAs substrate overgrown with a layer of low-temperature grown In0.51Ga0.49P (LT-InGaP). The structures were examined using x-ray reciprocal space mapping to determine the characteristics of the pseudosubstrates and buffer layers. For the sample grown on the LT-InGaP layer, the pseudosubstrate exhibited an asymmetric tilt around [11¯0] toward the [110] direction. Weak-beam dark-field electron imaging shows an imbalance of misfit dislocations with opposite sign Burgers vector. An explanation for this tilt is given and it is suggested that it may be responsible for the improved quality of epitaxial layers grown on such pseudosubstrates.

Three-dimensional analysis of dislocation networks in GaN using weak-beam dark-field electron tomography

We have developed a new method of tomographically reconstructing extended three-dimensional dislocation networks using weak-beam dark-field (WBDF) imaging in a TEM. A series of WBDF images is recorded every few degrees over a large tilt range, while ensuring that the dark-field reflection used for imaging maintains a constant deviation parameter. With suitable filtering of the WBDF images prior to tomographic reconstruction, the three-dimensional distribution of dislocations is reproduced with high fidelity and high spatial resolution. The success of this approach is demonstrated for heteroepitaxial Mg-doped GaN films. The fidelity of the tomographic reconstruction varies with the dislocation line-vector and elastic anisotropy of the material.

Weak-beam dark-field electron tomography of dislocations in GaN

By combining weak-beam dark-field imaging with tomography, we have been able to reconstruct the three-dimensional structure of dislocation arrays in GaN. With a mixture of both threading and in-plane dislocations, owing to plastic relaxation of the film, we look at how well each dislocation is reconstructed and what limits are imposed by way of dislocation density and material anisotropy.

Pd particles supported on ZnO (001) epitaxial thin films, evolution during HRTEM observations

Pd particles were condensed under UHV on ZnO (001) thin films epitaxially grown on mica single crystals. The epitaxial relationships between Pd and ZnO lattices were determined by electron diffraction, (111) Pd//(001) ZnO and [112] Pd//[100] ZnO. A tetrahedral shape of the particles larger than 20 nm was observed, using weak beam dark field electron microscopy. After strong irradiation conditions, an evolution of the structure was observed, which could be due to the formation of Pd Zn alloy.

HRTEM study of a Cu/Al2O3interface

Abstract The interface between Cu and faceted particles of a cubic Al2O3 phase in an internally oxidized CuAl alloy was studied by HRTEM. Image simulations indicate that the Al2O3 precipitates have a highly disordered spinel-type structure. The precipitates grow by ledge migration and possess internal defects, mainly coherent twin boundaries and stacking faults. At the Cu/Al2O3 interface, two orientation relationships were observed: ‘parallel topotaxy’ and ‘twin topotaxy’. The interface was always parallel to {111} planes of the Al2O3. No localized misfit dislocations were observed either by HRTEM or by weak beam dark field electron microscopy. The atomistic configuration at the interface is well described by a rigid lattice model assuming that a partially occupied oxygen layer terminates the Al2O3.

The use of weak-beam dark-field Electron Microscopy in the analysis of defects in intermetallics

Defect contrast under weak-beam (WB) conditions was understood in the late sixties. Until then, details on dislocation dissociation below a resolution of about ξg/3 was only matter of speculation. WB elucidated a number of problems on dissociation at a resolution of 2 to 6nm. WB became much less fashionable in the late seventies, leaving considerable domains almost undocumented. At this time, WB remained used mostly by materials scientists in the area of semiconductors and ceramics. A renewed WB activity took place in the mid eighties with the initiation of R &amp; D projects on ordered intermetallics in several countries. Ordered metal alloys are attractive since they offer high specific strengths that may increase with increasing test temperature above ambient up to 600°C-850°C. Unfortunately, most of these systems are so brittle that none of the expected substitutes for superalloys has been designed so far. Presently, Ti-Al is the system that receives the largest attention. Dislocation analysis is important in intermetallics since, excepted in multicomponent systems, the mobility of dislocations is dictated (i) by dissociation symmetries, (ii) by the transitions that dissociation may undergo dynamically and (iii) by the role of temperature on these. In view of the splitting distances that are currently observed, WB is perfectly adapted to this prospect. As a matter of facts, the potential of WB was originally exemplified by a report on four-fold dissociation in ordered Fe3Al. Upon request of the organizers of this session, the present contribution focuses on instrumental electron microscopy and on some aspects of contrast analysis under WB conditions. General information on the implications of core structure analysis on the mechanical properties of alloys together with limitations of dislocation analysis in TEM are reviewed elsewhere.

High-resolution electron microscopy of supported silver clusters

Abstract Small silver clusters produced by the gas aggregation technique and supported by thin carbon films or asbestos fibres have been investigated by high-resolution electron microscopy. Single crystals and twins have been found, and the proportion of the latter increased with increasing particle size. Both decahedra and cuboctahedra coexist for particles ranging in size between 2 and 10 nm. The cuboctahedra could also be characterized by weak-beam dark-field electron microscopy. Both symmetries were often found to have a few structural defects, the particles with threefold symmetries always containing fewer defects than those with the fivefold symmetries. The influence of contamination by air was also studied.

The structure of small, vapor-deposited particles: II. Experimental study of particles with hexagonal profile

“Multiply-twinned” gold particles with hexagonal bright field TEM profile were determined to be icosahedra composed of 20 identical and twin-related tetrahedral building units that do not have an fcc structure. The crystal structure of these slightly deformed tetrahedra is rhombohedral. Experimental evidence supporting this particle model was obtained by selected-zone dark field and weak beam dark field electron microscopy. In conjunction with the results of part I, it has been concluded that multiply- twinned gold particles of pentagonal or hexagonal profile that are found during the early stages of the vapor deposition growth process on alkali halide surfaces do not have an fcc crystal structure, which is in obvious contrast to the structure of bulk gold.