Search

High-resolution electron microscopy (HREM) was used to obtain information about structural variations in poly[1,6-di(N-carbazolyl)-2,4-hexadiyne) (polyDCHD) nanocrystals. Several particles in different crystallographic zones were imaged with high spatial resolution of up to 0.19nm, resulting in image information on a subunit-cell level (a = 1.74nm, b = 1.29nm and c = 0.49nm; β = 108.3°, P21/a; Z = 2). The average nanocrystal had a nominally rectangular shape and a size of about 40 nm × 80 nm with the major growth direction along the polymer chains (001). Most of the HREM images correspond to the [100] or [120] zone. In a few cases, it was also possible to image crystals oriented in the [001] zone with the polymer chain axis normal to the carbon substrate. Image interpretation was done by comparison with multislice HREM image simulations based on an approximation for the crystal structure and imaging conditions. These simulations performed for polyDCHD indicate that the HREM image is sensitive to ...

Disordering of both yttrium and boron atoms in YB56 was observed, in three dimensions, from difference images of high-resolution electron microscopy (HREM) and simulated images from an X-ray determined structural model. Averaged HREM images recorded using a slow-scan charge-coupled device camera from thin regions (less than 5 nm) along the [100], [110] and [111] directions of the YB56 were compared with calculated images based on the X-ray data. A residual index R HREM was used for image analysis to determine the crystal thickness and defocus value of the observed images. Based on the experimental conditions which minimized the R HREM values, difference images between observed and simulated images were calculated. These difference images observed along the three directions of the YB56 showed the atomic disorder, which are yttrium positions and boron clusters around the yttrium atoms. This approach is useful for the evaluation of disordered atomic positions in the crystal.

Further high-resolution electron microscopy studies have been performed on coherent thin non-equilibrium precipitates and unique nanocrystals with an identical orientation, which are produced from sputter-deposited amorphous Ti-48.2 at.° Ni thin films by heat treatments near or below the crystallization temperature Tc . Abundant evidence is provided for the previously proposed mechanisms for formation of the subnanometric precipitates near Tc and the nanocrystals below Tc ; it is as follows. For heat treatment below Tc , firstly a thin plate precipitate with the {100} habit plane, which has a bct lattice with its tetragonal axis perpendicular to the habit plane, is frequently observed at the crystal-amorphous interface, secondly the bct precipitates constituting the boundaries between nanocrystals are arranged in such a way that they can truncate an approximately spherical nanocrystal, while keeping the coherency with the B2 matrix at the {100} habit plane, thirdly enrichment of Ti atoms is detec...

A new method for analysing antiphase domains using high-resolution electron micrographs is described. A high-resolution electron microscope image taken at a zone axis is treated as the sum of many sets of interference fringes. The analysis extracts the information concerning the variations in the image periodicities as a function of position. In particular, the amplitude and phase variations of these fringes are analysed separately. The amplitude images reveal the antiphase boundaries in a similar way to a conventional dark-field experiment whilst the phase produces images of the translation domains and defines the associated translation vectors. The phase variations can also be related to the strain field associated with the corresponding lattice planes. Experimental images of antiphase boundaries in Cu3Au taken at [001] are used to demonstrate the technique. Antiphase boundaries perpendicular and parallel to the electron beam are characterized. The phase images are used to characterize disorder...

We present the results of a parallel study of the atomic structure of the ∑ = 5, (210), θ = 53·13° and (310), θ = 36·87°, [001] tilt axis grain boundaries in bcc Mo, by computer simulation and high-resolution electron microscopy (HREM). Excess energy values of different boundary configurations are obtained via a quasidynamical minimization scheme while cohesion is described by a new n-body central-force, phenomenological potential which satisfactorily reproduces static and dynamical properties of the bulk material. HREM observations and numerical modelling both show the symmetric configuration of the ∑ = 5, (210), θ = 53·13° boundary to be of the lowest energy. The calculations also show that the stable ∑ = 5, (310), θ = 36·87° configuration is nearly mirror symmetric, although the experimental verification in this case is still incomplete. After a few oscillations, the interplanar spacing near the boundaries converges rapidly to its bulk value while producing an overall expansion of the bicrysta...

In the alloy Ti3 Al-Nb, several phases appear after cooling at various rates from a high temperature. These are B2, D0 (alpha-phase), L1 (eta-phase) and an 19 2 0 omicro-type phase. A high-resolution electron microscopy observation is presented - 1 of an alloy continuously cooled at 10 K s from 1200oC, which shows very small regions of the eta-phase at the boundaries between the alpha-phase and B2 phase. An opportunity is thus provided to observe the early stages of the formation of the eta-phase. Orientation relationships for the phases are given.

In order to determine the mechanisms of twin growth, a high-resolution electron microscopy study of the tip morphology in (1012) twin in zinc has been carried out. In a new model presented here, the twin tip is described as a series of (1012)//(1012) step faces, containing one or more interfacial dislocations. They are separated by incoherent (0001)//(1010) interfaces resulting from a pile-up of twinning dislocations. The stress around the twin tip is accommodated through the emission of (c + a)⅓ 〈1123〉 lattice dislocations and formation of residual dislocations b D ≈ 0·47[1011] which are of the opposite sign to the twinning dislocation. This relaxation process can be associated with a blunting mechanism which finally stops the twin growth along the [1011] direction.

Twin boundaries in the massively transformed γ-TiAl phase, which is formed in a Ti-48 at.% Al alloy quenched from the high-temperature α-Ti phase field, have been investigated by high-resolution electron microscopy. At the twin boundaries and ledges, a characteristic periodic contrast with a spacing of about 0·7 nm is frequently observed, which corresponds to the three-times periodicity of d{111}γ. This contrast is quite similar to those reported previously for12Aug1996 severely deformed γ + α2 two-phase Ti-Al alloys, which was attributed to the formation of the 9R structure with ABC/BCA/CAB stacking sequence. From close examination of this contrast with the aid of image simulation, it is concluded that the contrast is caused by overlapping twin-related crystals along the incident beam direction and not by formation of the 9R structure. The configuration of overlapping twins is explained by complex ledge structures, which result from the formation of twin-related crystals in the α → γ massive tra...

Bi V O and Bi V O have been prepared by solid state reaction and their 4 2 11 6 3 16 structural chemistry has been investigated using X-ray powder diffraction, highresolution electron microscopy, solid-state nuclear magnetic resonance and electron-energy-loss spectroscopy. Their structures are closely related to each other and are both derived from-Bi V O with Bi O layers partially 4 2 11 2 2 depressed in an ordered manner, forming various superstructures and implying a location of oxygen vacancies in the Bi O layers, rather than the V-O layers as 2 2 previously believed. These results necessitate a reconsideration of the mechanism of ionic conductivity in these materials.

Atomic arrangements of austenite fcc and martensite bcc near the interface in an Fe Ni Mn alloy have been observed by high-resolution electron microscopy. It is found that atom rows along the close-packed directions of two phases are completely coherent at the interface, but there exist deformed lattices which are neither fcc nor bcc. These lattices are regarded as transient lattices changing continuously from fcc to bcc lattice. By observation of these transient lattices we can deduce the lattice deformation in the fcc to bcc martensitic transformation. It is concluded that the lattice deformation proposed by Kurdjumow and Sachs is perfectly consistent with the observed atomic arrangements near the interface. The Bogers and Burgers mechanism is inadequate to explain the present observations. It is emphasized that the austenite martensite interface in the present alloy is not a monoatomic layer but has a width of about 0 7 nm in which the transient lattices are contained.

The structure of the Al-Rh-Cu decagonal quasicrystal with periodicity of about 0.4 nm has been investigated by high-resolution electron microscopy. A high-resolution structure image taken with the incident electron beam parallel to the tenfold axis can be interpreted as an aperiodic tiling composed of three kinds of subunit. The atomic arrangements in these subunits have been proposed according to the structure of the Al-Co-Cu decagonal quasicrystal determined by single-crystal X-ray analysis. A calculated image using the present model reproduces well the contrast features of the high-resolution structure image.

For two γ-TiAl grains exhibiting a 180° twin relationship, the junction between a {111}γ1 // {111}γ2 twin plane and a perpendicular {121}γ1 // {121}γ2 interface has been investigated by high-resolution electron microscopy. The {111}-(121} junction exhibits a region of complex contrast due to dislocations at the end of both the {111} and the {121} facets. The {121}γ1 // {121}γ2, twin interface can be produced either by γ → γ shear or as the result of two α → γ transitions and subsequently undergoes a rigid-body shift at the interface in order to relax its structure. Both the structure and the mobility of {121}LI0 interfaces are discussed.

The formation process of the decagonal quasicrystalline phase (D-phase) with 1.2 nm periodicity from a crystalline approximant in the Al70Pd13Mn17 alloy has been identified by high-resolution electron microscopy. At the early formation stage, decagonal atom clusters, which are the most important structural units of the D-phase, were observed to segregate from the crystalline matrix with an atomic correlation determined by the structural characteristics of the crystal. A non-equilibrium structural state (a mixture of the segregated atom clusters and the crystalline matrix phase) was also observed, and its structural characteristics clearly revealed. The equilibrium D-phase, whose structure is characterized by an aperiodic arrangement of decagonal atom clusters with definite linkages and a long-range quasiperiodic correlation, was formed by fully annealing at 800degreeC for a long time. The linear phason strain in the equilibrium D-phase formed from the crystalline approximant may be related to the structur...

Lattice image simulations appropriate to high resolution electron microscopy (HREM) imaging conditions have been produced for partial dislocations in Si under the condition that the electron beam is parallel to the normal to the stacking fault plane. It is shown that the positions of partial dislocations can be determined by the difference in the lattice image between the faulted and unfaulted regions; hence the existence of kinks can be resolved under appropriate conditions. Image simulations have also been made for reconstructed 90°- and 30°-partial dislocations. The results show that the reconstruction may be detected by HREM for the 30° partial but not for the 90° partial. HREM experiments on dislocations in Si introduced by plastic deformation reveal that: (i) the positions of partial dislocations can be determined for thin samples, (ii) 30° and 90° partials are easily distinguishable, (iii) high density kinks are observed, but it is concluded that these are produced, during observation, by ...

Pressure-induced irreversible amorphization and crystallization were definitively observed at the atomic scale, using high-resolution electron microscopy, in the uniaxially compressed icosahedral phase of Al68Pd23Mn9, at pressures from 10 to 20 GPa at ambient temperature. The pressure-amorphized state was found to comprise a random packing of atomic clusters. The pressure-induced crystalline phase was identified to have a fcc structure with a lattice parameter a = 0·38 nm which is not an approximant of the icosahedral phase of Al[sbnd]Pd[sbnd]Mn.

A new method of high-resolution electron microscopy image analysis is presented which improves considerably the visibility of nanocrystals in amorphous materials. The technique is based on individual Bragg filtering of the lattice fringes in the image. The significant feature is that the amplitude and phase of the fringes as functions of position are calculated separately. This is similar to a holographic reconstruction with the Bragg interference representing the carrier wave. The phase information is shown to be particularly robust with respect to noise due to amorphous material. The images so produced are in a form suitable for statistical analysis, and criteria for judging quantitatively the extent of the crystal are proposed. The application of the technique to a wide range of similar problems concerning partially ordered materials is discussed.

Using high-resolution electron microscopy (HREM) the core structure of a 1/2(112) superdislocation in TiAl is examined and observed to have dissociated into an extrinsic stacking fault and an intrinsic stacking fault, bounded by three 1/6(112) partial dislocations. The core structure is non-coplanar, with the extrinsic stacking fault lying on two adjacent [111] planes bounded by a dissociated b = 1/6(112) partial dislocation whose deiocalization can pin the dislocation core and may lead to the formation of ±1/6(112)dipoles.

High-resolution electron microscopy observations have been performed on nickel silicide (Nisi2) precipitates in the ∑ = 25 grain boundary area of silicon bicrystal. The two orientation relationships type A and type B, usually encountered in monocrystalline silicon, have been recognized with respect to one of the grains. Details of the silicide/silicon interface structures are reported. In addition to most of the interfaces previously observed in the case of epitaxial growth, we have found evidence of two new ∑ = 1 Si(111)/(111)NiSi2 atomic structures. The interfacial Ni atoms, belonging to both new structures, have been determined to be sevenfold and eightfold coordinated. The growth of precipitates has been found to occur along the (111) planes, but ending by steps. For type A precipitates, 1/2[100] complete steps with the vector 1/2[101] have been identified while type B precipitates have 1/3[111] steps associated with a vector of 1/2[110].

A structure model of octagonal quasicrystals has been obtained using electron microscopy and crystallographic image processing. Octagonal quasicrystals in a rapidly solidified Mn-Si-Al alloy were studied by high-resolution electron microscopy (HREM). HREM images taken along the main symmetry directions, that is the eightfold and the two twofold axes were combined into a density map. The symmetry of the model is 84/mmc. Within each unit-cell repeat of 0.63 nm along the eightfold direction there are four layers. Alternating layers have eightfold and fourfold symmetry. Projections of the model match the HREM images. The shortest interatomic distances, 0.24 nm, and the density of 6.9 g cm−3 for the model are in good agreement with expected values.

The structure of a new crystalline phase (O-phase) in an Al72Pd18Cr10 alloy, which is closely related to an Al3Mn crystalline phase and the Al-Pd-Mn decagonal quasicrystal, was investigated by electron diffraction and high-resolution electron microscopy. The O-phase has an orthorhombic structure with the lattice parameters a=1.25 nm, b=3.28 nm and c=2.38 nm, and the space group is Pmmn (no. 59). This structure is characterized as a periodic tiling constructed with two types of tiles, a squashed hexagon and a star-shaped pentagon, formed with pentagonal atom columns which are structural units in the Al3Mn crystalline phase and in Al-Pd-Mn decagonal quasicrystal. Structural relationships among the O-phase, the decagonal quasicrystal and some crystalline phases are discussed.

Translation twins (T twins) of γ-TiAl (L1O-type structure) in an as-cast two-phase TiAl-Ti3Al intermetallic alloy with Cr addition are characterized by high-resolution electron microscopy and supported by image simulation. The T twin and the matrix are connected through a glide reflection plane at the {111} twin interface, and the glide vector is ½ [101] parallel to the interface. The formation mechanism of the T twins and their possible influence on the deformation mechanism of the material are discussed.

High-resolution electron microscopy indicates that the edge-type dislocations observed in small-angle boundaries on Al 62 Cu 20 Co 15 Si 3 decagonal quasicrystals dissociate into two partial dislocations. The projected Burgers vectors of these partial dislocations on the plane normal to the twofold A2D axis are b 1 =[a 1 /2, a 2 , 0, 0, 0, 0] and b 2 =[a 1 /2, -a 2 , 0, 0, 0, 0] respectively. The modulus of the total Burgers vector b=[a 1 ,0,0,0,0,0] is about 0.4 nm corresponding to the periodicity along the tenfold axis of the decagonal quasicrystal

The alloying behaviour of copper atoms into gold clusters has been studied by high-resolution electron microscopy. Preparation of nanometre-sized gold clusters and subsequent copper deposition onto the gold clusters has been carried out with the use of a miniature double-source evaporator that has been directly set at the tip of a specimen holder. Upon the deposition of copper, nanometre-sized gold clusters quickly changed into Au-Cu solid solution clusters even at ambient temperature. It has been confirmed that such spontaneous alloying takes place not via a quasi-melting process but via a purely solid-state process.

The morphology of supported Bi particles is studied using the technique of profile imaging at atomic resolution. These particles show a crystallized Bi core surrounded by an amorphous (or liquid) material. For small supported particles (diameter of the Bi core, about 6-9 nm) observed by high-resolution electron microscopy at room temperature, the lattice fringe orientation changes very rapidly during the observation. This fluctuating evolution is attributed to atomic rearrangements of the particle induced by the microscope electron beam. During this evolution, a decrease in the size of the crystallized Bi core is sometimes observed for the smaller particles.

An atomic arrangement of an Al-Pd-Mn decagonal quasicrystal with 1·2 nm periodicity has been proposed from high-resolution electron microscopy with the aid of computer simulation. A calculated image from the atomic model reproduced well the features of a high-resolution structure image taken from a thin sample.

High-resolution electron microscopy has been used to investigate the structure of coherent twin boundaries in ordered Cu 3 Au. Apart from confirming the previously established symmetric structure S and asymmetric structure A, the composition of atomic columns in the boundary has been examined. Evidence is presented for a new asymmetric structure which differs from structure A in the arrangement of Au and Cu atoms columns at the boundary

An incoherent {112} twin boundary in aluminium has been observed by high resolution electron microscopy. No rigid body translation parallel to the boundary plane has been found. The experimental structure is in agreement with one of the calculated models. The junction of the {112} segment with a coherent {111} boundary is also studied.

In the 〈110〉{001} slip system in L12 ordered alloys, both screw and edge dislocations may become locked by dissociating into nonplanar structures. In this paper the structure of edge dislocations in the [110](001) primary cube system in Ni3Ga and Ni3(Al, Ti) single crystals has been observed with weak-beam and lattice-imaging microscopy. Near 600°C the edge superdislocations are locked in the double Lomer—Cottrell lock; near 700°C the edge dislocations are locked in nonplanar structures formed by climb dissociations. Both structures have been observed directly by high-resolution electron microscopy. The implications of the locking of the edge dislocations on the operation of 〈110〉{001} and 〈110〉{111} slip systems are discussed.

High-resolution front views of reconstructed (111) gold surfaces have been obtained by electron microscopy, using several layer-diffracted beams. From two different models for the reconstructed surface, we have computed images taking into account the interaction of the electrons with the reconstructed surface and with the bulk gold. Comparison between these computed images and the experimental ones allows us to choose which of the models corresponds best to the experimental evidence.

The incommensuration of two high Tc Bi-Sr-Ca-Cu-O compounds has been investigated by electron diffraction and high resolution electron microscopy. The results suggest discommensuration rather than sinusoidal modulation of the atomic positions.

Five different malignant tumors and two nasal polyps were examined by electron microscopy for diagnosis purposes. In all cases occasional large cells containing lysosomes interconnected by characteristic junctions were observed. The interlysosomal junctions appeared organized in zig-zag and ladder like configurations. The ultrastructural and morphometric analyses of these junctions postulated that the two configurations represented the same structure viewed in two different sectioning planes. High-resolution electron microscopic evidence suggests that the interlysosomal junction is organized as a 35-40 A thick pleated ribbon extending between a 200-A wide interlysosomal space, in close contact with the adjoining lysosomal membranes, without apparent integration into the membrane.

A technique is presented for enhancing and combining electron microscope images of small clystalline areas. Phases obtained by Fourier transforming electron micrographs are merged with available more precise amplitudes, in a Fourier synthesis, to obtain a final estimated image. The procedure is illustrated with 42 individual images of the purple membrane from Halobacterium halobium. To show the power of combination, results based on 1, 2, 4, 8, 16, 32 and 42 images are presented. An estimate based solely on the micrograph data, i.e. ignoring the precise amplitudes, is also presented and is seen to be notably poorer. The level of uncertainty of the final image is assessed by stimulating 10 final images and superposing the results.