Search

Convergent beam electron diffraction (CBED) at vertical grain boundaries (parallel to the electron beam) can be used to determine the symmetry of bicrystals. It can also be used to investigate the structure of the boundary region itself when subnanometer probe sizes are employed. In this paper, the influence of the electron-beam geometry on the CBED pattern is discussed in terms of the defocus distance between the probe position and the specimen midplane, D (set by the second condenser lens), the probe size, the beam-convergence angle, and the microscope mode (micro- or nanoprobe). Theoretically expected CBED patterns for an electron probe on a vertical grain boundary are observed up to a certain maximum defocus | D c |, which is shown to increase linearly with probe size and specimen thickness. The experimental uncertainty in positioning the probe in the specimen midplane is independent of the probe size but increases linearly with specimen thickness. A comparison of this uncertainty in | D | with | D c | shows that for decreasing probe sizes it is increasingly difficult to obtain the theoretically expected CBED pattern from a vertical boundary. The effect of a defocused probe (| D |>| D c | up to | D |≫| D c |) upon the CBED pattern is discussed and experimentally illustrated for two electron beam directions parallel to a vertical twin boundary in Si.

Intersections of deformation twins with α plates in a two phase γ TiAl alloy deformed at room temperature have been investigated in detail by transmission electron microscopy (TEM). It has been found that two types of configurations, depending on the orientation of the intersection line, should be distinguished. For a type-I intersection with the intersection line parallel to 〈110] y , the deformation twins are blocked by the α 2 plate. At these intersections ordinary glide dislocations were emitted back into the γ matrix on cube slip planes, together with the production of twinning in γ parallel to the γ / α 2 interface. No shear transfer across the γ / α 2 interface was observed in this case. For a type-II intersection with the intersection line parallel to 〈011] γ , the incident twinning shear is usually accommodated by a -type slip on {11¯00} planes in the α 2 plate. In one particular case, {22¯01} twinning was observed in the α 2 plate at the intersection. The observations were analysed by taking into account all possible dissociations of a twinning partial (or a number of them) into two glissile dislocations, or into one glissile dislocation (glissile in matrix or plate) and a residual interface dislocation. This was done by considering a number of conditions for dissociation of twinning partials at the intersection and subsequent relaxation by slip. These conditions are similar as reported earlier for twin—twin intersections in the γ phase, and for slip transfer at grain boundaries in metals and alloys. The analysis showed that for a particular dissociation not all conditions could be fulfilled at the same time. The combination of the analysis and the observations showed the following order of priority of conditions: (i) a common line of intersection with the γ / α 2 interface of the incoming twinning plane and the relaxation slip planes; (ii) the slip directions of the glissile product dislocations are consistent with the applied load, which was checked by taking into account the sign of the Schmid factor S ; (iii) a zero or small residual interface dislocation, and (iv) a high value of ‖ S ‖ (sometimes S was low).

Characteristic splitting configurations occur at the intersection of mirror-related deficit HOLZ lines in the CBED central disc from a centro-symmetric crystal. In this paper essentially two types of splitting are found for special combinations of specimen thickness, structure factors and accelerating voltage in the three-beam approximation, referred to as the horizontal and vertical splitting, respectively. From the magnitude of these splittings, structure factors can be determined, and this has previously been discussed extensively for the horizontal splitting. For vertical splittings this paper discusses two methods for structure factor determination. The first method, based on a direct angle measurement in the CBED pattern, yields a value with a relative error equal to the relative error in specimen thickness and an accuracy up to 5% for higher-order structure factors. The second method is based on a least-squares refinement of both the structure factor and specimen thickness and consists of fitting a measured intensity profile in the central CBED disc to the calculated profile. This method does not suffer from an uncertainty in specimen thickness, but is more sensitive to random noise and the influence of spurious reflections which makes it more applicable (better than 5%) to lower-order structure factors. Energy filtering did not significantly improve the accuracy of this method.

Two phase {gamma}-TiAl-based alloys composed of ({gamma} + {alpha}{sub 2}-Ti{sub 3}Al) are currently under extensive investigation because their mechanical properties are superior to either single phase {alpha}{sub 2} or {gamma} alloys. The alloys usually have a duplex microstructure consisting of equiaxed {gamma}-grains with various volume fractions of lamellar colonies in them. The lamellar colony, which consists of alternating layers of ordered {gamma} and {alpha}{sub 2} plates with the orientation relationship {l_brace}111{r_brace}{sub {gamma}}//{l_brace}0001{r_brace}{sub {alpha}{sub 2}}, {gamma}// {sub {alpha}{sub 2}}, is formed either by a phase transformation {alpha} {r_arrow} {alpha}{sub 2} and then {gamma} precipitates from the {alpha}{sub 2} matrix, or by {alpha} {r_arrow} {alpha}/{gamma} {r_arrow} {alpha}{sub 2}/{gamma} upon cooling. The microstructure of the colony has been studied in detail by many investigators. It has been suggested that the precipitation of the {gamma} phase essentially involves a HCP {r_arrow} FCC structure change together with long-range transport of atoms to obtain the composition of the two phases. The structure change can be brought about if a/3 Shockley partials travel on alternate basal planes of the HCP phase. Thus, the {gamma} nuclei are essentially basal stacking faults in the {alpha}{sub 2} phase. This means that gliding of a/3 partial appears tomore » play an important role in the transformation. If it is so, it would be expected that an external stress would probably enhance the process. More recently, Gao et al. have claimed that stress-induced phase transformation may occur in two-phase TiAl-based alloys during deformation at room temperature, which is related to a stage on stress-strain curves. In this note, the authors are briefly reporting on evidence of the stress-induced {alpha}{sub 2} {r_arrow} {gamma} transformation in a {gamma}-TiAl-based alloy.« less

Two new methods for accurate determination of local foil thickness for a multilayer in plan view have been introduced. One is the zone-axis CBED method, which is based on the fact that the distance between a weak line and a strong bright line in a satellite HOLZ CBED disc is sensitive to foil thickness. A simple formula is derived, which makes the foil-thickness determination very easy. Another method is based on the observation of fringes relating to the periodic structure of the multilayer. The local foil thickness for a fringe is simply equal to the product of the multilayer period and the fringe number counted from the edge of the specimen. Factors affecting the accuracy in thickness determination are discussed. These two methods are compared with some classical methods.

By using computer simulation based on the dynamical diffraction theory, local interface irregularities (interface sharpness and period uniformity) of a multilayer are investigated in plan-view observation by convergent-beam electron diffraction (CBED) with high spatial resolution. It is found that the intensity of some bright lines in satellite higher-order Laue zone (sHOLZ) reflection discs is very sensitive to interface irregularity (interface sharpness and period non-uniformity). The effect of various other parameters is also discussed. The investigation shows that, if specific bright lines in different rings of sHOLZ discs are used, interface irregularity can be evaluated accurately.

Fine details of satellite higher-order Laue zone (sHOLZ) reflection discs from a GaAs/AlAs multilayer in plan view are observed and investigated. Computer simulation based on the dynamical theory of electron diffraction shows good agreement with the experimental observations. Apart from the two lines reported previously, some weak lines and a few other features in sHOLZ reflection discs are found and investigated. The investigation confirms the previous two-line explanation and also reveals the origin of the additional weak lines and the other features. The weak lines are due to the interference between Bloch waves, and their location depends on specimen thickness. Some other features are determined to be caused by multiple diffraction from neighbouring sHOLZ reflections. The phenomenon of the interruption of lines in sHOLZ reflection discs, in comparison to the lines in fundamental higher-order Laue zone (fHOLZ) discs, originates mainly from sHOLZ reflection coupling. Furthermore, intensities of sHOLZ reflections are shown to depend on their structure factors rather than on the excitation of their corresponding fundamental reflections. Details of the computer simulation are given. Finally a simple method, not relying on computer simulation, to identify the lines in sHOLZ reflections is presented.

In this paper the structure of a (011)-Σ=3 twist boundary in ordered Cu3Au is analysed geometrically. Wrong nearest-neighbour bonds can be avoided by facetting on an atomic scale along common {112} planes, together with a rigid-body translation parallel to one of the facets as measured in earlier work. In the specimen different translational states were found in different areas of the [165]-Σ=3 boundary by transmission electron microscopy (TEM). One of the translations was in agreement with the model, the other was associated with a presumably energetically more unfavourable structure. The in situ observation at 230 °C of the motion of the dislocation separating the different boundary areas was associated with a transformation of the boundary structure with the higher energy into the more favourable structure. Therefore, it is likely that the driving force of the dislocation motion was a difference in boundary energy.

The interaction of 1/2 screw dislocations with symmetric [1 1 0] tilt boundaries was investigated by atomistic simulations using many-body potentials representing ordered compounds. The calculations were performed with and without an applied shear stress. The observations were: absorption into the grain boundary, attraction of a lattice Shockley partial dislocation towards the grain boundary and transmission through the grain boundary under the influence of a shear stress. It was found that the interaction in ordered compounds shows similarities to the interaction in fcc.

The interaction between screw dislocations and [1 1 0] symmetric tilt boundaries is investigated by atomistic calculations. In order to study the differences between fcc and ordered alloys and to study the effect of increasing ordering tendency, many-body potentials representing Cu, Cu3Au and Ni3Al were used. For the ordered alloys, the different possible ordering configurations of the boundaries that were studied are discussed.

The glide behaviour of superdislocations at a coherent twin boundary in ordered Cu3Au was examined in a transmission electron microscope for the case when the superdislocations are sessile in the boundary. Possible schemes for dissociation of a superdislocation in the boundary were analysed geometrically. The leading superpartial of each superdislocation dissociated into a superpartial in the matrix and a residual Shockley partial in the boundary of glissile type. The trailing superpartial remained undissociated in the boundary. The superpartial in the matrix glided on a cube plane, and a ribbon of APB connected to the boundary was left in its trail. The cube slip occurs as a result of (i) a maximal resolved shear stress for the observed slip system and (ii) the geometric criteria for slip applied to all possible slip systems in the matrix. The Schmid factors for the slip systems in the matrix could be calculated by assuming a uniform tensile axis in the foil. The tensile axis was deduced from the observed slip systems in the twin.

The interaction between a screw dislocation and symmetric [1 1 0] tilt boundaries under the influence of an applied shear stress was investigated by atomistic simulation. Many-body potentials representing Cu and Ni3Al were used for the description of the interatomic forces. A comparison will be made with in-situ observations in a transmission electron microscope of the interaction of screw dislocations with a coherent twin boundary.

The quaternary semiconducting compound GaxIn1-xAsyP1-y has important applications in optoelectronic devices, in the form of heterostructures and quantum wells. Lattice matching between two materials with a composition difference (Δx,Δy) is achieved if Δy≈2.2Δx. Furthermore, decomposition structures have been described in these compounds; the decomposition is expected to follow Δy≈=Δx (Fig. 1). In the present study the sensitivity of high-resolution electron microscope (HREM) images for these composition variations is investigated.GaxIn1-xAsyP1-y layers lattice-matched to InP (x=0, y=0) have been studied with HREM in [001] plan view specimens, and in and cross-sections. The experimental images were interpreted using image simulations. The observations can be summarized as follows: (1) Even small differences in composition between lattice-matched layers (Δx≈0.17, Δy≈0.37) lead to appreciable HREM-image contrast in cross-section specimens (Fig. 2). -orientations are less sensitive, but interfaces associated with Δx≈0.26, Δy≈0.56 still show good composition contrast (Fig. 3).

Convergent-beam electron diffraction patterns were obtained from GaAs/AlAs multilayers, with the electron beam perpendicular to the layers. We report the first observation of extra higher-order Laue zone reflections originating from the superlattice. The fine structure of these reflections is compared with the first-order Laue zone reflections originating from the average lattice.

Utilisation de la technique de la diffraction des electrons a faisceau convergent pour etudier la structure des joints de grains. Discussion de l'influence de la position d'un joint de macle, situe parallelement a la surface de l'echantillon, sur les cliches de diffraction

Etude de la structure du joint de mâcle Σ=3 dans l'alliage Cu 3 Au. Determination des plans du joint. Analyse de la translation de corps rigide entre les deux cristaux aux joints. Mise en evidence de quatre regions de joints de grains ayant differentes translations, separees entre elles par une paroi d'antiphase sur l'un des cotes du plan du joint

Introduction The interaction between dislocations and grain boundaries is generally believed to be of great importance to the mechanisms of mechanical failure in metals. Especially the difference between ductile and brittle fracture might be explained by variations in the way of interaction between dislocations and grain boundaries. In this paper we concentrate on the detailed phenomena, at the atomic level, which occur when extrinsic lattice dislocations impinge on the grain boundary. One of the topics of interest is the meaning of the well known structural unit model [1] for the interaction between dislocations and grain boundaries. The mechanisms for the interaction between dislocations and grain boundaries have to do with the DSC lattice [2]. A grain boundary dislocation has a Burgers vector which is a vector of the DSC lattice associated with the grain boundary. Its core may be associated with a step in the boundary plane. In general two mechanisms for the interaction between a lattice dislocation and a grain boundary can be discriminated. The first mechanism is referred to as dislocation absorption, which can be explained by the dissociation in the grain boundary plane of the lattice dislocation into DSC dislocations. As the Burgers vectors of the DSC dislocations are relatively small, the dislocation can reduce its energy in this way. The second mechanism for the interaction is referred to as dislocation transmission. A lattice dislocation in one grain can create another lattice dislocation in the other grain, usually leaving a residue at the boundary. The residue is always a DSC dislocation. A number of experimental studies of dislocation-grain boundary interactions has been carried out in the past. See e.g. ref. [3,4].

Convergent-beam electron diffraction (CBED) has been applied in grain boundary structure determination, both for horizontal and vertical boundaries in thin specimens. First the relation between bicrystal symmetry and CBED pattern symmetry is developed for the two types of boundaries. These results are subsequently employed for a structure determination of twin boundaries in Au and Si. The CBED patterns from twinned bicrystals obtained experimentally are compared with computer-simulated patterns, for the case of a horizontal twin boundary in Au and Si. Finally the symmetry of CBED patterns from vertical twin boundaries in Si is analysed and found to depend on whether a static incident beam or a rocking-beam geometry is employed.

Presentation d'un calcul numerique de la structure des joints de torsion dans un metal c.f.c. Observation de cette structure en microscopie electronique a transmission dans des lames minces d'or

Synopsis Measurements are carried out on the viscosity coefficient of heliumgas at liquid helium temperatures as a function of temperature and pressure. Three different types of apparatus based on the oscillating disc method have been used. The experimental values obtained for the viscosity coefficient as a function of temperature are approximately 10% too high as compared with the theoretical values calculated by De Boer (Physica 10 (1943) 348). The rate of change in the viscosity as a function of pressure agrees very well with the rate of variation found by Ubbink (Physica 13 (1947) 659) for the heat conductivity.

A vertical twin boundary in silicon has been examined using both large angle convergent beam electron diffraction and double-rocking zone-axis pattern techniques. The resulting patterns have different symmetries. This is due to a fundamental difference between the two techniques in the geometry of the electron beam-specimen interaction.

The difference in the lattice parameter between the non-superconducting tetragonal phases of YBa2Cu3O6.9 and YBa2Cu3O6.2 is accompanied by two observed modes of structural differences; (a) The T2 exhibits a 2 X 2 supercell distortion evidenced by weak extra diffraction spots which are not observed in the T1 phase diffraction pattern; and (b) The 001 convergent beam diffraction of T2 exhibits a fine dynamic structure which is not seen in the T1 phase. The superlattice is explained by a possible Jahn-Teller periodic distortion of the CuO4 units and the vanishing of the dynamic extinction can be attributed to the dynamic fluctuation of the oxygen atom in the (003) planes. The two phenomena are explained in terms of the lattice contraction and are possibly coupled.

Employing computer simulation techniques several studies of the relaxation of atoms in coincidence type grain boundaries have been performed in recent years. Often it is difficult to obtain a clear representation of the relaxed boundary structure, especially in the case of small atomic displacements. In this paper a method of representing the relaxed grain boundary structure is described using a Fourier analysis of the atomic configuration after relaxation. Computer experiments were carried out on Σ=19, 46.8°/[111] in Al. Next, results obtained from computer experiments dealing with the interaction between He and a tilt grain boundary in Mo are presented. Direct motivation for this study stems from the transmission electron microscopic observation of helium trapping at a set of [100] edge dislocations defining a low angle tilt boundary in Mo.

Superlattice higher-order Laue zone (HOLZ) rings of diffraction are observed to occur in CBED patterns taken in plan view from GaAs/AlAs superlattices. The fine structure of these superlattice reflections is characteristically different from the corresponding structure of fundamental reflections originating from the average lattice. These diffraction effects, induced by composition modulation, are discussed and compared with strain modulation effects occurring in CBED patterns from other systems such as GaAs/InGaAs or Si/GeSi.

In recent years, the influence of the establishment of long-range order in cubic alloys on the structure of grain boundaries in L12 alloys has been considered. Thus, for example, for the ⅀=5 (310) tilt boundary the various possible structures have been investigated that are generated upon ordering, starting from plausible structures in the disordered state. However, apart from some rough energy estimates based upon nearest neighbour interactions, no reliable energy calculations have been performed of these different possible structures. In this paper, computer calculations based upon interatomic pair potentials constructed in such a way that the L12 structure is stable with respect to disordering, are reported for the ⅀=5 (310) boundary. The relative stability of various possible structures, with associated different boundary compositions, has been investigated.

The application of the reciprocity theorem in the field of electron diffraction has led to a number of useful relations between intensities of various diffracted electron beams [Pogany, A. P. & Turner, P. S. (1968). Acta Cryst. A24, 103]. Apart from perfect crystals, the theorem can also be applied to images of individual lattice defects, e.g. planar faults and dislocations [Howie, A. (1972). Proc. Fifth European Congress Electron Microscopy, Manchester, p. 408; Schapink, F. W. (1973) Phys. Stat. Sol. (b), 56, K61]. The symmetry relations for dislocation images, both in bright field and dark field, are investigated in some detail. They may be divided into two groups: (i) Symmetry due to application of the reciprocity theorem, followed by a symmetry operation of the crystal associated with the diffraction vector (mirror inversion or central inversion). The relations found do not depend on the specific properties of the dislocation displacement field. (ii) Special relations due to symmetry properties of the displacement field, for particular dislocation geometries. The resulting symmetry depends on the degree of elastic anisotropy of the crystal.