• Volume 6, Issue 3

      July 1984,   pages  415-623

    • Foreword

      Placid Rodriguez

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    • High resolution electron microscopy of long range ordered alloys

      S Amelinckx G Van Tendeloo J Van Landuyt

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      In this paper a survey is given of the application of high resolution electron microscopy and selected area electron diffraction to the study of long range ordered alloys. The characteristic features of the diffraction effects and of the imaging of ordered alloys are discussed. Group theoretical considerations allow prediction of microtexture of such alloys.

      A number of simple alloy structures are imaged and it is shown, by means of simulated images, that under suitable diffraction conditions the columns of minority atoms are represented as bright dots. The columns of majority atoms are only visible at very small thicknesses. Subsequently a number of one- and two-dimensional long period superstructures derived from these simple structures are analysed, using high resolution micrographs. Finally the observed microtexture of the same alloy superstructures is compared with the predictions of group theory. Strongly non-conservative antiphase boundaries are found to dissociate inapb’s which are as close to conservative as is compatible with thefcc lattice.

    • High resolution electron microscopy of superconductors

      C S Pande

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      Until recently, in the field of superconductivity, electron microscopy has played only a minor role in studies of structure and properties of existing superconductors (except Nb and its alloys) and in the development of new and improved superconductors. This situation has changed in recent years. Electron microscopy is now being extensively used to study the growth, microstructure, and kinetics of many superconductors, especially of A15 type. Electron microscopy has also contributed to the understanding of the nature of radiation damage in these materials. These contributions will be reviewed with selected examples.

    • High resolution electron microscopy as a tool for structural investigations

      G Singh R S Rai

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      Lattice imaging technique has been used to study an unknown high period polytype structure of SiC. A known structure like 6H has been studied by tilted beam two-dimensional images to determine optimum conditions for deriving structural information. The technique has then been used to determine a new structure (411R and its intergrowth structures), and the lattice imaging technique as a tool for structural investigation has been critically evaluated.

    • Scanning transmission electron microscopy and microdiffraction techniques

      J M Cowley

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      At the moment scanning transmission electron microscopy (stem) instruments are not competetive with conventionaltem instruments for high resolution bright field imaging. For studies of the structure and defects of crystalline materials, their special virtues lie in the application of dark field imaging modes combined with observations of microdiffraction patterns from regions of diameter comparable with the microscope resolution limit (currently about 5 Å). They also offer capabilities for microanalysis by use of energy dispersive x-ray spectroscopy (eds) or electron energy loss spectroscopy (els). In principle the spatial resolution of these microanalysis methods is comparable to that of the imaging modes but in practice it is limited by poor signal-to-noise ratios or by the nonlocalized nature of the inelastic scattering process.

      The capabilities for microdiffraction are illustrated by sequences of diffraction patterns obtained as the incident beam is moved within the unit cell of a crystal of large (20 Å) periodicity. Applications of more immediate practical significance include diffraction studies of small crystallites of gold 20 to 50 Å in diameter and of the near-amorphous, thin oxide layers formed on chromium and iron films at room temperature.

      Microdiffraction, combined with reflection electron microscopy andels analysis, provides a powerful new approach to the study of the surface structure of crystals, including bulk samples, and the investigation of surface reactions. In particular, if a beam of small diameter (10–20 Å) is made to run along the face of a small crystal, the diffraction pattern andels curves are very sensitive to the form of the potential distribution at the surface and the excitations of the surface states of the crystal.

    • Applications of scanning transmission electron microscope microanalysis in the study of materials

      Gary R Purdy

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      The dedicatedstem equipped with field emission electron source, affords opportunities for the acquisition of microchemical and microstructural data on a scale approaching the nanometer level. In this presentation, a number of applications of microchemical analysis and microdiffraction to engineering materials will be discussed, with major emphasis on high-strength low-alloy steels, steels for high formability in automotive applications, and aluminium alloys. Some advantages and limitations of the technique are noted, especially for application to analysis of electrochemically prepared thin foils, and to the analysis of extracted particles.

    • X-ray microanalysis in the transmission electron microscope

      G W Lorimer

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      The technique of x-ray microanalysis of thin specimens in the transmission electron microscope is well established. Quantitative analyses can be obtained by converting observed x-ray intensity ratios into weight fraction ratios by incorporating suitable correction factors.

      Flourescence corrections in thin specimens are always significantly less than in bulk specimens and, except where strong characteristic radiation fluorescence enhancement is predicted in the bulk, can be safety ignored. As analysis probes less than 100 Å diameter have become available, the beam spreading has become an important parameter in determining the spatial resolution for analysis.

      Present developments are directed at improving the quality of the analyses which can be obtained. This requires the generation of high quality correction factors, particularly forZ<13, rapid and accurate methods of determining sample thickness, so that absorption corrections can be made, and the development of reliable procedures for determining the shape of the activated volume in a thin specimen.

    • Analytical electron microscopy of aluminium alloys

      T R Ramachandran D C Houghton J D Embury

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      X-ray microanalysis and electron energy loss spectroscopy of thin foils constitute the important techniques of high resolution chemical analysis using the electron microscope. The technique of x-ray microanalysis is discussed in this paper with particular emphasis on the study of aluminium alloys using a dedicated scanning transmission electron microscope (stem).

      The principle of determining chemical composition from observed x-ray peak intensities including the absorption of x-rays and beam broadening in thin foils are considered. The accuracy of peak intensity measurement and detection limits in x-ray microanalysis are illustrated with reference to Al-Mn alloys. The Cliff-Lorimer (k) factors for manganese, iron and copper with respect to aluminium were obtained from standard samples. Identification of phases in 1100 and 1200 aluminium and 3008 (Al-Mn-Zr) alloy were carried out from measured intensities of x-ray peaks. The experimental results emphasize the value of developing techniques for extracting the particles from the aluminium matrix. The transition phases formed in Al-6%Zn-3%Mg and Al-4% Cu were investigated by micro-diffraction and x-ray microanalysis.

    • Electron microscopy of defect clusters produced by radiation damage

      S Banerjee

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      The methods of characterization of different types of radiation-induced defect clusters bytem have been reviewed. Point defects produced in irradiated materials agglomerate in two or three dimensional clusters to reduce the strain energy associated with them. Two-dimensional clusters assume the configuration of vacancy or interstitial type dislocation loops which can be resolved if the size of the loops is large compared to the extinction distance associated with the imaging reflection. The small loops give rise to a black dot contrast under the kinematical and a black-white contrast under the dynamical imaging conditions. The method of characterization of dislocation loops which include the determination of the nature of the loop, the Burgers vector and the loop plane normal is discussed taking examples from the work done on the ion irradiated Ni4Mo samples. A summary of available experimental results on the characterization of dislocation loops in different metals and alloys having fcc, bcc and hcp structures is presented. The contrast from stacking fault tetrahedra which form in some fcc metals and alloys after a certain degree of annealing is also discussed. The optimum conditions for imaging three-dimensional clusters or voids are derived on the basis of the contrast theory proposed for such defects. Special reference is made to the usefulness of “through focus analysis” in the imaging of very small cavities (with diameters as small as about 10 Å).

      It is shown that the formation of disordered zones resulting from displacement cascades in the ordered matrix can be utilized in determining the shape and the volume of cascades in the virgin state. The importance of different contributing factors like the strain contrast and the structure factor contrast in producing the overall contrast from the disordered zones is discussed. Detailed observations on the shape of the disordered zones are shown to be important to establish the occurrence of the replacement collision sequence and the formation of sub-cascades.

    • Electron microscopy of layered single crystals grown by direct vapour transport method

      M K Agarwal P D Patel J V Patel J D Kshatriya

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      Besides interesting properties such as optical, transport, structure, etc. possessed by crystals of transition metal dichalcogenides, they have also been found to have a potential application in the fabrication ofpec solar cells. These crystals are normally grown by carrier gas transport technique but are always contaminated by carrier gases. A new method of direct vapour transport has been developed and successfully applied to grow these crystals including those of off-stoichiometric varieties.

      The crystals thus grown have been characterized structurally using the techniques of x-ray powder, rotation and Weissenberg photographs and electron diffraction. Perfection studies have been made by techniques like chemical etching and electron microscopy.

      This review describes the electron microscopic studies made on the single crystals of the layered compounds. High resolution technique of weak beam has been employed to study dislocation pattern. Dissociated dislocations have been used to estimate stacking fault energy. Such measurements have also been carried out at different temperatures and the variation of stacking fault energy with temperature has been worked out. Interesting information regarding phase transformation for TaS2 and W3Se4 in the temperature range 109 to 580 K has been derived from the electron diffraction studies and the implications have been discussed.

    • Details of an imaging atom probe

      V R Seshadri K Neelakantan E Mohandas V S Raghunathan

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      An imaging atom probe has been incorporated in a conventional field ion microscope in our laboratory. The principle and instrumentation of the probe has been discussed.

    • Field ion microscopic observations of LaB6 on tungsten

      D B Joag P L Kanitkar M M Kanitkar V M Shukla

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      The importance of field ion microscopy as a unique surface microscopic technique has been pointed out with particular reference to the lanthanum hexaboride (LaB6)-deposited refractory metal cathodes. In the core, field ion microscopic observations of LaB6 deposited tungsten are described. The observations are discussed in relation to the field electron emission microscopy of LaB6/W system. The paper ends with a few comments on the scope of further study of this or a similar system using the field ion microscopy and the atom probe field ion microscope.

    • Metallic glasses

      C Suryanarayana

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      Metallic glasses are a new class of materials produced by rapidly quenching the melts at rates often exceeding a million deg/sec. These have been found to display an optimum combination of properties such as high strength, good bend ductility, improved corrosion resistance and excellent soft magnetic properties. Thus, metallic glasses are emerging as economically viable competitors to conventional engineering materials.

      A clear understanding of the atomic structure of metallic glasses and its change during annealing are of prime importance. Although techniques such as field-ion microscopy (fim) x-ray diffraction and small angle x-ray scattering have been employed for this purpose, high resolution electron microscopic (hrem) investigations conducted during the last few years aided greatly in deriving information on the atomic scale. Availability of high-voltage high-resolution electron microscopes has been mainly responsible for this happy situation.

      Studies on several metallic glasses have revealed thathrem images contain well-defined pattern of fringes over local regions, even though the diffraction pattern is that of a typical amorphous structure. The details of structure in the early stages of crystallization derived throughhrem, fim and analytical microscopic techniques are presented and discussed.

    • High voltage electron microscope irradiation and observations in metallic glasses

      R V Nandedkar A K Tyagi

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      High voltage electron microscope (hvem) has been extensively used to produce radiation damage and to study the characteristics of defects so produced in crystalline solids. To understand the defect production in metallic glasses and to evaluate the influence of such defects on physical properties like crystallisation temperature etc., high voltage microscopy and subsequentin situ heating and observation has been extremely useful technique. This paper gives a qualitative overview of such work performed in metallic glasses. In particular results obtained on a nickel based metallic glass using ahvem and an electron accelerator are presented. The advantages and limitations ofhvem irradiation are highlighted.

    • High resolution electron microscopic studies of HoAl3, Er0·5Gd0·5Al3 and Y0·91Er0·09Al3 in thin film form

      Anand Kumar Singh Ajay Kumar Singh M S Gupta O N Srivastava

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      Rare earth trialuminides (RAl3) exhibit an interesting series of structures changing from 2H to 3C in the bulk form. Many of the rare earth trialuminides have been recently found to exhibit curious structural characteristics such as the occurrence of the modulated phases. A detailed investigation of the formation synthesis and characterization of some binary and ternary alloys of the rare earth-aluminium system has been carried out. High resolution microscopic technique has been employed to study the modulated phases for some alloyse.g. HoAl3, Er0·5Gd0·5Al3 and Y0·91Er0·09Al3. With the help of lattice imaging technique, several new modulated phases have been investigated. A possible mechanism for the formation of these phases has been suggested. The details of the results obtained by lattice imaging technique are discussed.

    • Electron diffraction analysis of antimony films

      P D Prabhawalker Amarjit Singh

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      Antimony films of about 3000 Å thickness are grown on NaCl substrate at various substrate temperatures. The films were subsequently analysed for electron diffraction and for grain size on electron microscope. At lower substrate temperatures, the film showed a single crystalline growth, and the crystal grain size was small. However at higher substrate temperatures a cubic phase along with hcp phase was found to exist and also the enlargement of the grain size was observed.

    • Formation of lamellar M23C6 on and near twin boundaries in austenitic stainless steels

      B Sasmal

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      Thin foil electron microscopy studies were made on the precipitation of lamellar M23C6 during aging at 973 K and 1073 K in water-quenched specimens of two austenitic stainless steels. After the precipitation on incoherent twin boundaries M23C6 formed on coherent twin boundaries and in the regions adjacent to incoherent twin boundaries. These precipitates showed lamellar morphology and were aligned in a specific manner with respect to the twin boundaries. Such lamellar precipitates were absent in the specimens which were isothermally treated at 1073 K after being transferred from the solution treatment temperature. The lamellar morphology of M23C6 is suggested to be developed by the influence of residual specific stress field around twin boundaries resulted from quenching.

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