Analytische Chemie
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- Electron backscatter diffraction (30)
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The main component of this program is a simultaneous representation of the unit cell and the calculated powder pattern. It allows the manipulation of the Crystal structure by moving selected atoms of the asymmetric unit. The resulting powder pattern can be directly compared to experimental data in order to obtain reliable starting values for further computations in refinement programs.
PowderCell 2.0 for Windows
(1998)
PowderCell contains a comfortable, user friendly visualization and modification tool for crystal structures. It provides on-line calculation of the corresponding powder diffraction patterns simulating a variety of experimental conditions. The common ICSD and Shelx file formats are supported for importing crystal structure information. It has control of automatic cell transformation and also derivation of subgroups. More than 740 different settings of the 230 space-group types are supported. Up to ten crystal structures can be considered simultaneously. A full pattern refinement enables the direct comparison with experimental diffractograms for quantitative phase analysis, lattice parameter refinement, polynomial background estimation, etc.
PowderCell as teaching tool
(1998)
PowderCell represents a user friendly program which supports the solution of scientific problems as well as teaching and education. Especially for the last one the program offers a lot of information regarding the space-group type as well as crystal structure used. Therefore, on some universities the program is used successfully to make students familiar with x-ray crystallography. The quasi-simultaneous diffraction pattern simulation visualized the changes caused by the respective crystal structure. However, it is also possible to vary different diffraction parameters and investigate the resulting changes in the interference intensity or the reflection position. In principle, the aim of the program is the intuitive generation of structure models. Therefore, special tools have been implemented to move (rotate or shift) or transform the crystal structure.
The program PowderCell is a crystallographic tool for visualization of Crystal structures. However, it offers also non-conventional features like the fulautomatical generation of subgroups or the consideration of non-standard settings of space-group types. On the one Hand the program is very useful for non-crystallographers who like to get an impression of the atomic arrangement within the unit cell. But also for crystallographers it is recommendable because it contains a lot of additional information which can be extracted and used like data given in the International Tables for Crystallography, Vol. A. However, the most important advantage of the program is the simultaneous calculation of the X-ray or neutron diffraction powder patterns for a mixture of up to 10 crystalline phases. Between more than 7 different characteristic radiations can be chosen and their influence on the resulting powder pattern can be studied. Furthermore, experimental diffractograms can be analysed using a refinement procedure. The implemented LeBail-algorithm allows the investigation of unknown phases. In combination with the refinement algorithm an interface to Shelx offers the possibility for a step by step ab initio structure analysis. Certainly, the user-friendly shell is one reason that especially users who are not so familar with space-group symmetry, crystal structure data or diffractometry use this powerful tool for the solution of scientific or analytical problems as well as in teaching.
X-ray stress analysis on crystalline materials is based on the determination of elastic lattice strains. The strains described by the shift of peak positions are converted to stresses by the means of theory of elasticity. The development of the sin2ψ-method of X-ray stress analysis introduced an enormous progress in X-ray stress analysis during the last decades. This report outlines some important elements of X-ray physics and fundamentals of theory of elasticity. The Standard measuring procedure are described and special experimental differences to the common powder diffractometry are outlined.
The orientation relationship (OR) between the bcc and fcc phase in the plessite microstructure of the iron meteorites Watson, Agpalilik and Gibeon has been analysed in a scanning electron microscope using electron back-scattered diffraction (EBSD). A very strong OR exists, independently on the analysed plessite type and the observed spreading of single orientation data. The agreement between the experimental orientation distribution and existing models varies for each meteorite. The black plessite in the Agpalilik corresponds to the Nishiyama-Wassermann model whereas the Duplex plessite of the Gibeon meteorite shows an OR close to the Kurdjumov-Sachs model. The Watson meteorite is strongly deformed so that a general OR is difficult to determine due to the blurred experimental orientation distribution.
A Co-Cr-Mo laser cladding applied on railway wheels is characterised by a combination of EBSD and EDX. A complete pass of the cladding is investigated to achieve a better understanding of the microstructure evolution during laser cladding. A microstructure with columnar grains extending over the whole thickness of the cladding is observed. The grains have a <001>-fibre texture with the fibre axis parallel to the normal of the substrate surface, and a substructure consisting of cells/dendrites. During the cladding process, two different kinds of precipitates form in the cell walls, which can be identified as M6C and a non-equilibrium phase. Furthermore, stacking faults are observed to occur in the cladding grains and are discussed with respect to the laser cladding process.
Electron backscatter diffraction and energy-dispersive X-ray spectrometry were used to investigate the intermixed interface produced during laser cladding of a Co-Cr-Mo alloy on a steel substrate. A multi-component system and rapid solidification conditions together lead to a complex microstructure at the interface. The solidification of the cladding starts with the formation of an interface layer, which is about 75µm in thickness and consists of randomly oriented equiaxed grains of Co-Cr-Fe solid solution and martensite. Orientation analysis of the grains in the interface layer revealed that some grains have a special orientation relationship with the former austenite grains in the heat affected zone but the cladding is not formed by epitaxial growth on the substrate. Intermixing of the materials at the interface is providing a strong bond between the substrate and the cladding. For a grain from the interface layer to emerge as columnar grain in the cladding, it was determined that its <001> crystallographic direction is not supposed to deviate more than 25° from the sample normal direction.
During focused ion beam microscopy (FIB) of crystalline materials imaging contrast and milling result are effected by orientation of the crystals with respect to the incident ion beam. This is due to the possibility of ion channeling along preferred crystal directions which effects the depth at which interaction between ions and specimen atoms takes place. As a result of channeling emission of ion induced secondary electrons (iiSE) and secondary ions (SI) as well as the sputter rate decreases.
Theoretical channeling orientations and critical angles can be calculated.
These effects have been studied quantitatively for polycrystalline recrystallized Cu as a typical model case.
The competitive growth of columnar grains in a single-grain selector, which is used for directional solidification of single-crystal blades from nickel-based superalloys, has been investigated by electron backscattered diffraction and local X-ray diffraction analysis. It has been found that the competitive grain growth in a starter block is determined by the crystallographic factor: rapidly growing grains with the axial orientation close to the [001] direction dominate in this part of the casting. For the competitive grain growth in a helicoidal separator, the geometric factor (the position of a grain at the input of the separator) is also important. The results obtained suggest that an appropriate geometry of the single-grain selector was chosen. In addition, the distribution of the orientations of columnar grains obtained by electron backscattered diffraction, can be used for approximate estimation of the yield of suitable (i.e., with the deviation of the axial orientation from the [001] direction within a specified tolerance) single-crystal blades.