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Paper des Monats
- ja (1)
A survey is presented on some characteristic features of meteoritic (Fe,Ni)3P which is an abundant and important minor phase of most iron meteorites. This mineral (named schreibersite/rhabdite) plays a decisive role during the formation of the so-called Widmanstätten pattern. Different transmission as well as scanning electron microscopic techniques have been applied to get more precise information about the real structure of the phosphide crystals, their chemical composition and the metal distribution across the phoshide/ kamacite interface. X-ray crystal structure determinations have been performed for selected (Fe,Ni)3P - cystals from various iron meteorites (Toluca, North Chile, Watson, Orange River, Morasko, Agpalilik, Odessa, Canyon Diablo). These experiments revealed a metal ordering, i.e. for the three non-equivalent metal positions a different substitution of Fe by Ni has been found. The perfection of the brittle (Fe,Ni)3P samples differs appreciably and seems to be dependent on the thermal history of each individual meteorite. Moreover, inside Ni-rich rhabdite crystals small monocrystalline inclusions of CrN (carlsbergite) have been detected.
Characterization of the fcc/bcc orientation relationship by EBSD using pole figures and variants
(2004)
The orientation relationship (OR) between fcc and bcc lattices are described by crystallographic fundamentals using the example of Kurdjumov-Sachs (K-S) and Nishiyama-Wassermann (N-W). Complete pole figures containing all variants will be used to distinguish even between slightly different ORs. EBSD on iron meteorites and duplex steel has been used to analyse a large number of crystal orientations with regard to a high statistical significance and a high probability to capture all variants in a single measurement. It is shown that the use of fixed OR models like K-S, N-W, Bain, Pitsch, or Greninger-Troiano does not satisfacturally reflect the observed experimental pole distributions. It is not convenient to use high-indexed lattice planes and directions to describe the small deviations from the given models. The Euler subspace representation offers a readily comprehensible tool to get an idea about the characteristic of the experimentally detected OR.
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.
Improved determination of fcc/bcc orientation relationships by use of high indexed-pole figures
(2006)
The use of variants to analyze fcc/bcc orientation relationships is demonstrated by EBSD data. Because of multiply occupied poles in the stereographic projections low indexed pole figures are not always suitable. This is mostly caused by the convolution of all scattered individual orientation data as the single poles cannot resolved in the pole figure. Pole figures of higher indexed lattice planes more reliably reflect the character of the orientation relationship since no overlapping of poles occurs.
Residual stress depth profiles of a cast nickel-base superalloy were measured by regarding the deflections occurring in plate-shaped specimens while successively removing layers from the machined or treated surface by electropolishing. The results are in good agreement with previous findings showing (i) the influence of grinding parameters on the width of so-called white layers, which correspond to steep gradients of residual tensile stress, and (ii) a broad zone of compressive residual stress in the case of shot peening.
The suitability of the electron back-scatter diffraction technique (supported by EDS) in order to study the complex microstructures of iron meteorites is demonstrated on the example of the Watson meteorite. The orientation relationships between the main phases kamacite, taenite and schreibersite/rhabdite as well as effects of the real structure have been investigated. In kamacite bands highly deformed blocks appear which show a contineous change of orientation. Plessitic regions are surrounded by deformed taenite lamellae. Also these lamellae show the typical M-profile of the Ni concentration in cross section. In the center a martensitic microstructure has been proven. The white plessite is characterized by a high number of individual kamacite grains which however are separated mainly by low-angle boundaries. So an orientation clustering occurs. The determination of orientation relationships was only possible for a single plessite region comparing the intensity distribution in pole figures with simulations. Schreibersite is brittle and shows a high number of microcracks. However, the strong deformation of kamacite does not allow us to decide whether an orientation relationship between the phosphides and the surrounding kamacite exists or not.
Interphase Boundary Characterization in Duplex Steel and Iron Meteorites Using EBSD Technique
(2006)
FIB Milling and Canneling
(2007)