Establishment of new analytical technique applicable for local atomic arrangements in various multi-component materials

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This study aims at establishing a new TEM (transmission electron microscope) techique to analyze local atomic arrangements quantitatively which is applicable for various multi-component systems, such as aluminides or oxides. The optimum experimental conditions and formulations of ALCHEMI (atom location by channelling enhanced microanalysis) for B2 or L1_0 type-ordered aluminides were investigated experimentally and theoretically. The structure factors of superlattice reflections in the aluminides were measured by some different methods, and the preciseness and applicability of the methods were examined. The main conclusions are drawn as follows.1. In case of alloys and compounds containing a large amount of light elements, such as Al or O, the result of ALCHEMI depends strongly on the diffraction condition. If a positive excitation error is set for the channelling condition, the characteristic X-ray intensity may give incorrect occupation probabilities of the constituent elements. The … More fact is mainly due to delocalization of the inner-shell ionization for the X-ray emission and absorption of the X-rays within the specimen. The differences between the experimental values of occupation probabilities and the correct ones become large for alloys and compounds which have considerable small structure factors of superlattice reflections, such as B2-ordered Ti-Al-Mo.2. A new procedure of ALCHEMI which gives reliable data for such aluminides are proposed as follows : (1) set the crystal orientation into the systematic row excitation. (2) measure the characteristic X-ray intensity of constituent elements with tilting the incident electron beam to obtain a "diffraction condition-characteristic X-ray intensity" profile, as well as the foil thickness. (3) fit the profile by the computer simulation of ALCHEMI to determine the occupation probabilities of the constituent elements which give the best fitting result.3. The simulation of ALCHEMI must consider (i) dynamical diffraction of elastic-scattered electrons attenuated by inelastic scattering, (ii) characteristic X-ray emission by the elastic- and inelastic-scattered electrons, (iii) delocalization of the inner-shell ionization and (iv) absorption of the X-rays within the specimen.4. For alloys which have small structure factors of superlattice reflections, such as B2-ordered Ti-Al-Mo, the quantitative electron diffraction and its fitting-simulation using the many-beam dynamical diffraction theory give much reliable values of the structure factors than the intersecting Kikuchi-line method and the thickness-fringe one. Less