Morphological and Structural Characterization of the Spinodal Decomposition of FE-CR Alloys.
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MORPHOLOGICAL AND STRUCTURAL CHARACTERIZATION OF THE SPINODAL DECOMPOSITION OF FE-CR ALLOYS. A. Cerezo*, M.G. Hetherington*, J.M. Hyde* and M.K. Miller**. *Department of Materials, University of Oxford, Oxford, England. "**Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 ABSTRACT The position sensitive atom-probe (POSAP) is capable of extremely high resolution (subnanometer) microanalysis. Its 3-dimensional capabilities are unique and offer a new opportunity to study the topology and structure of materials. This paper is an initial overview of the new types of parameters and analysis that this technique makes possible. INTRODUCTION TO 3-D ATOM-PROBES A new generation of atom-probes is being developed which allows a material to be reconstructed with sub-nanometer resolution in 3-dimensions. The POSAP at the University of Oxford [1] is the first of these instrument to be built and applied. The conventional field ion microscope can image individual atoms; for example, each of the spots which make up the image shown in Fig. 1 is a single atom. The specimen is an Fe- 45 at. % Cr alloy which has been aged for 192 h at 525"C. This alloy has approximately the critical composition and is therefore expected to decompose spinodally at all temperatures below the critical temperature. The darkly imaging phase is Cr-enriched and the brightly imaging phase is Fe-rich. Viewing sequences of such micrographs allows qualitative assessment of the morphology if the microstructure is sufficiently coarse, but the field ion microscope does not measure composition. The conventional atom-probe can measure the composition of layers but deconstructs the 3dimensional crystal into a 1-dimensional sequence of atoms, thereby losing information about the spatial configuration of the atoms in the crystal.
Fig. 1. Field ion micrograph of Fe- 45 at. % Cr aged for 192 h at 525"C A 3-D atom-probe combines time-of-flight measurements with a position sensitive detector. The time-of-flight measurements identify the elemental species of an atom (as in a conventional atom-probe) and the position sensitive detector gives its position in the specimen (as in the field ion microscope). A computer can reconstruct the composition fluctuations in the specimen in 3-dimensions from these data, and permit the microstructures to be examined qualitatively and quantitatively. Mat. Res. Soc. Symp. Proc. Vol. 186. 01991 Materials Research Society
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Fig. 2. Sequence of grey-scale cross-sections computed from the POSAP data from an Fe-45 at. % Cr specimen aged for 192 h at 525'C.
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REPRESENTATION TECHNIQUES The feasibility of combining time-of-flight mass spectrometry with x-y positioning has coincided with the commercial availability of graphics supercomputers. The high computing performance and special hardware in these machines allows complex 3-dimensional objects to be rendered graphically in 3-dimensions. The images and computations presented in this paper were calculated on a Stardent GS 1000 graphics supercomputer. Another o
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