Solid-State Alloying in Binary Systems with a Positive Heat of Mixing
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background removal, extraction of the EXAFS signal, X (k), Fourier transform of X (k), and inverse transform to isolate the EXAFS contribution from a selected region in real space [16,19]. RESULTS AND DISCUSSIONS EXAFS: Cu-Fe versus Ag-Fe Fourier transforms of the EXAFS signals of Cu-Fe powders before and after milling are shown in Fig. 1 (left graph). For milled Cu 20Fe 80 , the Fourier transform of Cu-K edge no longer match that of pure fcc Cu but instead bears a bcc signature similar to that of bcc Fe. For milled Cu 50 Fe 50 , both the Fe-K edge and Cu-K edge transforms closely resemble the "fingerprint" of fcc Cu, suggesting fcc coordination surrounding both Cu and Fe. These results provide direct evidence that after milling Cu and Fe have dissolved into each other forming substitutional bcc (Cu 20 Fe 80 ) and fcc (Cu 50 Fe 50 ) solid solutions. Nearest-neighbor structural parameters (coordination number of like and unlike atoms and bond distances) obtained from the best fit for ball-milled Cu 5oFe 50 have been presented elsewhere [20]. Such quantitative simulation confirms atomic-level alloying and rules out the possibility that Fe is in fcc coordination not because of alloying but mainly because of formation of fcc Fe domains coherent with Cu [4,20]. EXAFS data obtained for ball-milled Ag-Fe show very different alloying behavior. The Fourier transforms are displayed in Fig. 1 (right) for Ag50Fe 50 as an example. Both the Ag and Fe edge transforms are compared with those of unmilled elemental fcc Ag and bcc Fe. Apparently Fe and Ag give almost identical "signatures" before and after milling. Simulation of the inverse transform of the first structural peak in the Fe transform shows that the best fit is obtained when all nearest neighbors are Fe atoms with a bond distance very close to that of elemental Fe (2.482A). Adding unlike (Ag-Fe) pairs degrades the quality of the fit and leads to unrealistic bond distances. In contrast to Cu-Fe, apparently only minor alloying, if any, occurred between Ag and Fe. The conclusions obtained for both Cu-Fe and Ag-Fe systems are in agreement with those drawn from other characterization techniques [2,3,11,14]. However, it should be emphasized that EXAFS provides strong proof of alloying (or lack thereof) on an atomic scale as it directly probes the local environment of the absorbing atom. The technique is atom specific and short-range-order sensitive. Hence it avoids several problems associated with regular x-ray diffraction studies, such as overlapping of broadened peaks in diffraction patterns of milled powders [2-5], disappearance of Bragg peaks of one of the elements due to absorption effects [15] or the formation of small 1.0
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