X-ray absorption studies of Fe-based nanocrystalline alloys

Local structural changes are studied occurring during transformation from amorphous to nanocrystalline phases of Fe85Zr7B6Cu2, Fe73.5Cu1Nb3Si15.5B7 and Fe66Cr8Cu1Nb3Si13B9 alloys. The nanocrystalline state of a formerly amorphous alloy was achieved by heat treatment at temperatures between 420 and 600°C. The high-resolution transmission electron microscopic images have shown the average size of the Fe and Fe(Si) nanograins in nanocrystalline alloys ranging from 5 to 15 nm in alloys at different stages of crystallisation. X-Ray absorption spectra at the K-edge of Fe, Cu and Cr atoms for nanocrystalline alloys measured using synchrotron radiation have shown a different local structure, namely the structure bcc Fe around Fe atoms, fcc Cu around Cu atoms and amorphous around Cr atoms. The growth of fcc Cu clusters before the nucleation of bcc Fe grains in nanocrystalline alloys was observed. It can be considered as a basic mechanism responsible for nanocrystalline grain formation. The FEFF8 program was used for calculations of X-ray absorption spectra up to about 260 eV above the Fe K-edge for bcc Fe metal and Fe3Si compound in terms of self-consistent muffin-tin potential approximation and full multiple scattering approach. The theoretical spectra well explained the experimental result that the amplitude of EXAFS oscillations in Fe3Si was considerably decreased by the presence of Si atoms in comparison to that for Fe metal.