TRANSMISSION MOSSBAUER-SPECTROSCOPY AND X-RAY-DIFFRACTION STUDIES ON THE STRUCTURE OF NANOCRYSTALLINE FE-CU-SI-B ALLOYS

Nanocrystalline Fe-Cu-Si-B alloys with grain sizes of 25-90 nm were synthesized by crystallization of the amorphous alloy. Two nanocrystalline phases of alpha-Fe(Si) and Fe2B were observed in all tested samples. Transmission Mossbauer spectroscopy investigation on the structure of nanocrystalline Fe-Cu-Si-B alloys showed that Si atoms are completely and substitutionally dissolved in Fe bcc lattice and the arrangement of the Si atoms in the alpha-Fe(Si) phase shows short range order (SRO), whereas 8.5-9.7 at.% of the B atoms were found as Fe2B and the remainder were located in the interfaces. When grains grow larger, the arrangement of the Si atoms in the alpha-Fe(Si) phase changes. Interestingly, x-ray diffraction results reveal that the lattice constant of alpha-Fe(Si) phase decreases rapidly with grain coarsening. Based on the thermodynamic analysis, the changes in the lattice constant of the alpha-Fe(Si) phase and SRO of Si atoms in bcc Fe lattices were attributed to the lattice expansion as a result of the variation of vacancy concentration in alpha-Fe(Si) solid solution. Meanwhile, owing to the interface contribution, the alloy with small grain size is found to exhibit large values of half linewidth (HLW) and isomer shift (IS) in the Mossbauer spectra. The results from electrical resistivity measurements agree and confirm the strong effects of the lattice distortion and interfaces.