Thermal behavior of substituted FeCo-based metallic glasses

Three compositions of metallic glasses, Fe₅₈Co₂₄Nb₃Ta₁Mo₁B₁₃, Fe_61.5Co_20.5Nb₃Ta₁Mo₁B₁₃ and Fe₆₆Co₁₈Si₁B₁₅ were prepared by rapid quenching from the melt and subsequently annealed for 1 hour at 450, 650 and 750 °C. The samples were analyzed by X-ray diffraction (XRD) and Mössbauer spectroscopy. All Mössbauer spectra were fitted with a six-line pattern corresponding to the crystalline α-(FeCo) phase and a hyperfine magnetic field distribution representing the amorphous component. The Mössbauer spectra of annealed Fe₆₆Co₁₈Si₁B₁₅ revealed the presence of a secondary crystalline phase, namely (FeCo)₃(BSi). Moreover, the last Mössbauer spectrum in the set was fitted with an additional sextet, corresponding to the appearance of hematite in the system. It is inferred that the addition of Nb, Ta and Mo considerably delays the onset of bulk crystallization and iron oxidation in these systems. The XRD patterns are in qualitative agreement with the Mössbauer results and are consistent with a surface layer of hematite nanoparticles in the system. The activation energy of 4.34 eV for oxidation was estimated from the Mössbauer results.