Phase and structural transformations in a nanocrystalline alloy Fe72.5Cu1Nb2Mo1.5Si14B9

The material for investigation was a ribbon with an amorphous structure obtained by the melt spinning technique from a melt of molybdenum-modified Finemet-type high-permeability Fe72.5Cu1Nb2Mo1.5Si14B9 alloy. Using the methods of non-ambient X-ray diffraction, calorimetry, and dilatometry, temperature intervals of transformations during the transition of material from the amorphous state to the nanocrystalline one with subsequent recrystallization were determined. Each method was characterized by its own heating rate: 1 K / min for the non-ambient X-ray diffraction, 30 K / min for calorimetry, and 20 K / min for dilatometry. Regardless of the heating rate, phase and structure transformations (crystallization and recrystallization, respectively) were observed sequentially. With decreasing heating rate, the crystallization temperature significantly decreased and the recrystallization temperature slightly decreased. Specific heats of crystallization (386 kJ / mol) and recrystallization (88 kJ / mol) were calculated from calorimetry data. Basing on the results of X-ray diffraction and calorimetric studies, the possibility of using the structural unit model was analyzed to describe the amorphous state. It was supposed that any condensed state of the material (amorphous, nanocrystalline and recrystallized) were distinguished by different sizes of coherent scattering regions (CSRs). The lower estimate of coherent scattering regions size was made from the X-ray diffraction halo width for the amorphous state and (110) diffraction line broadening for polycrystals. The Wigner-Seitz cell (truncated octahedron) containing one atom has been adopted as a structural unit. Specific heats of transformations were compared to the values of energies related to the transitions of atoms from CSR borders to lattice sites. Satisfactory applicability of the structural unit model for the description of the amorphous state was demonstrated.