PSI - Issue 40

N. Nikul’chenkov et al. / Procedia Structural Integrity 40 (2022) 354–358 N. Nikul'chenkov et al. / Structural Integrity Procedia 00 (2022) 000 – 000

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It was assumed that all condensed states of the material (amorphous, nanocrystalline, and recrystallized) have different CSR sizes (Fig. 2), following the model adopted by Nikul’chenkov et al. (2019) . The Wigner-Seitz cell, which is a truncated octahedron for the iron bcc lattice, was taken as the minimum structural unit, including one atom (Fig. 2.(a)). CSRs for various structural states were represented as areas filled by cells of the same type. The CSR size is several Wigner-Seitz cells along the one cluster length (Fig. 2.(b), (c), and (d)). This idea simulates nanocrystalline and recrystallized states, i.e., nanograins and grains. The lower estimate of the size of the coherent scattering regions (L) was made from the broadening of the bcc iron {110} α line and the halo for the amorphous state using the Scherrer formula (see e. g. Stoltz et al. (2005)): L x = K ∙ λ / (β ∙ cos θ), where К = const ≈ 0.9; β is the integral width of the diffraction line, expressed as radians; cos (θ) corresponds to the position of the center of gravity of the diffraction line. The interplanar distance (d) was calculated along the {110} line, according to B ragg’s law for the material in the crystalline state. The position of the barycenter of the halo was used to determine the average interatomic distance for the first coordination sphere for the amorphous state, Scanning electron microscopy (SEM) structure studies was performed on Tescan Mira3 SEM.

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Fig. 2. Model of structural units: minimal structural unit (a) amorphous (b), nanocrystalline (c) and recrystallized (d) states.

3. Results and Discussion The XRD results were used to calculate the sizes of CSR regions for the amorphous, nanocrystalline, and recrystallized states. These sizes were 2, 14, and 86 nm, respectively. At present, it is not possible to study the structure of the amorphous state of Fe 72.5 Cu 1 Nb 2 Mo 1.5 Si 14 B 9 since the scanning electron microscope is unable to show the structure of such a dispersion level. SEM image (secondary electron analysis) was used (Fig. 3.(a)) to determine the grain size of the nanocrystalline Fe 72.5 Cu 1 Nb 2 Mo 1.5 Si 14 B 9 after annealing at 550 ° C. The separated regions were taken as nanograins having practically the same contrast at the image. The images were processed in SIAMS software (Fig. 3.(b)) for particle size analysis. The average grain size (d av ) was 17 nm (Table 1). SEM image (backscattered electron analysis) was used (Fig. 4) to determine the grain size of the Fe 72.5 Cu 1 Nb 2 Mo 1.5 Si 14 B 9 alloy after annealing at 770 ° C. The following statistical metallography method was used to calculate the average grain size. A known area in an arbitrary rectangular figure was chosen. A rectangular area with random sizes was selected, then the area of the figure was calculated. Areas of almost the same contrast were marked by dots, and they were taken as individual grains. The number of grains (dots) at the boundaries and in the inner part of the selected area was calculated separately. The area per point (grain) was determined by the formula d = S / (n 1 + 0.5n 2 + 1), where n 1 is the number of grains within the region, n 2 is the number of grains run across by the boundaries of the regions. The average grain size (d av ) was 85 nm (Table 1)