Mechanism of nanocrystalline microstructure formation in amorphous $\mathrm{Fe}\text{−}\mathrm{Nb}\text{−}\mathrm{B}$ alloys

To understand the mechanism of the high number density of $\mathrm{bcc}\text{−}\mathrm{Fe}$ nanocrystals in a partially crystallized ${\mathrm{Fe}}_{84}{\mathrm{Nb}}_7{\mathrm{B}}_9$ alloy, we have investigated detailed local structural and compositional changes on annealing amorphous ribbons using transmission electron microscopy, three-dimensional atom probe, and high-energy x-ray diffraction techniques. Nanobeam electron diffraction patterns from an as-quenched amorphous ribbon indicated a local nanoscale atomic ordering. On annealing, reduced interference functions showed a clear change just below the crystallization temperature $(\sim 773\mathrm{K})$. At this stage, local compositional fluctuations started to appear, and medium-range ordering with a $\mathrm{bcc}\text{−}\mathrm{Fe}$ structure as small as $2\mathrm{nm}$ was clearly observed in high-resolution electron micrographs with an extremely high number density. Pair distribution function analyses suggested a structural change at this stage of annealing to increase the chemical bonds in the residual amorphous matrix around the $\mathrm{bcc}\text{−}\mathrm{Fe}$ regions. The increase of atomic chemical bonds in the residual amorphous matrix is considered to retard the growth of the $\mathrm{bcc}\text{−}\mathrm{Fe}$ nanocrystals after the coalescence of $\mathrm{bcc}\text{−}\mathrm{Fe}$ MRO regions in the later stage of annealing. These results suggest that $\mathrm{bcc}\text{−}\mathrm{Fe}$ nanocrystallization with the extremely high number density is ascribed to primarily (i) the presence of highly dense $\mathrm{bcc}\text{−}\mathrm{Fe}$ MRO regions and (ii) the increase of chemical bonds of matrix atoms on annealing.

Figure 1

(Color online) Reduced interference functions of as-quenched and annealed $\left(773\mathrm{K}\right)$ specimens obtained by (a) high-energy x-ray diffraction and (b) energy-filtered electron diffraction techniques.

Figure 2

(a) HREM image and (b)–(e) NBED patterns taken from an as-quenched specimen.

Figure 3

(a) HREM image taken from an annealed specimen $\left(773\mathrm{K}\right)$, together with NBED patterns in (b), (c), and (d). Simulated HREM image is also shown in the inset of (a). (e) HREM image of minor regions in the specimen annealed up to $773\mathrm{K}$.

Figure 4

(Color online) Frequency distribution diagrams of 3D elemental mapping for each element obtained from (a) as-quenched and (b) annealed $\left(773\mathrm{K}\right)$ specimens.

Figure 5

(Color online) Pair distribution functions of (a) as-quenched and (b) annealed $\left(773\mathrm{K}\right)$ specimens obtained by the high-energy x-ray diffraction technique.

Figure 6

(Color online) Schematic diagram of structural and compositional changes in the present nanocrystallization model.