Mechanism of nanocrystalline microstructure formation in amorphous Fe-Nb-B alloys

To understand the mechanism of the high number density of bcc-Fe nanocrystals in a partially crystallized Fe84Nb7B9 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 (similar to 773 K). At this stage, local compositional fluctuations started to appear, and medium-range ordering with a bcc-Fe structure as small as 2 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 bcc-Fe regions. The increase of atomic chemical bonds in the residual amorphous matrix is considered to retard the growth of the bcc-Fe nanocrystals after the coalescence of bcc-Fe MRO regions in the later stage of annealing. These results suggest that bcc-Fe nanocrystallization with the extremely high number density is ascribed to primarily (i) the presence of highly dense bcc-Fe MRO regions and (ii) the increase of chemical bonds of matrix atoms on annealing.