Development of short-range order in Fe-B-C amorphous alloys with thermal treatment: an NMR study

In an earlier work, 11B NMR spectra obtained from a series of as-quenched Fe-B-C amorphous alloys revealed two distinct peaks centered at 34.7 and 36.3 MHz, which were attributed to regions with short-range order (SRO) characteristic of body-centered tetragonal Fe3B (bct-Fe3B) and orthorhombic Fe3B (o-Fe3B), respectively. It was determined that the addition of C favors the formation of o-Fe3B-like SRO and this was correlated with the enhancement of the soft magnetic properties. In this report, 11B NMR is utilized to study the evolution of the above phases, along with bcc α-Fe, in the Fe80B20-xCx alloys (x = 0, 2, 4, 7 and 9) for various thermal treatments. Consistent with the earlier work, it is found that the addition of C is necessary for o-Fe3B-like SRO formation upon thermal treatment. In particular, annealing the x = 0 alloy at both 460°C and 840°C results in bct-Fe3B precipitates plus bcc α-Fe. However, for the alloys with x > 0, the annealing treatment at 460°C results in some o-Fe3B precipitates along with bct-Fe3B and bcc α-Fe; the amount of o-Fe3B increases with the C content. The same situation occurs for the 840°C heat treatment with the o-Fe3B precipitates occurring to a greater degree and the bct-Fe3B SRO being completely suppressed for x ≥ 4. These results show that the C atoms play a crucial role in the formation and stability of the o-Fe3B SRO which is developed during the thermal treatments.