Crystallization behavior of amorphous Fe_90-XNb₁₀B_X (X = 10 and 30) alloys

The phase transformation of Fe_90-XNb₁₀B_X (X = 10 and 30) amorphous alloys by annealing was studied by differential scanning calorimetry (DSC) and X-ray diffraction. The Fe₆₀Nb₁₀B₃₀ alloy exhibits a large supercooled liquid region (i.e. the difference between the glass transition temperature (T_g) and the onset of crystallization temperature (T_x)), ΔT_x (=T_x - T_g) of 67 K, whereas the Fe₈₀Nb₁₀B₁₀ amorphous alloy transforms directly into crystalline phases. New metastable crystalline phases were found during the crystallization process of these alloys. In the crystallization process of the Fe₈₀Nb₁₀B₁₀ alloy, the structure of the primary precipitation phase is α-Mn type, which transforms into α-Fe phase at a higher temperature. In case of Fe₆₀Nb₁₀B₃₀ alloy, another metastable phase, Fe₂₃B₆-type structure, is formed corresponding to the first exothermic peak in the DSC curve. Although the metastable phases in the two alloys are completely different, the dissociated phases of α-Fe, Fe₃B and Fe₂B are formed in both alloys after the final stage of crystallization. The local atomic ordering structure in the α-Mn type is similar to that in an amorphous state of the Fe₈₀Nb₁₀B₁₀ alloy. On the other hand, the formation of the Fe₂₃B₆-type structure requires a significant change in the geometrical rearrangements with relatively long-range ordering of the Fe₆₀Nb₁₀B₃₀ alloy, which may be attributed to the high stability of the supercooled liquid.