Phase transformations of Fe sub(73.5)Cu sub(1)Nb sub(3)Si sub(15.5)B sub(7) amorphous alloy upon thermal treatment

The structural transformations of Fe sub(73.5)Cu sub(1)Nb sub(3)Si sub(15.5)B sub(7) amorphous alloy under non-isothermal as well as isothermal conditions were studied. Differential scanning calorimetry (DSC) showed that slow heating rates induce a series of stepwise structural transformations consisting of endothermic peaks and more pronounced exothermic peaks in the broad temperature range from 350 to 970 K. Over this range the system changed from an as-deposited amorphous alloy of higher excess free energy to an annealed sample exhibiting lower excess of free energy. X-ray diffraction (XRD) analysis found that primary crystallization started with formation of a face-centred Fe sub(3)Si phase in an amorphous matrix. At higher temperatures (between 780 and 920 K) we detected, in addition to the Fe sub(3)Si phase, which reached an almost constant value of 85 wt%, three new phases, FeCu sub(4), Fe sub(16)Nb sub(6)Si sub(7) and Fe sub(2)B. Further annealing above 923 K led to, with Si initially migrating from the Fe-Si phase to the Nb-rich grain boundaries, formation of two new phases, Fe sub(5)Si sub(3) and Nb sub(5)Si sub(3). The Fe content in the cubic Fe-Si phase was estimated by means of a change in lattice parameter. Below 923 K the size of crystallites for the major Fe sub(3)Si phase was less than 10 nm. It was shown that further heating induced rapid crystallite growth, reaching a size greater than 500 nm at 1123 K.