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

Abstract

The structural transformations of Fe 73.5Cu 1Nb 3Si 15.5B 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 3Si phase in an amorphous matrix. At higher temperatures (between 780 and 920 K) we detected, in addition to the Fe 3Si phase, which reached an almost constant value of 85 wt%, three new phases, FeCu 4, Fe 16Nb 6Si 7 and Fe 2B. 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 5Si 3 and Nb 5Si 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 3Si 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.