Irreversible structural relaxation in Fe–B–Si amorphous alloys

Structural relaxation in Fe78B7Si15 and Fe75B10Si15 amorphous alloys with annealing has been studied by means of the measurements of electrical resistance and Mössbauer spectroscopy. The electrical resistance of as-prepared alloys decreased abruptly in the initial stages of annealing and then attained a quasi-equilibrium value that depends on annealing temperature. A model, combining the Ziman theory for resistance and free-volume model for viscous flow, was used to analyze the irreversible decrease of resistance. The observed resistance changes were well reproduced with the model. The activation energy for annihilation of free volume was evaluated to be 154 kJ mol−1 for the Fe78B7Si15 alloy and 222 kJ mol−1 for the Fe75B10Si15 alloy from resistance changes. On the other hand, we observed a significant difference in the annealing behavior of the average isomer shift and hyperfine field with time between these two alloys. This indicates that the local structural change during relaxation process differs much in both amorphous alloys. The observed difference in relaxation behavior between the two alloys can be explained in terms of Dubois and Le Caerʹs structural model of Fe–B–Si amorphous alloys.