Glass forming ability and nanocrystallization kinetics of Fe₆₅Nb₁₀B₂₅ metallic glasses

Abstract

In this work the mechanisms controlling the nanocrystallization kinetics of the Fe 65 Nb 10 B 25 metallic glasses have been determined by the combination of the analysis microstructural data from XRD and TEM, and the kinetic analysis performed using the Master Curve method of the continuous heating and isothermal calorimetric curves. The results show that the transformation starts by the nucleation and interface controlled growth of the Fe 23 B 6 -type phase that changes to diffusion controlled growth as the transformation advances until is stopped by the soft-impingement effect. The transformation is modeled in the framework of the Kolmogorov-Johnson-Mehl-Avrami (KJMA) theory using constant activation energy expressions for the nucleation frequency and interface-controlled growth and taking into account the reduction of those quantities with the transformed fraction due to the change in the matrix composition using a mean-field approximation. The parameters of the modeling are determined from the coupling between the isothermal and constant heating rate calorimetric analysis and from the quantitative analysis of microstructural data. This is the outset for the determination of the viscosity, driving force for crystallization, and interfacial energy when replacing the constant activation energy expressions by the classical nucleation and growth ones. Both the glass forming ability in Fe-Nb-B based bulk metallic glasses and the temperature dependence of the interfacial energy are discussed in terms of the influence of the minor alloying elements.