Controlling devitrification in the FeSiB system without alloying additions

Abstract

Efficient electric machines require soft magnetic materials with superior properties, motivating research on the processing-structure-response correlation in Fe-based metallic glass systems. Here, significant differences in the primary devitrification process and the resulting microstructure of amorphous metallic Fe79Si11B10 alloys synthesized into two different forms by different rapid solidification methods are confirmed. Melt-spun ribbons and water-quenched microwires were investigated with calorimetric, structural, and magnetic probes. It is found that the primary devitrification process of the ribbons occurs at a higher temperature (by 80 K) with a faster exothermic heat release relative to that of the quenched microwires, despite their common chemical composition and fully devitrified structural state. Analysis of the primary devitrification reveals that the ribbons exhibit a ∼70% higher effective activation energy and a ∼110% larger value of Avrami exponent relative to those characterizing the microwires, indicating different devitrification routes taken by these two types of material. In specific, the ribbons crystallize via a continuous nucleation process that partly relies on pre-existing surface nuclei, with an interface-controlled growth mechanism. In contrast, the quenched microwires devitrify solely from pre-existing nuclei, with diffusion-controlled growth. These differences are attributed to unique quenched-in structures that are created by the specific rapid solidification conditions. These results suggest approaches to control the microstructure in FeSiB compositions without the need for non-magnetic alloying additions.