Assessment of the Fe-Si-B eutectic composition using calorimetry and microstructural analysis

Eutectic compositions in potentially glass forming binary or ternary alloy systems are usually the starting point to identify metallic liquids in search of an increased glass-forming ability (GFA). The Fe-Si-B ternary system has great potential if it could be additively manufactured via laser bed powder fusion into large, intricate soft magnetic parts for using in highly energy-efficient devices such as motors. However, the GFA of the currently available alloys is low to print fully amorphous parts, making alloy development necessary. Therefore, this study assesses Fe-rich Fe-Si-B alloys in term of eutectic point, which has not been accurately explored over the past four decades. A combination of differential thermal analysis, phase analysis via X-ray diffraction, and microstructural characterization through optical microscopy and scanning electron microscopy in the relevant composition range reveals that the eutectic composition of the Fe-Si-B system with a high Fe content is found to be at or close to Fe 77 Si 11 B 12 at% with a eutectic temperature of 1388 K (1115 °C). Thermal analyses and microstructural studies reveal a sharp melting peak and a well-defined phase transition into the distinct eutectic lamellae, composed of Fe 2 B and α-Fe/Fe 3 Si. Deviations from the eutectic composition either at Si-rich or at B-rich side result in the formation of primary simple dendritic (Fe 3 Si) or faceted dendritic (Fe 2 B) phases, respectively, which precede eutectic solidification. The GFA study using high-energy XRD of the cast plates reveals that the eutectic composition does not necessarily exhibit the maximum critical casting thickness and GFA can be enhanced by strong glass-forming elements in off-eutectic compositions.