Eutectic α-Fe2Si5 and Ε-FeSi formed in an arc furnace showed a three-layer structure

We melted stiochiometrically mixed (FeSi2) Fe and Si powders in a tri-arc furnace in an argon atmosphere and allowed them to cool naturally to room temperature and obtained a eutectic crystal of Ε-phase embedded in a α-phase of iron silicide. We determined three kinds of layers: the grain size of Ε-phase was more than 100 μm, about 50 μm and less than 1 μm. Only the first layer did not contain α-phase. The interfaces between the layers were so definite that the grain size was changed abruptly between the layers. When the crystal was annealed in vacuum at 1150 K for 70 h, the last two layers were transformed to β-FeSi2 but the bottom layer containing only Ε-phase was not changed by the thermal treatment.