Effects of Cu addition on the β-phase formation rate in Fe2Si5 thermoelectric materials

Abstract

We proposed Fe2Si5 based alloys with a small amount of Cu as new Fe–Si thermoelectric materials. A few acicular structures enriched in Cu were newly formed in slowly solidified alloys containing Cu above 0.2 at%. α single phase structure was formed by a conventional solidification process in alloys below 0.2 at% Cu. β phase was only formed by the eutectoid reaction (α→β+Si). Differential thermal analysis, X-ray diffraction and structure observation clearly confirmed that the eutectoid reaction rate was drastically enhanced by the addition of a small amount of Cu and its rate decreased with decrease of Cu content. Its rate also depended on the annealing temperature and it was maximum at about 1073 K for most alloys. The addition of only 0.1 at% Cu was still very effective in Mn or Co doped alloys. The final structure after the eutectoid reaction in these alloys was duplex composed of β and Si. The size of Si decreased with decrease of Cu content and the annealing temperature. Transmission electron microscope observation showed that β was transformed from α with many planar faults (stacking fault) that will act as a drag resistance for the transformation. We speculated that the addition of Cu probably decreased the stacking fault energy so as to decrease the drag force and to enhance the formation rate.