beta-FeSi2 Phase formation from a unidirectionally solidified rod-type eutectic structure composed of both alpha and epsilon phases

Kinetics of the transformation from the alpha and epsilon phases to the beta phase in an iron silicide thermoelectric material was examined. The specimens with a rod-type eutectic structure composed of the alpha and epsilon phases, prepared by a unidirectional solidification technique, were used as a diffusion couple. The beta phase formation from alpha and epsilon consisted of two processes, depending on temperature. The first process which occurred below 1143 K proceeded in the following three stages: (1) the ring-like beta phase was formed on the interface between the matrix alpha and rod shaped epsilon by the conventional peritectoid reaction, (2) the lamellar structure composed of the beta and Si phases was formed by the eutectoid reaction of the remaining alpha phase after an incubation period, (3) finally, the beta phase was formed by the subsequent reaction between the remaining epsilon and Si phases formed by the eutectoid reaction described in stage (2). In the second process above 1153 K, where most of the beta phase was formed by the peritectoid reaction, the eutectoid decomposed slowly. A rapidly solidified specimen has a very fine eutectic structure and the beta phase was mostly formed by the peritectoid reaction, irrespective of temperature. These two apparently different processes were explained by the combination of the simultaneous progress of the peritectoid and eutectoid reactions of which the kinetics depended on temperature. It was also found that a small addition of Mn remarkably decreased the beta phase formation rate.