Eutectoid decomposition in rapidly solidified α-Fe 2Si 5-based thermoelectric alloys

Abstract

The β phase transformation from the rapidly solidified Fe 2Si 5 based alloys was examined by SEM and TEM observation, cyclic differential thermal analysis (DTA) and X-ray diffraction. The rapid solidification enhanced the transformation rate to a value about 3 times higher than that in conventionally solidified alloys. A sharp endothermic peak at the equilibrium eutectoid temperature was observed only once during the first heating cycle and no peaks were observed in further heating or cooling cycles. The most important reaction for the accelerated transformation was the rapid decomposition of the supersaturated α phase with Si, formed by rapid solidification, into the β-Si eutectoid. A few part of the metastable ε phase contributed to the β phase formation by the peritectic reaction with the α phase. TEM observation showed that most of the ε phase was dissolved into the α phase before the α phase was decomposed into the β phase and Si. The result of a Johnson–Mehl–Avrami analysis for the β phase formation by isothermal annealing showed that there was no significant difference in the transformation mechanism between the rapidly solidified specimen and the slowly solidified specimen. This suggests that nucleation became saturated in the first stage of the annealing and subsequently growth controlled the transformation.