Temperature dependence of β-phase transformation in Cu added Fe2Si5 thermoelectric material

Abstract

Abstract The time–temperature–transformation diagrams for the eutectoid decomposition (α→β+Si) in slowly solidified Fe 2 Si 5 alloys with small amounts of Mn and Cu were obtained in the temperature range 873 and 1173 K. The shape of the diagrams can typically be described by the character C. The nose temperature in a binary Fe 2 Si 5 alloy was about 1023 K and increased by about 50 K in the Cu added alloy. The addition of small amounts of Cu drastically shifted the diagram to shorter times. The start of the eutectoid decomposition in the Cu added alloy was more than 240 times faster than that in the Cu free alloy at the nose temperature. The shape of the Si dispersoids formed by the eutectoid decomposition changed from lamella in the Cu free alloy to granular in the Cu added alloy. In the FeSi 2 alloy, the peritectoid reaction (α+e→β) was also enhanced by the addition of Cu. These results suggest that the existence of Si is not significant for the acceleration of the β formation. The kinetic analysis of the β-phase transformation based on the Johnson–Mehl–Avrami equation suggests that the addition of Cu does not change the transformation mechanism but changes the growth kinetics. The size of the Si dispersoids was quite fine when heating below 1023 K.