Resistivity Study of Eutectoid Decomposition Kinetics of α-Fe<SUB>2</SUB>Si<SUB>5</SUB> Alloy

Abstract

The kinetics of the eutectoid decomposition a → β + Si in a sintered α-Fe 2 Si 5 alloy under isothermal conditions has been studied by using electrical resistivity technique. X-ray diffraction was applied to determine therelation of resistivity and transformed volume fraction by quantitative analysis. The time-temperature-transformation (TTT) diagram for the reaction was obtained in the temperature range of 873-1148 K. The TTT diagram shows a typical C shape and gets nose at 1073 K. A general expression of Johnson-Mehl-Avrami (JMA) equation was proposed by introducing a constant, which is associated with spatial distribution of nucleation. The mechanism of the transformation was discussed in the theoretical frame of the modified JMA theory. The Avrami exponent was found to change with temperature, n = 3.8 above 1073 K and n = 3.0 in the region of 873-1073 K. The results indicated that an interface controlled three-dimensional growth is responsible for the β formation in the eutectoid decomposition under the conditions of decreasing nucleation rate above 1073 K but zero nucleation rate (site saturation of nucleation) in the region of 873-1073 K. The activation energy associated with the eutectoid decomposition, obtained in the lower temperature range, was 132.8 kJ/mol.