Effect of titanium on the kinetics of the sigma-phase formation in a smallgrain Fe-Cr alloy

Using Mossbauer Spectroscopy the kinetics of the sigma-phase formation was studied in situ on Ti-doped quasi-equiatomic Fe-Cr alloys having an average grain size equal to 31(2) mu m. The kinetics was studied both with a tradi tional approach i.e. the amount of the sigma-phase precipitated, A(sigma), was determined from the spectral area, as well as with a novel approach i.e . in terms of the average isomer shift. In both cases the data could be wel l fitted in terms of the Johnson-Avrami-Mehl equation which yielded the kin etics parameters: the time constant, k, and the form factor, n. Assuming th e k-values follow the Arrhenius law, the Ti-induced change in the activatio n energy, E, was determined. In particular, it was revealed that addition o f 1.5 at% Ti or less accelerates the transformation, with the highest rate for 0.3 at% Ti, and the difference in E equals to 18.8 kJ/mol. In alloys wi th Ti-content higher than 1.5 at%, the formation of the sigma-phase was ret arded. The value of E = 207 +/- 20 kJ/mol was found for the Fe53.8Cr46.2 al loy. (C) 2000 Elsevier Science S.A. All rights reserved.