On the kinetics of the α-σ phase transformation in an Al-doped Fe-Cr alloy

The influence of aluminium on the kinetics of the sigma -phase formation in a quasi-equiatomic Fe-Cr alloy with fine- and coarse-grained structure was studied using Fe-57 Mossbauer Spectroscopy. The amount of the sigma -phase precipitated could be well described by use of the Johnson-Avrami-Mehl equation which yielded the kinetics parameters k and it. Assuming the k-values follow the Arrhenius law, the effective activation energy, E, was determined. It was found that addition of 0.2 at% Al slightly accelerated the kinetics in the coarse-grained samples (E smaller by similar to 1.7 kJ mol(-1)), and practically did not affect it in the fine-grained samples. On the other hand, doping with 1 at%: Al resulted in a significant retardation of the sigma -phase formation both in the fine-grained samples tan increase of E by similar to 16.5 kJ mol(-1)) as well as in the coarse-grained samples (E enhanced by similar to 20.6 kJ mol(-1)). The presence of Al also affected the value of the form factor, II, hence the underlying mechanisms responsible for the transformation. (C) 2000 Elsevier Science B.V. All rights reserved.