Kinetics of the isothermal A2 to sigma phase transformation in Fe-Cr alloy

The transition of the A2 phase with random Fe and Cr atoms distribution to the sigma (σ) phase in Fe-46 wt%Cr alloy during isothermal annealing near 655 and 700 °C is investigated both in the bulk material by in situ neutron diffraction and on the surface of a sample by X-ray diffraction method. Kinetics of the σ-phase formation is quantitatively described with the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. According to neutron data, the A2 phase fully transformed to σ-phase at 655 °C for 5 h. In X-ray diffraction experiments, the partial A2 to σ transition and subsequent reverse transformation of the σ-phase into the A2 phase are observed, and the probable reason for this phenomenon is discussed.