Inetics of phase formation during the decay of wustit

On the basis of high-temperature X-ray phase analysis of wustite-containing scale on iron subjected to isothermal tempering at temperatures below 575 °C, the kinetics of the formation of wustite decay products is investigated: stoichiometric wustite, a primary and a secondary magnet, as well as particulate iron. It is established that stoichiometric wustite is formed as an intermediate phase at tempering temperatures below 400 °C and diffusion control. It is possible that stoichiometric wustite is the result of kinetic difficulties in the decay of the original non-stoichiometric wustite. Depending on the tempering temperature, the decay of wustite proceeds by two mechanisms. At temperatures above 400 °C, wustite decays with the simultaneous formation of magnetite and iron. At temperatures below 400 °C, the decay occurs in two stages. At the first stage, primary magnetite and stoichiometric wustite are formed. After the complete decay of the original non-stoichiometric wustite, the second stage occurs - the decay of the stoichiometric with the formation of secondary magnetite and iron. At tempering temperatures of less than 400 °C, two magnetites are formed: primary and secondary. The differences between these phases are that the first is formed as a result of the decomposition of the initial nonstoichiometric wustite, and the second is a decomposition product of the intermediate stoichiometric wustite. This, in turn, causes significant differences in the kinetics of phase formation. In particular, the growth rate of crystalline nuclei of secondary magnetite is very low, which suggests the absence of crystal defects in stoichiometric wustite, or a very small number of them. It was also found that the formation of all decomposition products includes several successive stages. The initial stages of the formation of new phases proceed under diffusion control and the tempering temperature is the factor determining the rate of the process. All final stages are controlled kinetically, and the speed of the stages is determined by the amount of hypothermia.