Chemical potentials of tetragonal ferrite and its equilibrium with $\gamma$-phase in steels

For steels with non-carbide bainite equilibrium of tetragonal ferrite/austenite occurring in the decay of $\gamma$-phase on the basis of Fe-C alloys is considered. Chemical potentials of the components are calculated by summing the corresponding expressions for $\alpha$-phase with the cubic lattice and tetragonal additive according to Zener–Khachaturyan theory. The condition of equality of chemical potentials of the components for two phases makes it possible to calculate the boundary concentrations of carbon in $\alpha$- and $\gamma$-phases. It is determined that in a tetragonal $\alpha$-phase the calculated carbon concentration is 40–60 times higher than for conventional cubic ferrite. This opens up new possibilities for the construction of high-strength steel with non-carbide bainite structure.