*3.1. Para Equilibrium*

PE [16] describes the equilibrium state where only interstitial atoms are free to redistribute while substitutional atoms remain configurationally frozen during transformation, i.e.,

$$\frac{u\_M^\alpha}{u\_{Fe}^\alpha} = \frac{u\_M^\gamma}{u\_{Fe}^\gamma} = \frac{u\_M^0}{u\_{Fe}^0} \tag{1}$$

where *uFe* and *uM* are molar fractions of Fe and M with respect to substitutional sites which are termed u-fraction. The superscript 0 stands for bulk concentration and α or γ denotes the ferrite or austenite phase. PE is a constrained equilibrium which is defined as

$$\begin{cases} \mu\_{\rm C}^{\alpha} = \mu\_{\rm C}^{\mathcal{V}}\\ \left(\mu\_{\rm Fc}^{\mathcal{V}} - \mu\_{\rm Fc}^{\alpha}\right) + \frac{\mu\_{\rm M}^{0}}{\mu\_{\rm Fc}^{0}} \left(\mu\_{\rm M}^{\mathcal{V}} - \mu\_{\rm M}^{\alpha}\right) = 0 \end{cases} \tag{2}$$
