Solid solutions in Fe-C, Fe-N, Nb-O, Ta-O or W-C systems consist of a body-centered lattice containing C, N or O interstitials in octahedral sites. Among these systems, carbon-supersaturated iron has a major industrial interest. At low temperature, it can be found in the form of ferrite (bcc-Fe) or martensite (bct-Fe and bco-Fe). Based on a mean-field elasto-chemical model of the interstitialstrain interaction, we investigated numerically the thermodynamic stability of the three possible crystal forms. The phase stability depends not only on temperature and solute carbon content but also on the mechanical stress state of the crystal. We computed phase diagrams mapping the equilibrium phase and orientational variant as function of the components of the stress tensor. These maps are intended to help understanding the spatial distribution of orientational variants in martensite nano-crystals and in Cottrell atmospheres around dislocations.