- Thermodynamic study of interstitial solid solutions gives the necessary information about solubility of an alloying element. The main energy contribution determining solubility is the energy of the deformational interaction. This interaction is linked to the nonuniform static displacements and Kanzaki forces. We calculate the displacements and show the decrease of the values efforces for growing concentration of carbon atoms. This effect leads to strong deviation of solubility from the regular solution model.