Thermodynamic stimulus of martensitic transformation in Fe-C alloys

Martensitic transformation is the basis of the practically important phenomenon of steel hardening. Formation of martensite crystals requires excess consumption of free energy of  phase for elastic and plastic deformation and surface energy of martensite plates. Its source is the difference of free energies of prior and new phases that is called driving force, or thermodynamic stimulus, of martensitic transformation. The purpose of this paper is to apply thermodynamic analysis to study the thermodynamic stimulus of martensitic transformation as a function of carbon content in steel. The change of free energy associated with martensite formation in Fe-C alloys of different concentration is calculated using statistical theories of interstitial solid solutions, modern thermodynamic data on free energy of ferrite and austenite and the Zener-Khachaturyan theory of martensitic transformation. This allowed to estimate thermodynamic stimuli of martensitic transformation and their concentration dependence. The latter was found to be nonmonotonic with a pronounced peak at about 3 at. %. The stimulus grows with increasing carbon content up to 3 at. % (0.66 wt. %) and then starts to fall. The conceivable reason is the start of formation of tetragonal martensite lattice.