Thermodynamics of the martensitic transformation in Fe-C and Fe-N alloys

The martensitic transformation in Fe-C and Fe-N alloys, respectively, canno t be described consistently by applying thermodynamic parameters of the aus tenitic fee and the ferritic bcc phase. A model has been developed for the transformation of austenite into body centred tetragonal (bct) martensite w here the bet phase is considered a mixture of FeVa(3) and Fe(X,Va(2)) group s (Va = vacant sites; X = C, N). Using experimental data for start temperat ures of twin martensite and a constant driving force - Delta G(Ms) = 2100 J /mol thermodynamic parameters of carbon and nitrogen in the bet phase were derived. Concerning lath martensite, the calculation yielded a linear depen dence of the driving force on the concentration for both the start (M-s) an d the finish (M-f) of transformation. Concerning the end of transformation of twin martensite, a constant driving force of about 5000 J/mol was obtain ed.