Thermodynamics of the Martensitic Transformation in Fe-C and Fe-N Alloys

The martensitic transformation in Fe-C and Fe-N alloys, respectively, cannot be described consistently by applying thermodynamic parameters of the austenitic fcc and the ferritic bcc phase. A model has been developed for the transformation of austenite into body centred tetragonal (bct) martensite where the bct phase is considered a mixture of FeVa 3 and Fe(X, Va 2 ) groups (Va = vacant sites; X = C, N). Using experimental data for start temperatures of twin martensite and a constant driving force - ΔG Ms = 2100 J/mol thermodynamic parameters of carbon and nitrogen in the bct phase were derived. Concerning lath martensite, the calculation yielded a linear dependence of the driving force on the concentration for both the start (M s ) and the finish (M f ) of transformation. Concerning the end of transformation of twin martensite, a constant driving force of about 5000 J/mol was obtained.