Abstract: Expressing the excess partial molar free energies and integral excess free energy as poly- nome of the composition,then the parameters in the expression of excess partial molar free energy of C were evaluated based on:the excess partial free energy of C in C-Fe bi- nary alloy:the integral excess free energy of Fe-X binary alloy:the C saturation curve and the iso-activity curve of C (α_c=0.025) in the ternary system.Then the activity coefficient of X was evaluated with the so-called “I-D” method,by which the component activities of every element in the whole molten ternary alloy can be evaluated,and the results are in good agreement with literature data based on experiment.

Key words: component activity coeffcient, C-Fe-Si system, C-Fe-Mn system, C-Fe-Cr system, C-Fe-Ni system