Coefficients for equilibrium partition of a third element between solid and liquid in iron-carbon base ternary alloys and their relation to graphitization during iron-carbon eutectic solidification

During iron-carbon eutectic solidification, the coefficients for partition of a third element between the eutectic liquid and its solid were evaluated thermodynamically. The coefficientk_M^A/L for the equilibrium partition of the third element (M) between austenite and liquid iron largely depended on the interaction between carbon and the third element and a simplified method for the evaluation ofk_M^A/L was introduced. The coefficients,K^S andK^M, for the partition of the element between the eutectic liquid and its solid in the stable and metastable eutectic solidification, respectively, were also calculated fromk_M^A/L and the coefficientk_M^C/A for the equilibrium partition of the element between cementite and austenite. It was indicated by the thermodynamics of the free energy for the co-existing phases that the effect of a third element on graphitization occurring during eutectic solidification was related quantitatively to the value of K which was represented byK^S-K^M. The effect of a third element on the difference between the stable and metastable eutectic temperatures and on the carbon activity of liquid iron was closely related to K or the equilibrium partition coefficient,k_M^C/A.