EXPERIMENTAL INVESTIGATION OF THE THERMODYNAMICS OF THE FE-TI-C AUSTENITE AND THE SOLUBILITY OF TITANIUM CARBIDE

The thermodynamics of Fe-Ti-C austenite has been investigated in the temperature range of 1273 to 1473 K using a dynamic gas equilibrium technique. Hydrogen-methane mixtures have been used for fixing carbon potentials, and the carbon contents have been determined as dynamic weight changes via a sensitive Cahn microbalance. The effect of titanium-carbon interaction in austenite has been observed (1) as a minimum in the carbide solubility curve, (2) as increases in carbon content due to titanium additions at constant carbon activity, and (3) as the variation of solubility limit of the carbide with carbon content at high carbon contents. The results on the isoactivity measurements in the ternary Fe-Ti-C austenite have been analyzed using the modified Wagner formalism. The ternary interaction parameter epsilon(C)Ti has been quantitatively related to the solubility minimum and the relative increase in carbon content at constant carbon activity. The variation of the solubility limit of the carbide with carbon content has been described using an additional term to the classical solubility relationships. This additional term is related to the self- and cross-interaction of carbon. Using the solubility relations, the dissolution free energy of body-centered cubic (bcc) Ti in face-centered cubic (fcc) Fe has also been determined. Very good agreement between the thermodynamic calculations and the experimental results was found.