Thermodynamics of Fe-Ti-C and Fe-Nb-C Austenites and Nonstoichiometric Titanium and Niobium Carbides

Abstract

The thermodynamics of Fe-Ti-C and Fe-Nb-C austenites and that of nonstoichiometric titanium and niobium carbides have been experimentally investigated using a dynamic gas equilibration technique in the temperature range 1273K-1473K. Methane-hydrogen gas 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 effects of titanium-carbon and niobium - carbon interactions in austenite have been observed (i) as a miniumum in the carbide solubility curve, (ii) as increases in carbon content due to Ti/Nb additions at constant carbon activity and (iii) as the variation of solubility limit of the carbide with carbon content at high carbon concentrations. The results on the isoactivity measurements in the ternary Fe-Ti-C and Fe-Nb-C austente have been analyzed using the modified Wagner formalism. The ternary interaction parameter ∈CM(M=Ti, Nb) 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 bcc Ti in fcc Fe has also been determined. The results on the activity - composition relationship in the binary nonstoichiometric titanium and niobium carbide phases have been analyzed using the sublattice - subregular model suggested by Hillert and Staffansson and the interaction parameters in the model determined. A correlational relationship between the ternary interaction parameter and the free energy of formation of carbides has been established.