Experimental investigation of the thermodynamics of Fe-Nb-C austenite and nonstoichiometric niobium and titanium carbides (T = 1273 to 1473 K)

The thermodynamics of Fe-Nb-C austenite and nonstoichiometric binary niobium carbide and titanium carbide in the temperature range of 1273 to 1473 K has been investigated using a dynamic gas equilibration technique. Methane-hydrogen mixtures have been used for fixing carbon potentials, and the carbon contents have been determined as dynamic weight changesvia a sensitive Cahn microbalance. The niobium-carbon interactions, similar to the titaniumcarbon interactions, are manifested as a minimum in the niobium carbide solubility in austenite, as increased amounts of dissolved carbon with niobium additions, and as the variation of solubility limit of the carbide with carbon content at high carbon levels. The results on the isoactivity measurements in the ternary Fe-Nb-C have been analyzed using the modified Wagner formalism, and the ternary interaction parameter eCNb has been evaluated. The solubility of niobium carbide in Fe-Nb-C in austenite has been determined as the deflection in the variation of carbon content with Nb additions at constant carbon activity. The dissolution free energy of body-centered cubic (bcc) Nb in face-centered cubic (fcc) Fe has also been determined from the solubility data. Rational correlation between the dissolution free energies of transition metal solutes in fcc Fe and the group number in the Periodic Table has been shown to exist. A correlational relationship between the ternary interaction parameter and the free energy of formation of carbides has been established. These relationships are utilized in the assessment, as well as the systematization of thermodynamic data. The results on the activity-composition relationship in the binary niobium and titanium carbides have been analyzed using the sublattice-subregular model proposed by Hillert and Staffansson, [2] and the interaction parameters in the model were determined. The thermodynamic calculations based on this model and our experimental results were carried out, and very good agreement between experimental and calculated results was obtained.