Date of Award
Doctor of Philosophy (PhD)
Professor J.S. Kirkaldy
The thermodynamics of Fe-Nb-C, Fe-Ti-C, Fe-Nb-N austenites and that of binary carbides and nitrides of Ti and Nb essential in the understanding of the precipitation behaviour in microalloyed steels has been investigated in this study. A dynamic gas equilibration technique has been used in the experimental investigation of the thermodynamics of ternary austenites and binary carbides of Ti and Nb. The results obtained in this study have been analyzed using the modified Wagner formalism for dilute ternary austenites and the sublattice - subregular model suggested by Hillert and Staffansson for the interstitial carbides and nitrides. The solubilities of NbC, TiC and NbN in austenite have been determined from the isoactivity measurements done in this study. The solubility minimum and the variation of solubility limits of carbides of Ti and Nb and the increase in carbon content at constant carbon activity have been quantitatively related to the ternary interaction parameter. The C-Nb, C-Ti and N-Nb interaction parameters have been determined. The dissolution free energies of Nb and Ti in fee Fe have been obtained from the analysis of the solubilities of NbC, NbN and TiC in their respective austenites.
Expressions describing the variation of the partial molar free energies of the components in the binary carbides and nitrides of transition metals have been obtained via statistical mechanical considerations. The pair interaction free energies involved in the statistical mechanical description of interstitial solutions have been related to the interaction parameters in the sublattice model. The carbon activity measurements in the NbCy and TiCy phases and the nitrogen activity measurements in the NbNy phase obtained in this study together with the data available in the literature have been analyzed to obtain the interaction parameters in the sublattice model. The limitations in applying the classical Wagner-Schottky model to highly nonstoichiometric compounds have been discussed. The necessary modifications to this classical model have been made by referring to the expressions obtained via the statistical mechanical approach.
The microalloyed ternary austenite - nonstoichiometric carbide equilibrium has been evaluated for the Fe-Nb-C and Fe-Ti-C systems. The effects due to solute interactions and the nonstoichiometry of the precipitate have been clarified. The solubility of TiN in Fe-Ti-N austenite has been calculated using the dissolution free energy of Ti evaluated in this study. Rational correlations between the ternary interaction parameters and the free energies of formation of carbides and nitrides have been established. Using this correlation an approximate value of the nitrogen-titanium interaction parameter has been evaluated.
Balasubramanian, Krishnamurty, "Thermodynamics of Microalloyed Austenites and Nonstoichiometric Carbides and Nitrides" (1988). Open Access Dissertations and Theses. Paper 2129.