Computed multicomponent phase diagrams for hardenability (H) and HSLA steels with application to the prediction of microstructure and mechanical properties

Approximate methods for the calculation of the phase equilibria of multicomponent dilute systems designed for minimal claims on computer time and memory are reviewed. These results are required as the constraints on semiempirical kinetic models for the prediction of the TTT and CCT diagrams, which are used in turn for the prediction of microstructure and mechanical properties. Comparisons of predictions and observations based on the work of Kirkaldy and coworkers in hardenability and HSLA steels are presented. In the theoretical part, expressions for the chemical potentials of the elements in substitutional and substitutional- interstitial solid solutions are developed via the partition function method assuming random positioning of the atoms. This approach leads for substitutional solid solutions to the BalePelton “unified formalism” in thermodynamically consistent second- order approximation. It is shown that the quadratic term contributed by the solvent in substitutional solutions does not appear in the expression of the activity coefficient for the interstitial elements in the substitutional-interstitial solid solution as reported in 1962 by Kirkaldy and Purdy. This paper was presented at the International Phase Diagram Prediction Symposium sponsored by the ASM/MSD Thermodynamics and Phase Equilibria Committee at Materials Week, October 21-23,1991, in Cincinnati, Ohio. The symposium was organized by John Morral, University of Connecticut, and Philip Nash, Illinois Institute of Technology.