Evolution of CALPHAD models and databases

One goal of CALPHAD is the development of multicomponent databases that are suitable for design and process development of commercial alloys. This requires reliable descriptions of all phases occurring in the alloy system. The reliability of the descriptions is strongly linked to the quality of the models being employed for the individual phases. Since its inception, the CALPHAD method has continued to evolve by improving model descriptions to better reflect the underlying physics. The performance of new model formalisms is traditionally evaluated on the basis of how well the new formalism describes the thermodynamic properties and phase equilibria and how well these are extrapolated to higher component systems. However, it is not trivial to change the model for a phase in a large multicomponent database and thorough testing of a new model is required to justify its implementation. In addition, the list of the properties being described using the CALPHAD methods is being expanded. For the description of the compositional dependence and, maybe also, temperature dependence of diffusion mobility and other phase-based properties, it is generally agreed to use the same formalism as for the Gibbs energy, but it is often not clear whether the new formalism will be well suited to describe these properties. The current state of CALPHAD models and their implementation in newly developed descriptions will be evaluated.