CalPhaD Coffee Lecture – Defect Phases in CALPHAD - From Bulk to Microstructure

CALPHAD is the state-of-the-art theory that enables the description of multi-component multi-phase equilibria. Thousands of engineers and scientists use CALPHAD databases and software regularly to design engineering materials and their processing. However, CALPHAD has been confined to descriptions of bulk phases. These descriptions have been used to predict also some behavior of defects (e.g. stacking fault energies from the Gibbs energy difference between FCC and HCP phases), but do not describe a multi-component multi-phase equilibrium. Established models of solute partitioning at defects, such as the Langmuir-McLean and Fowler-Guggenheim models, rely on idealized descriptions of sites and interactions, inherently limiting their applicability to multi-component (several elements, especially several principal components) and multi-phase (several defects) systems.Building on the work of SFB-1394 "Structural and Chemical Atomic Complexity: From Defect Phase Diagrams to Material Properties", it is shown here how defect phases can be described in the CALPHAD framework using the concept of constraints. On the example of grain boundaries, it is argued that constraints can be understood as descriptions of the macroscopic degrees of freedom, while regions with different microscopic degrees of freedom are considered different phases. Thus, phase transformations at the grain boundary can be described. The presentation will conclude with an outlook on how several defects can be included in the framework, allowing for the description of multi-component multi-phase multi-defect equilibria.