First-principles-based CALPHAD phase diagrams

Phase diagrams calculated using the CALPHAD method typically use experimental phase diagram and thermodynamic data to obtain parameters describing the Gibbs energy of each phase. The combination of these Gibbs energies is minimised in order to determine which phase (or phases) is stable at a given point in temperature-composition space. In recent years, it has become common to complement the experimental data with ab initio calculations, however phase diagrams using only first-principles calculations to describe the system has been relatively unsuccessful due to the wealth of approximations used in the CALPHAD approach, particularly ideal solution approximations.

By making careful consideration of all approximations used in the description, a description of the solid aluminium-nickel system was obtained at a practical application level using only first-principles calculations and obtaining all parameters in the database directly without optimisation considering the known phase diagram. This method makes use of the Bragg-Williams-Gorsky pair approximation for bond energies used within the four substitutional sublattice order-disorder partitional model. Derivations of the necessary models are available here.

This work has been accepted for publication in Calphad [1] – preprint coming soon!

Further developments to this method, including extension to other systems and considering magnetic effects and interacting defects, are underway.

1. Davey T, Tran N-D, Saengdeejing A, Chen Y. First-principles-only CALPHAD phase diagram of the solid aluminium-nickel (Al-Ni) system. Calphad. 2020