3rd generation Calphad – review and future

Thermodynamic models and databases serve as the fundamental framework for materials design within the Calphad approach. The Calphad type databases are built based on reliable descriptions of elements at both stable and metastable states. In the 1995 Ringberg workshop [1], modelling thermodynamic properties with more physical basis was discussed and some models were proposed. In the past decades, these models have been used or modified to describe many elements, which has been the work for the third generation Calphad.

A summary of the work and status until 2022 for the third generation Calphad was provided in [2], some key points are mentioned here. For unaries, the Einstein model is used for the harmonic vibrations while polynomials are to describe the electronic and anharmonic contributions. For some elements, multiple Einstein functions may be needed. The unary liquid is described using the two-state model. For compounds and stable and metastable end-members of the sublattice model for solution phases, different methods are suggested to model their thermodynamic properties based on the availability of experimental and DFT data.

In recent years, focus has been on how to extrapolate the heat capacity of unary solid phases above their melting temperature. The most recent suggestion is to use two temperature ranges and if necessary, also the Equal Entropy Criterion [3] which disregards any solid with higher entropy than the most stable liquid. It is also discussed how liquid can be described with the limited data usually available, how DFT-calculated data can be used, and modelling of the chemical ordering contribution that is important in many systems.

[1] M. W. Chase et al., Calphad, vol. 19, no. 4, 1995.

[2] Z. He, PhD thesis, KTH Royal Institute of Technology, 2022.

[3] B. Sundman et al., Calphad, vol. 68, p. 101737, 2020.