A Modified-CALPHAD Approach for Phase Diagrams without Experimental Data

Theresa Davey1, Nguyen-Dung Tran1, Arkapol Saengdeejing1, Ying Chen1

1. School of Engineering, Tohoku University, Japan

ICMAT 2019, Marina Bay Sands, Singapore

Contributed oral presentation in Materials Design and Accelerated Development

Phase diagrams calculated entirely from first principles have potential to reduce time and expense in investigations for materials design by providing important thermodynamic information on new material systems at the prediction stage. However, it remains challenging to create a thermodynamic description of most systems using only calculated data in place of experimental data with conventional phase diagram methods. An approach is proposed that considers several theoretical techniques to inform a CALculation of PHAse Diagrams (CALPHAD)-based thermodynamic description derived only from first principles data.

Commonly, thermodynamic descriptions using the CALPHAD approach use the Bragg-Williams approximation to describe the configurational entropy of a solid, which is a point correlation model ignoring the pair and higher order interactions [1]. Generally, other configurational entropy contributions are indirectly included in the excess energy terms that have optimized parameters fitted to experimental data. With only computational data, this model does not give a proper description of the phase diagram, which is partly attributed to the lack of consideration of short range ordering. In this work, various techniques have been implemented to modify the Gibbs energy descriptions of solid phases, such as reciprocal interaction parameters in a structure based on the Compound Energy Formalism (CEF) [2,3].

We show that the effects of short range ordering are introduced with this framework, and further configurational entropy contributions can be directly included by comparison with other theoretical techniques such as the Cluster Variation Method (CVM) [4]. Using this approach, a satisfactory solid phase diagram reproducing all topological features of the experimental phase diagram was produced using only first principles data.

[1] W. Shockley; J. Chem. Phys. 6 (1938) 130
[2] A. Kusoffsky, N. Dupin and B. Sundman; Calphad 25 (2001) 549
[3] T. Abe and M. Shimono; Calphad 45 (2014) 40
[4] R. Kikuchi; Physical Review 81 (1951) 988