Direct consideration of point defects in CALPHAD modelling of zirconium carbide

Theresa Davey1, Ying Chen1

1. School of Engineering, Tohoku University, Japan

ICACC 2020, Daytona Beach, FL, USA

Invited oral presentation in Ultra-High Temperature Ceramics - UHTC Phase Equilibria and Properties

Zirconium carbide is of interest in nuclear and aerospace industries due to its stability at extremely high temperatures. Its properties are strongly affected by the presence of significant structural vacancies. Conventional CALPHAD-type phase diagram models do not directly consider such defects; instead they are implicitly considered via thermodynamic data relating to non-stoichiometric compounds. The widely-used C-Zr phase diagram from Fernandez-Guillermet has been previously shown to be intrinsically incompatible with our physical understanding of structural vacancies.

Defect-related properties such as formation energies and defect-defect interaction energies are challenging to obtain experimentally with the required accuracy. In this work, state-of-the-art first-principles calculations of defect-related properties are used to inform development of Gibbs energy models that may be used in cases where many structural point defects are present. This is done both by incorporating such information directly into existing thermodynamic databases with conventional Gibbs energy descriptions, and through development of new Gibbs energy models.

Directly considering defect-related properties in the development of the thermodynamic database produces a more physically consistent description and may allow further predictive ability of the phase diagram in regions where experimental information may be scarce.