Structural vacancies in CALPHAD modelling of zirconium carbide

Zirconium carbide is of interest in the nuclear and aerospace industry due to its extremely high melting point. Its properties are strongly affected by significant structural vacancies. Conventional thermodynamics-based phase diagram (CALPHAD) models are not currently able to consider the effects of such defects directly, and the widely-used C-Zr phase diagram model has been shown to be intrinsically incompatible with our physical understanding of structural point defects [1]. The vacancy formation energy as described by the widely used CALPHAD assessment [2] is not consistent with the same quantity as obtained using fully anharmonic first principles calculations [3].

Nonetheless, phase diagrams are a key tool in materials understanding and design, and having accurate descriptions is vital in materials development. In this work, state-of-the-art high accuracy first principles calculations of defect-related properties [4] are used to inform development of specific Gibbs energy models for cases where there are many structural point defects (for example, vacancies, interstitials, and Frenkel pairs) are present. By incorporating such information directly into the CALPHAD-type thermodynamic database, the description becomes more physically consistent and may allow further predictive ability than current models.

Additionally, using such models will allow greater use of first principles calculations in phase diagrams, which has potential to save time and expense in materials development.

References [1] T Davey. PhD thesis (Imperial College London), 2017
[2] A Fernandez Guillermet. Journal of Alloys and Compounds, 217:69–89, 1995.
[3] AI Duff, T Davey, MW Finnis et al.. Physical Review B, 91(21):214311–8, 2015.
[4] TA Mellan, AI Duff, MW Finnis. arXiv:1809.03281. 2018.