Regensburg 2025 – scientific programme

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MM: Fachverband Metall- und Materialphysik

MM 11: Topical Session: Defects of Defects

MM 11.6: Talk

Tuesday, March 18, 2025, 12:15–12:30, H10

A computationally highly efficient analytical model for the description of defect phase diagrams — •Jing Yang, Mira Todorova, and Jörg Neugebauer — Max Planck Institute for Sustainable Materials, Düsseldorf, D-40237, Germany

In this work, we propose an analytical thermodynamic model for constructing defect and surface phase diagrams. The model is capable of accurately describing the composition-temperature dependence of phase transitions on the surface, including order-disorder ones. It provides a promising alternative to the sublattice model, which is commonly used in the CALPHAD framework to describe solution phases with ordering, as we demonstrate using the system of Mg surface with Ca substitutions. First, as a foundational reference we construct the surface phase transition with grand-canonical Monte Carlo simulation coupled with cluster expansion. The system undergoes a transition from a solid solution (disordered) phase at high temperature, Ca-poor condition to an ordered defect phase with 1/3 Ca coverage. We then show that it is possible to accurately reproduce the critical transition condition with an analytical model assuming a Boltzmann distribution of the phase fractions. Finally, we compare our method with the sublattice model. The proposed method provides a computationally highly efficient and easy-to-parametrize analytical model for constructing defect phase diagrams.

Keywords: surface; Monte Carlo; thermodynamic modeling; alloy; defect