Phase Stability, Short-Range Order, and Defect Thermodynamics in High-Entropy Covalent Ceramics

High-entropy ceramics (HECs), particularly carbides and borides, have emerged as a promising class of materials for extreme environments due to their exceptional thermal stability, mechanical strength, and compositional tunability. However, the vast compositional space and complex bonding in these systems present fundamental challenges for predicting phase stability and thermodynamic behavior.

In this talk, we present recent advances in first-principles–based free energy modeling of high-entropy carbides and borides, enabling quantitative predictions of phase stability across composition and temperature. These predictions are validated through comparison with both existing data and newly synthesized materials, highlighting the predictive capability of ab initio thermodynamics while also revealing kinetic limitations that often hinder equilibration in these systems.

Beyond phase stability, we demonstrate the presence of chemical short-range order (CSRO) in covalently bonded HECs—an effect well established in metallic systems but previously unconfirmed in ceramics. Using machine-learning interatomic potentials in combination with high-resolution electron microscopy, we provide direct evidence of CSRO and show that its nature is strongly dependent on composition and thermal history.

We further examine the thermodynamic and kinetic implications of CSRO for point defect behavior. In contrast to metallic systems, defect transport in HECs is governed by a complex interplay of ionic and covalent bonding. Our results reveal that CSRO plays a critical role in modulating defect energetics and diffusivities, with direct consequences for radiation tolerance.

Finally, we present new insights into radiation-induced segregation (RIS) at interfaces in multicomponent ceramics. We show that RIS is strongly influenced by local chemical ordering, underscoring the coupling between phase stability, defect dynamics, and interfacial chemistry.

Together, these results establish fundamental links between local chemical order, thermodynamics, and defect behavior in high-entropy ceramics, and provide new pathways for the design of advanced materials in extreme environments.