Phase Stability and Short-Range Order in High-Entropy Ceramics

High entropy ceramics are a new class of materials with properties that make them promising for applications in extreme environments, including high-temperature, radiation, and corrosion. One major challenge in advancing discovery of high-entropy ceramics lies in accurately predicting their single-phase stability and formation ability. Traditional CALPHAD approaches can be unreliable because of limited experimental data needed to fit free energy models used for predicting phase diagrams. Another type of approach that has recently gained popularity in literature is to identify descriptors that correlate with single-phase stability and that can be calculated in a high-throughput manner using <i>ab initio</i> methods. Examples of such descriptors include entropy forming ability, lattice strain, and disordered enthalpy-entropy descriptor. While these descriptors are easy to calculate, they are only proxies for the physics that controls phase stability and its transferability to new systems is an open question.<br/><br/>In the first part of the talk, we demonstrate that phase stability in high-entropy ceramics can be predicted directly using approximate free energies determined from <i>ab initio</i> calculations based on the density functional theory (DFT). We demonstrate applicability of this direct method on the examples of High Entropy Borides (HEBs) and High Entropy Carbides (HEBs). Our approach shows good agreement with existing experiments on these material systems. Our predictions also show a relatively good agreement with CALPHAD calculations and the discrepancies between DFT-based free energy calculations and CALPHAD can be traced to lack of thermodynamic data for mixing terms in CALPHAD. Using our model, we predicted several new single-phase compositions that have not been previously synthesized. Our approach based on DFT calculations has the advantage of being derived from fundamental thermodynamics. Consequently, the path to future refinements of the model is clear as it involves improving approximations made in DFT calculations of free energies.<br/><br/>In the second part of the talk, I will discuss our recent experimental discovery of chemical short-range (CSRO) order in HECs. The presence of CSRO has been previously reported in metal alloys, and it has been postulated in HECs based on results of atomistic simulations. Here, we demonstrate the existence of CSRO in two HECs using high resolution scanning transmission electron microscopy (HR-STEM) and 4D-STEM. We also demonstrate that the degree of CSRO can be altered by high-temperature annealing. The impact of the chemical short-range order on radiation resistance of HECs will also be discussed, providing scientific foundation for potential design of radiation resistance HECs.