Theoretical predictions and experimental verification on the phase stability of enthalpy-stabilized HE TMREB2s

Transition metal diborides (TMB2s) are the materials of choice in extreme environments due to their excellent thermal and chemical stabilities. However, the degradation of oxidation resistance of TMB2s at elevated temperature still hinders their applications. To cope with this challenge, it is effective to incorporate rare earth elements to form high-entropy transition and rare-earth metal diborides (HE TMREB2s). To obtain thermodynamically stable single-phase structures for HE TMREB2s, a “16 × 16 mixed enthalpy matrix” is constructed using first-principles calculations to predict the single-phase formation ability of 120 two-component diborides (TCBs). Through the use of the “16 × 16 mixed enthalpy matrix” of TCBs, specific combinations of TMB2s and REB2s that are most likely to form single-phase HE TMREB2s are confirmed. Subsequently, based on the energy distribution of the local mixing enthalpies of all possible configurations, the enthalpy and entropy descriptors of HE TMREB2s (RE = Sc, Lu, Tm, Er, Ho and Dy) are investigated. It is found that the mixing enthalpy plays a critical role in the stability of the single-phase HE TMREB2s, i.e., HE TMREB2s are enthalpy-stabilized materials. The experimental results further confirm that enthalpy dominates the thermodynamic domain and drives the stability of REB2s in HE TMREB2s. This study validates that enthalpy-stabilized HE TMREB2s can further expand the compositional space of ultrahigh temperature ceramics (UHTCs) and is expected to further improve the oxidation resistance and high temperature properties of UHTCs.