Unlocking ultrastrong high-temperature ceramics via non-equimolar refractory metal high-entropy nitrides

Abstract Refractory ceramic materials are critical for applications in extreme high-temperature environments. Refractory high-entropy ceramics belong to this class of materials and show great potential due to their remarkable combination of properties. Traditionally, increasing compositional complexity and chemical diversity of high-entropy ceramics whilst maintaining a stable single-phase solid solution has been a primary design strategy for developing new ceramics. Here, we unveil an alternative strategy based on deviation from conventional equimolar composition towards non-equimolar composition space, enabling tuning the metastability level of the supersaturated single-phase solid solution. By employing high-temperature micromechanical testing and post-mortem microstructural characterization of refractory metal-based high-entropy nitrides, we observed the activation of an additional strengthening mechanism upon spinodal decomposition of the metastable phase into coherent cubic-phase domains exhibiting compositional modulation. This process propels the yield strength of a non-equimolar nitride at 1000 °C to a staggering 6.9 GPa, that is 43% higher than the most robust equimolar nitride.