Unlocking ultrastrong high-temperature ceramics: Beyond Equimolar Compositions in High Entropy Nitrides

Traditionally, increasing compositional complexity and chemical diversity of high entropy alloy ceramics whilst maintaining a stable single-phase solid solution has been a primary design strategy for the development of new ceramics. However, only a handful have shown properties that justify the increased alloying content. Here, we unveil a groundbreaking 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 of refractory metal-based high entropy nitrides, we found that the activation of an additional strengthening mechanism upon metastable phase decomposition propels the yield strength of a non-equimolar nitride at 1000 C to a staggering 6.9 GPa, that is 40 % higher than the most robust equimolar nitride. We show that the inherent instability triggers the decomposition of the solid solution with non-equimolar composition at high temperatures, inducing strengthening due to the coherency stress of a spinodally modulated structure, combined with the lattice resistance of the product solid solution phase. In stark contrast, the strength of equimolar systems, boasting diverse chemical compositions, declines as a function of temperature due to the weakening of the lattice resistance and the absence of other strengthening mechanisms.