The highest melting point material: Searched by Bayesian global optimization with deep potential molecular dynamics

The interest in refractory materials is increasing rapidly in recent decades due to the development of hypersonic vehicles. However, the substance that has the highest melting point (T_m) keeps a secret, since precise measurements in extreme conditions are overwhelmingly difficult. In the present work, an accurate deep potential (DP) model of a Hf–Ta–C–N system was first trained, and then applied to search for the highest melting point material by molecular dynamics (MD) simulation and Bayesian global optimization (BGO). The predicted melting points agree well with the experiments and confirm that carbon site vacancies can enhance the melting point of rock-salt-structure carbides. The solid solution with N is verified as another new and more effective melting point enhancing approach for HfC, while a conventional routing of the solid solution with Ta (e.g., HfTa₄C₅) is not suggested to result in a maximum melting point. The highest melting point (~4236 K) is achieved with the composition of HfC_0.638N_0.271, which is ~80 K higher than the highest value in a Hf–C binary system. Dominating mechanism of the N addition is believed to be unstable C–N and N–N bonds in liquid phase, which reduces liquid phase entropy and renders the liquid phase less stable. The improved melting point and less gas generation during oxidation by the addition of N provide a new routing to modify thermal protection materials for the hypersonic vehicles.