High-temperature oxidation kinetics of a metastable dual-phase diboride and a high-entropy diboride

The processing of multicomponent (Ti0.25V0.25Zr0.25Hf0.25)B2 ultra-high temperature hexagonal transition metal diboride in dual-phase and single-phase microstructures and investigation of oxidation behavior in the air at 1000 and 1500 degrees C are reported. The dual-phase diboride is a metastable phase composed of Hf-Zr-rich and Ti-Vrich phases that undergo phase transformation to a single-phase high-entropy diboride after thermal annealing. At 1000 degrees C, a B2O3 layer was formed on the material's surface, and the oxidation kinetics followed a para-linear behavior. At 1500 degrees C, a porous oxide layer was formed, facilitating oxygen diffusion and reaction with the diboride, resulting in linear oxidation kinetics. The prediction of the lifetime of the materials during hightemperature oxidation suggested that the high-entropy material outperforms the dual-phase diboride, making it most suitable for related applications. The superior performance of the high-entropy single-phase diboride was associated with the high-entropy and sluggish diffusion effects.