Developing superior oxidation resistance of high-entropy diborides is critical for extending their potential applications in harsh environments. Herein, we developed non-equimolar (Hf,Zr,Ta,W)B 2 high-entropy diborides with superior oxidation resistance by adjusting W contents for the first time. The as-fabricated (Hf 0.28 Zr 0.28 Ta 0.28 W 0.15 )B 2 samples are predicted to possess superior oxidation resistance at 1473–1773 K through the oxidation depth quantitative analysis obtained by machine learning, which is attributed to the reason that the moderate WO 3 can effectively inhibit the volatilization of B 2 O 3 . Furthermore, a four-layered structure is found in the generated product layers at 1773 K, which is due to the preferential oxidation of Hf and Zr elements at low oxygen partial pressure and the different diffusion activation energies of oxide products. The preferential oxidation of Hf and Zr elements is further demonstrated by the computational phase stability diagrams, and oxygen adsorption energies and charge transfer between oxygen atoms and different metal adsorption sites via first-principles calculations. Such superior performance endows (Hf 0.28 Zr 0.28 Ta 0.28 W 0.15 )B 2 with potential applications as ultrahigh-temperature structural materials.
High-entropy diboride-based (MeB2-based) ceramics are promising high-temperature structural materials because of their excellent mechanical properties, high-temperature stability, and oxidation resistance. In order to achieve low-temperature sintering of the high-entropy ceramics, a novel preparation method of high-entropy (Ti,Zr,Nb,Mo,W)B2-SiC ceramics based on reactive sintering of pre-alloyed solid-solution metals and nonmetals of Si, C, B4C was conducted in the current work. Mechanical alloying behavior of the mixed metal powders, as well as the phase composition, microstructure, mechanical properties, and oxidation behavior of the as-sintered MeB2-SiC ceramic were investigated. The XRD, SEM, and EPMA results indicated that the primary MeB2solid-solution and SiC phases could be successfully formed during reactive sintering at a relatively low temperature of 1650 °C. The as-sintered MeB2-SiC ceramics had a high relative density of 97.8% and high mechanical properties (hardness of 19.74 ± 0.8 GPa, flexure strength of 533 ± 38 MPa, and fracture toughness of 6.01 ± 0.77 MPa·m1/2). Combining the oxidation behavior and microstructure evolution of the oxidation layer, a continuous and relatively dense MeOx-SiO2oxidation layer was gradually formed and covered on the external surface, leading to decelerating oxidation behavior after an oxidation exposure time of 10 min.
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