High-entropy diboride has been arousing considerable interest in recent years. However, the low toughness and damage tolerance limit its applications as ultra-high-temperature structural materials. Here we report that a unique SiB₆ additive has been first incorporated as boron and silicon sources to fabricate a high-entropy boride (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂–SiC composite though one-step boro/carbothermal reduction reactive sintering. A synergetic effect of high-entropy sluggish diffusion and SiC secondary phase retarded the grain growth of the (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂–51SiC composites. The small grain size was beneficial to shorten the diffusion path for mass transport, thereby enhancing the relative density to ~99.3%. These results in an increase of fracture toughness from ~5.2 in HEBS-1900 to ~7.7 MPa·m^1/2 in HEBS-2000, which corresponded to a large improvement of 48%. The improvement was attributed to a mixed mode of intergranular and transgranular cracking for offering effective pinning in crack propagation, resulting from balanced grain boundary strength collectively affected by improved densification, solid solution strengthening, and incorporation of SiC secondary phase.

MAB phases are layered ternary compounds with alternative stacking of transition metal boride layers and group A element layers. Until now, most of the investigated MAB phases are concentrated on compounds with Al as the A element layers. In this work, the family of M₅SiB₂ (M = IVB–VIB transition metals) compounds with silicon as interlayers were investigated by density functional theory (DFT) methods as potential MAB phases for high-temperature applications. Starting from the known Mo₅SiB₂, the electronic structure, bonding characteristics, and mechanical behaviors were systematically investigated and discussed. Although the composition of M₅SiB₂ does not follow the general formula of experimentally reported (MB)_2zA_x(MB₂)_y (z = 1, 2; x = 1, 2; y = 0, 1, 2), their layered structure and anisotropic bonding characteristics are similar to other known MAB phases, which justifies their classification as new members of this material class. As a result of the higher bulk modulus and lower shear modulus, Mo₅SiB₂ has a Pugh’s ratio of 0.53, which is much lower than the common MAB phases. It was found that the stability and mechanical properties of M₅SiB₂ compounds depend on their valence electron concentrations (VECs), and an optimum VEC exists as the criteria for stability. The hypothesized Zr and Hf containing compounds, i.e., Zr₅SiB₂ and Hf₅SiB_2, which are more interesting in terms of high-temperature oxidation/ablation resistance, were found to be unfortunately unstable. To cope with this problem, a new stable solid solution (Zr_0.6Mo_0.4)₅SiB₂ was designed based on VEC tuning to demonstrate a promising approach for developing new MAB phases with desirable compositions.