Toughened (Ti 0.2Zr 0.2Hf 0.2Nb 0.2Ta 0.2)B 2–SiC composites fabricated by one-step reactive sintering with a unique SiB 6 additive

High-entropy diboride has been arousing considerable interest in recent years. However, the low toughness and damage tolerance limit its application 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 a balanced grain boundary strength collectively affected by improved densification, solid solution strengthening, and the incorporation of SiC secondary phase.