Achieving Superhardness and Enhanced Toughness in High-Entropy Boride-Based Composites by Tailoring Their Multi-Scale...

Unlike strong yet tough high entropy alloys, high entropy ceramics normally exhibit good hardness but poor strength and fracture toughness. To overcome this obstacle, B₄C-(Zr_0.2Hf_0.2Nb_0.2Ta_0.2Ti_0.2)B₂ composites with a unique hierarchical microstructure are designed and prepared by boronizing reaction sintering of dual-phase multicomponent carbides. In the as-obtained composites, massive platelet-like aggregations assembled by core-rim structured (Zr_0.2Hf_0.2Nb_0.2Ta_0.2Ti_0.2)B₂ fine grains are distributed randomly in the B₄C matrix. Such special microstructure makes B₄C-(Zr_0.2Hf_0.2Nb_0.2Ta_0.2Ti_0.2)B₂ composites exhibit excellent mechanical properties. An extra toughening mechanism of crack bridging is provided in as-obtained composites (fracture toughness of 4.70 ± 0.08 MPa m^1/2) by the interaction between cracks and platelet-like diboride aggregations whilst fine-grained microstructures guarantee high flexural strength (633 ± 25 MPa). More importantly, during producing indents, homogenization of core-rim structured (Zr_0.2Hf_0.2Nb_0.2Ta_0.2Ti_0.2)B₂ alongside more difficult lattice glides caused by short-range ordering and rough glide planes containing different-dimension transition metal atoms cooperatively induce increased indentation volume work and consequently unparalleled Vickers hardness (>54 GPa at 1.96 N), which is confirmed by in-depth transmission electron microscopy characterizations. This work gives a new inspiration to design high-performance high-entropy ceramics via multi-scale microstructure tailoring and composition tuning.