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

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-(Zr0.2Hf0.2Nb0.2Ta0.2Ti0.2)B-2 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 (Zr0.2Hf0.2Nb0.2Ta0.2Ti0.2)B-2 fine grains are distributed randomly in the B4C matrix. Such special microstructure makes B4C-(Zr0.2Hf0.2Nb0.2Ta0.2Ti0.2)B-2 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 (Zr0.2Hf0.2Nb0.2Ta0.2Ti0.2)B-2 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.