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

Abstract Unlike strong yet tough high entropy alloys, high entropy ceramics normally exhibit good hardness but poor strength and fracture toughness. To overcome this obstacle, B ₄ C‐(Zr _0.2 Hf _0.2 Nb _0.2 Ta _0.2 Ti _0.2 )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 (Zr _0.2 Hf _0.2 Nb _0.2 Ta _0.2 Ti _0.2 )B ₂ fine grains are distributed randomly in the B ₄ C matrix. Such special microstructure makes B ₄ C‐(Zr _0.2 Hf _0.2 Nb _0.2 Ta _0.2 Ti _0.2 )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 (Zr _0.2 Hf _0.2 Nb _0.2 Ta _0.2 Ti _0.2 )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.