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Revealing the hardening and strengthening mechanisms is crucial forfacilitating the design of superhard and high-strength high-entropy ceramics(HECs). Here, we take high-entropy diborides (HEB$_2$) as the prototype tothoroughly investigate the hardening and strengthening mechanisms of HECs.Specifically, the equiatomic 4- to 9-cation single-phase HEB$_2$ ceramics(4-9HEB$_2$) are fabricated by an ultra-fast high-temperature sintering method.The as-fabricated 4-9HEB$_2$ samples possess similar grain sizes, comparablerelative densities (up to ~98%), uniform compositions, and clean grainboundaries without any impurities. The experimental results show that thehardness and flexural strength of the as-fabricated 4-9HEB$_2$ samples have anincreasing tendency with the increase of metal components. The first-principlescalculations find that lattice distortion is essential to the hardness andstrength of HEB$_2$. With the increase of metal components, an aggravation oflattice distortion accompanied by B-B bond strengthening is determined,resulting in the enhancement of the hardness and flexural strength. Moreover,the correlation between other potential indicators and the hardness/flexuralstrength of HEB$_2$ has been disproved, including valence electronconcentration, electronegativity mismatch, and metallic states. Our resultsunravel the hardening and strengthening mechanisms of HECs by intensifyinglattice distortion, which may provide guidance for developing superhard andhigh-strength HECs.

Unrevealing hardening and strengthening mechanisms in high-entropy ceramics from lattice distortion