Transition metal borides are commonly
hard and incompressible,
offering great opportunities for advanced applications under extreme
conditions. Recent studies show that the hardness of high-entropy
borides may exceed that of their constituent simple borides due to
the “cocktail effect”. However, how high-entropy borides
deform elastically remains largely unknown. Here, we show that two
newly synthesized high-entropy diborides are ultra-incompressible,
attaining ∼90% of the incompressibility of single-crystalline
diamond and exhibiting a 50–60% enhancement over the density
functional theory predictions. This unusual behavior is attributed
to a Hall–Petch-like effect resulting from nanosizing under
high pressure, which increases the bulk moduli through dynamic dislocation
interactions and creation of stacking faults. The exceptionally low
compressibility, together with their high phase stabilities, high
hardness, and high electric conductance, renders them promising candidates
for electromechanics and microelectronic devices that demand strong
resistance to environmental impacts, in addition to traditional grinding
and abrading.