Ultra-incompressible High-Entropy Diborides

Abstract

Transition metal borides are commonly\nhard and incompressible,\noffering great opportunities for advanced applications under extreme\nconditions. Recent studies show that the hardness of high-entropy\nborides may exceed that of their constituent simple borides due to\nthe “cocktail effect”. However, how high-entropy borides\ndeform elastically remains largely unknown. Here, we show that two\nnewly synthesized high-entropy diborides are ultra-incompressible,\nattaining ∼90% of the incompressibility of single-crystalline\ndiamond and exhibiting a 50–60% enhancement over the density\nfunctional theory predictions. This unusual behavior is attributed\nto a Hall–Petch-like effect resulting from nanosizing under\nhigh pressure, which increases the bulk moduli through dynamic dislocation\ninteractions and creation of stacking faults. The exceptionally low\ncompressibility, together with their high phase stabilities, high\nhardness, and high electric conductance, renders them promising candidates\nfor electromechanics and microelectronic devices that demand strong\nresistance to environmental impacts, in addition to traditional grinding\nand abrading.