Shear-induced structural transformation and plasticity in ultraincompressible ReB₂ limit its hardness

The recent search for superhard materials concentrated on diborides of the 5d transition metals because of their high elastic moduli. Using density-functional theory we report a systematic study of the shear-induced irreversible structural transformations and plasticity in ReB₂ which limits its strength and hardness. The plasticity is due to breaking and rearrangement of bonds accompanied by crystal-field splitting of 5d orbitals from the stabilized to destabilized states during the finite shear that takes place at the atomic level in plastic deformation. Such effects are expected to occur also in diborides of other 5d transition metals.