  Bottom-up design of high-entropy ceramics is a promising approach for
realizing materials with unique combination of high hardness and
fracture-resistance at elevated temperature. This work offers a simple yet
fundamental design criterion - valence electron concentration (VEC) > ~9.5
e-/f.u. to populate bonding metallic states at the Fermi level - for selecting
elemental compositions that may form rocksalt-structure (B1) high-entropy
ceramics with enhanced plasticity (reduced brittleness). Single-phase B1
(HfTaTiWZr)C and (MoNbTaVW)C, chosen as representative systems due to their
specific VEC values, are here synthesized and tested. Nanoindentation arrays at
various loads and depths statistically show that (HfTaTiWZr)C (VEC=8.6 e-/f.u.)
is hard but brittle, whilst (MoNbTaVW)C (VEC=9.4 e-/f.u.) is hard and
considerably more resistant to fracture than (HfTaTiWZr)C. Ab initio molecular
dynamics simulations and electronic-structure analysis reveal that the improved
fracture-resistance of (MoNbTaVW)C subject to tensile and shear deformation may
originate from the intrinsic material's ability to undergo local lattice
transformations beyond tensile yield points, as well as from relatively facile
activation of lattice slip. Additional simulations, carried out to follow the
evolution in mechanical properties as a function of temperature, suggest that
(MoNbTaVW)C may retain good resistance to fracture up to ~900-1200K, whereas
(HfTaTiWZr)C is predicted to remain brittle at all investigated temperatures.
