Carbon deficiency introduced plasticity of rock-salt-structured transition metal carbides

High-hardness rock-salt structured transitional metal carbides (TMC) are attracting substantial interest as potential next-generation thermal protection materials. However, the intrinsic brittleness of TMC ceramics impedes their performance in aerodynamically harsh environments. In this work, a promising strategy is proposed to introduce plasticity in TaC-HfC solid solutions by manipulating carbon deficiency. The approach combines density-functional theory (DFT) with experiments and takes Pugh's ratio ( k ) as the criteria. Depletion of carbon atoms in TaC-HfC solid solutions results in the de-localizing of valence electrons, deviation of spatial modulus along different crystal plane directions, and leading to significant elastic anisotropy. The carbon deficient Ta 0.8 Hf 0.2 C 0.8 is predicted to be a 'softer phase' with reduced modulus and Pugh's ratio ( k = 0.58). A series of Ta 1-x Hf x C y ( x = 0.2 and 0.8 , y = 0.8, 0.9, and 1.0) bulk ceramics are experimentally fabricated by an excessive metal alloying method. Trigonal and hexagonal close-packed structured carbides are derived when the carbon deficiency y decreased to 0.7. The indentation modulus drops from 641.8 & PLUSMN; 14.8 GPa for Ta 0.8 Hf 0.2 C 1.0 to 555.8 & PLUSMN; 9.9 GPa for Ta 0.8 Hf 0.2 C 0.8 . The specific stoichiometric composition of Ta 0.8 Hf 0.2 C 0.8 is experimentally verified to possess both plasticity ( k = 0.41) and ultra-high nanohardness (41.3 & PLUSMN; 1.3 GPa).& COPY; 2023 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.