DFT-guided alloying and enhanced toughness in Ti-Nb-B2 ceramic composites

• Reactive liquid-phase sintering yields dense Ti-Nb-B 2 for extreme environments. • Nb boosts toughness via crack deflection and refined multi-phase microstructures. • DFT design identifies Ti 0.5 Nb 0.5 B 2 as the optimal balance of toughness & strength. • Combined DFT and experiments workflow advances design of damage-tolerant UHTCs. In this study, the effect of niobium substitution on the microstructure and mechanical properties of TiB 2 -based ceramics was systematically investigated through combined experimental and first-principles density functional theory (DFT) calculations. TiB 2 -NbB 2 /NbB composites were fabricated via a two-stage reactive sintering process with liquid-phase-forming additives, achieving relative densities up to 98 %. DFT results confirmed the formation of continuous Ti 1-x Nb x B 2 solid solutions. Among these, the equiatomic composition was predicted as the most thermodynamically stable, exhibiting the minimum mixing energy across the series. Theoretical elastic constants revealed a progressive decrease in stiffness and hardness and a rise in ductility indicators with increasing Nb content. Experimentally, hardness decreased as NbB 2 content increased, while fracture toughness and flexural strength exhibited a non-monotonic trend. Notably, Ti 0.5 Nb 0.5 B 2 achieved the optimum mechanical performance, with a fracture toughness of 9.16 MPa.m 0.5 , hardness of 22.6 GPa, and flexural strength of 522.7 MPa, attributed to optimized crack deflection, multi-phase toughening, and uniform, dense microstructure. The strong qualitative agreement between theory and experiment highlights the importance of integrating computational predictions with microstructure engineering to design damage-tolerant, ultra-high-temperature boride ceramics for demanding structural applications.