Entropy enhancing toughness of diborides: First-principles insights and experimental validation

The intrinsically low toughness and high brittleness of diboride ceramics are major limitations restricting their widespread application. Toughness can be enhanced through increased configurational entropy; however, the physical mechanisms underpinning this entropy-induced toughening remain poorly understood. In this study, a comprehensive approach from element screening to mechanistic elucidation and experimental validation is undertaken to address these gaps. First, a theory-guided element screening strategy is employed. Starting from dilute solid-solution models and integrating calculation of phase diagrams (CALPHAD) composition–property fitting, Ta, Nb, Mo, Hf, and V are identified for their synergistic optimization of hardness and toughness. Subsequently, the evolution of toughness with increasing configurational entropy is assessed using bulk modulus/shear modulus (B/G), fracture toughness (K_IC), and related metrics. The calculations are validated against available experimental data, revealing an almost monotonic trend, with the six-component system exhibiting a K_IC exceeding 5.8 MPa·m^1/2—approximately double that of the single-component counterpart. A systematic analysis of the lattice distortion and crystal orbital Hamilton population is performed for diborides containing two to six alloying elements. From a bond-strength perspective, the toughening mechanism originates from increased thermodynamic disorder, which broadens and flattens the bond-strength distribution, giving rise to a “bond-strength trap”. Experimental validation is conducted on the (TiTa)B₂ and (TiNb)B₂ systems with pronounced bond-strength contrast, as well as the ternary (TiTaNb)B₂ system. The results corroborate the predicted electronic bonding evolution, while further analysis of phonon force constants and stacking fault energies indicates that the synergy between strengthened M–B bonds and reduced dislocation slip barriers underpins the enhanced toughness.