Synthesis, densification behaviour, and mechanical properties of high-entropy borides: Role of composition and sintering route

Abstract

Equiatomic (Ti0.2Nb0.2Ta0.2Hf0.2Zr0.2)B2 and non-equiatomic (Ti0.31Nb0.19Ta0.13Hf0.15Zr0.21)B2 high entropy borides were synthesized via boro/carbothermal reduction and consolidated by hot pressing and spark plasma sintering at 1900°C. Both compositions formed single phase hexagonal AB2 solid solutions, as confirmed by diffraction and microscopy. Densification followed Coble creep behaviour, with higher activation energies observed for hot pressing than spark plasma sintering. Microstructural analysis revealed a largely homogeneous matrix with localized Nb-rich and minor Ta-rich segregations, contributing to limited grain coarsening. The non-equiatomic composition showed slightly reduced hardness but improved fracture toughness (∼4.1 MPa m1ᐟ2). Hardness ranged from 16 to 23 GPa (Vickers) and up to ∼28 GPa by nanoindentation, with elastic modulus reaching ∼455 GPa. Flexural strengths of 500–525 MPa were obtained, with fracture dominated by transgranular brittle mechanisms. © 2026 Elsevier Ltd and Techna Group S.r.l. All rights are reserved, including those for text and data mining, AI training, and similar technologies.