The elastic, mechanical, and thermodynamic properties of polycrystalline MB ₂ (M = Hf and Ta) under high pressure

Abstract Ultra‐high‐temperature ceramics (UHTCs), especially hafnium diboride (HfB 2 ) and tantalum diboride (TaB 2 ), have become indispensable for extreme‐environment applications due to their unparalleled combination of mechanical strength, exceptional thermal stability, and ultra‐high melting temperatures exceeding 3200°C. While these refractory ceramics hold great promise for advanced technologies, their fundamental behavior under high‐pressure conditions remains insufficiently understood, posing a critical barrier to their full potential utilization. We successfully fabricated fully dense, phase‐pure polycrystalline HfB 2 and TaB 2 ceramics via precisely controlled high‐pressure, high‐temperature processing (5 GPa, 1300°C). Comprehensive microstructural characterization verifies the deliberate incorporation of advantageous lattice defects and tailored crystallographic distortions, which are engineered to optimize material performance. The mechanical, elastic, and thermodynamic properties of these materials were systematically investigated under varying pressures using in situ high‐pressure ultrasonic interferometry. Third‐order finite strain equation analysis of the experimental data at ambient temperature reveals distinct elastic properties for these UHTCs: HfB 2 exhibits superior stiffness with a bulk modulus B 0 = 313 ± 2 GPa (pressure derivative ∂ B /∂ P = 3.8 ± 0.1) and shear modulus G 0 = 247 ± 3 GPa (∂ G /∂ P = 2.7 ± 0.1), while TaB 2 shows relatively compliant behavior with B 0 = 232 ± 3 GPa (∂ B /∂ P = 4.3 ± 0.2) and G 0 = 151 ± 2 GPa (∂ G /∂ P = 3.6 ± 0.1). Thermodynamic analysis at high‐pressure conditions yields the following thermal transport properties: For HfB 2 , we obtain a Debye temperature Θ = 740.6 K, Grüneisen parameter γ = 1.242, and acoustic thermal conductivity λ acoustic = 391.2 W/mK. The corresponding values for TaB 2 are Θ = 584.9 K, γ = 1.418, and λ acoustic = 153.5 W/mK. Our results reveal pressure‐dependent behaviors in elastic moduli, hardness, fracture toughness, and thermal transport, emphasizing the role of microstructure and lattice dynamics. Both HfB 2 and TaB 2 exhibit nearly linear relationships between wave velocity, bulk modulus, shear modulus, and Debye temperature with pressure. Notably, hardness decreases with increasing pressure, while fracture toughness increases, showing an inverse relationship. The Grüneisen parameter rises with pressure, while acoustic thermal conductivity decreases, reflecting distinct pressure‐dependent thermal and mechanical behaviors. Our findings reveal previously unrecognized microstructure‐dependent mechanisms in UHTCs under extreme conditions, enabling transformative design strategies for future thermal protection systems and radiation‐tolerant structural components.