Radial X‐Ray Diffraction Study of Superhard Early Transition Metal Dodecaborides under High Pressure

The deformation behavior of the three metal dodecaborides (YB₁₂, ZrB₁₂, and Zr_0.5Y_0.5B₁₂) is investigated using radial X‐ray diffraction under nonhydrostatic compression up to ≈60 GPa with a goal of understanding how bonding and metal composition control hardness. Zr_0.5Y_0.5B₁₂, which has the highest Vickers hardness (Hv = 45.8 ± 1.3 GPa at 0.49 N load), also shows the highest bulk modulus (K₀ = 320 ± 5 GPa). The 0.49 N hardness for ZrB₁₂ and YB₁₂ are both lower and very similar, and both show lower bulk moduli (K₀ = 276 ± 7 GPa, and K₀ = 238 ± 6 GPa, respectively). Differential stress is then measured to study the strength and strength anisotropy. Zr_0.5Y_0.5B₁₂ supports the highest differential stress, in agreement with its high hardness, a fact that likely arises from atomic size mismatch between Zr and Y combined with the rigid network of boron cages. The (200) plane for all samples supports the largest differential strain, while the (111) plane supports the smallest, consistent with the theoretically predicted slip system of {111} $\overline{1}\overline{1}2$ ]. Strain softening is also observed for ZrB₁₂. Finally, the full elastic stiffness tensors for ZrB₁₂ and YB₁₂ are solved. ZrB₁₂ is the most isotropic, but the extent of elastic anisotropy for all dodecaborides studied is relatively low due to the highly symmetric boron cage network.