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The shear‐induced amorphization has been observed in many strong ceramics and is responsible for their cracking and fragmentation. But its underlying mechanism remains elusive due to the complex structure and bonding environment in strong ceramics. To illustrate the deformation mechanism of local amorphization in strong ceramics, we employed molecular dynamics simulations with a deep‐learning force field to examine the shear‐induced amorphization in B₁₂P₂. Surprisingly, we identified a stacking‐fault‐mediated amorphization mechanism along the most plausible slip system (1 1 1)/[1 1 ]. This mechanism is even more favorable at a higher temperature than room temperature. In contrast, the direct crystal to amorphization transition, due to the icosahedral slip, is observed for the other most plausible slip system (0 1 1)/[2 ]. We report the activation volume and the activation free energy for the amorphization along the (1 1 1)/[1 1 ] slip system. The derived activation volume is only 41.47 A³, which is roughly 2–3 icosahedra, suggesting that the localized amorphization in B₁₂P₂is mediated by stacking fault formation. The previous results suggest complex amorphization mechanisms in strong ceramics.

 

Grain boundaries, ubiquitous in real materials, play an important role in the mechanical properties of ceramics. Using boron carbide as a typical superhard but brittle material under hypervelocity impact, we report atomistic reactive molecular dynamics simulations using the ReaxFF reactive force field fitted to quantum mechanics to examine grain‐boundary engineering strategies aimed at improving the mechanical properties. In particular, we examine the dynamical mechanical response of two grain‐boundary models with or without doped Si as a function of finite shear deformation. Our simulations show that doping Si into the grain boundary significantly increases the shear strength and stress threshold for amorphization and failure for both grain‐boundary structures. These results provide validation of our suggestions that Si doping provides a promising approach to mitigate amorphous band formation and failure in superhard boron carbide.

 

The high strength of boron carbide (B₄C) is essential in its engineering applications such as wear‐resistance and body armors. Here, by employing density functional theory simulations, we demonstrated that the strength of B₄C can be enhanced by doping lithium to boron‐rich boron carbide (B₁₃C₂) to form r‐LiB₁₃C₂. The bonding analysis on r‐LiB₁₃C₂indicates that the electron counting rule (or Wade's rule) is satisfied in r‐LiB₁₃C₂whose formula can be written as r‐Li⁺(B₁₂)^2‐(CB⁺C). The shear deformation on r‐LiB₁₃C₂indicates that its ideal shear strength is larger than that of B₄C because of the existing of Li dopant. The failure process of r‐LiB₁₃C₂under ideal shear deformation initiates from breaking the icosahedral‐icosahedral B‐B bonds. Then these B atoms react with the middle B in the C‐B‐C chain, resulting in the disintegration of icosahedral clusters and brittle failure. More interesting, the nanotwinned r‐LiB₁₃C₂is even stronger than r‐LiB₁₃C₂because of the directional nature of covalent bonding at the twin boundaries. This suggests that the nanotwinned r‐LiB₁₃C₂has a significant enhanced strength compared to B₄C. Our simulation results illustrate the deformation mechanism of Li‐doped boron carbide and its nanotwinned microstructure. We proposed to improve the strength of boron carbide by doping Li into B₁₃C₂and increasing its twin densities.

 

The low fracture toughness of strong covalent solids prevents them from wide engineering applications. Microalloying metal elements into covalent solids may lead to a significant improvement on mechanical properties and drastical changes on the chemical bonding. To illustrate these effects we employed density functional theory (DFT) to examine the bonding characteristic and mechanical failure of recently synthesized magnesium boride carbide (Mg₃B₅₀C₈) that is formed by adding Mg into boron carbide (B₄C). We found that Mg₃B₅₀C₈has more metallic bonding charterer than B₄C, but the atomic structure still satisfies Wade's rules. The metallic bonding significantly affects the failure mechanisms of Mg₃B₅₀C₈compared with B₄C. In Mg₃B₅₀C₈, the B₁₂icosahedral clusters are rotated in order to accommodate to the extensive shear strain without deconstruction. In addition, the critical failure strength of Mg₃B₅₀C₈is slightly higher than that of B₄C under indentation stress conditions. Our results suggested that the ductility of Mg₃B₅₀C₈is drastically enhanced compared with B₄C while the hardness is slightly higher than B₄C.

 

Boron carbide is super-strong and has many important engineering applications such as body armor and cutting tools. However, the extended applications of boron carbide have been limited by its low fracture toughness arising from anomalous brittle failure when subjected to hypervelocity impact or under high pressure. This abnormal brittle failure is directly related to the formation of a tiny amorphous shear band of 2–3 nm in width and several hundred nm in length. In this Perspective, we discuss mitigating the amorphous shear bands in boron carbide from various strategies including microalloying, grain boundary engineering, stoichiometry control, and the addition of a second phase. Combined with recent theoretical and experimental studies, we discuss strategies that can be applied in synthesizing and producing boron carbide-based materials with improved ductility by suppressing the formation of the amorphous shear band.