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The authors recently reported that undercooled liquid Ag and Ag–Cu alloys both exhibit a first order phase transition from the homogeneous liquid (L-phase) to a heterogeneous solid-like G-phase under isothermal evolution. Here, we report a similar L–G transition and heterogenous G-phase in simulations of liquid Cu–Zr bulk glass. The thermodynamic description and kinetic features (viscosity) of the L-G-phase transition in Cu–Zr simulations suggest it corresponds to experimentally reported liquid–liquid phase transitions in Vitreloy 1 (Vit1) and other Cu–Zr-bearing bulk glass forming alloys. The Cu–Zr G-phase has icosahedrally ordered cores versus fcc/hcp core structures in Ag and Ag–Cu with a notably smaller heterogeneity length scale Λ . We propose the L–G transition is a phenomenon in metallic liquids associated with the emergence of elastic rigidity. The heterogeneous core–shell nano-composite structure likely results from accommodating strain mismatch of stiff core regions by more compliant intervening liquid-like medium. 

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.