Nanotwinning and Directed Alloying to Enhance the Strength and Ductility of Superhard Materials

Strength refers to a material's ability to withstand failure or yield, while ductility is its ability to permanently deform without fracture. Many important engineering applications require materials that have high strength yet ductile, such as cutting tools, body armor for soldiers, and manufacturing processes. One promising candidate is boron carbide (B4C), a superhard ceramic because of its high strength. However, B4C has a low ductility preventing its extended engineering applications. In this charter, we will summarize the recent progress of enhancing the strength and ductility of B4C-based superhard materials by imposing nanotwinning and directed alloying approaches. This chapter includes four sections: Introduction; Enhancing the strength of superhard materials through nanotwinning; Enhancing the ductility of superhard materials through directed alloying; Summary. In section (2), we first focus on identifying the atomic structures of nanoscale twins in B4C, boron-rich boron carbide (B13C2), boron suboxide (B6O), boron subphosphorus (B12P2), and β-B by combining quantum mechanics (QM) simulations and high-resolution transmission electron microscopy (HRTEM) experiments. Then we will discuss how these nanotwins affect the mechanical response of these materials under shear deformation. We found that for the B4C, the theoretical shear strength can be exceeded by 11% by imposing nanoscale twins. The origin of this strengthening mechanism is suppression of twin boundary (TB) slip within the nanotwins due to the directional nature of covalent bonds at the TBs. In contrast, for other ceramics such as B13C2, B6O, B12P2, and β-B, the ideal shear strength in the twinned structure is lower than crystalline structure, suggesting that the brittle failure initiates at the TBs for these materials. In section 3 we discuss how directed alloying in B4C affects the mechanical properties. We examined various alloying elements in B4C, e.g. Si and P, to establish the design principles for improving the ductility while keeping high hardness. We found two design principles to improve the ductility: replacing the three-atom C−B−C chains with two-atom chains to eliminate the highly reactive central atom; making sure that the strength of the two-atom chain is less than that of the icosahedron. We will also discuss the cocrystal of B6O and B4C approach to improve the ductility of pure B4C. These findings are essential for comprehensively understanding the nanotwins, directed alloying, and mechanical properties of B4C-based superhard ceramics at the atomic scale, and lays the foundation for developing high-performance superhard ceramics with excellent mechanical properties.