Microalloying boron carbide with silicon to achieve dramatically improved ductility

Boron carbide (B₄C) is a hard material whose value for extended engineering applications such as body armor; is limited by its brittleness under impact. To improve the ductility while retaining hardness, we used density functional theory to examine modifying B₄C ductility through microalloying. We found that replacing the CBC chain in B₄C with Si-Si, denoted as (B₁₁C_p)-Si₂, dramatically improves the ductility, allowing a continuous shear to a large strain of 0.802 (about twice of B₄C failure strain) without brittle failure. Moreover, (B₁₁C)-Si₂ retains low density and high hardness. This ductility improvement arises because the Si-Si linkages enable the icosahedra accommodate additional shear by rotating instead of breaking bonds.