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 (B11Cp)-Si-2, 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)-Si-2 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.