Microalloying Boron Carbide with Silicon to Achieve Dramatically Improved Ductility

Boron carbide (B_4C) 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 B_4C ductility through microalloying. We found that replacing the CBC chain in B_4C with Si–Si, denoted as (B_(11)C_p)–S_i_2, dramatically improves the ductility, allowing a continuous shear to a large strain of 0.802 (about twice of B_4C failure strain) without brittle failure. Moreover, (B_(11)C)–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.