First principles predicting enhanced ductility of boride carbide through magnesium microalloying

Abstract

The low fracture toughness of strong covalent solids prevents them from wide engineering applications. Microalloying metal elements into covalent solids may lead to a significant improvement on mechanical properties and drastical changes on the chemical bonding. To illustrate these effects we employed density functional theory (DFT) to examine the bonding characteristic and mechanical failure of recently synthesized magnesium boride carbide (Mg_3B_(50)C_8) that is formed by adding Mg into boron carbide (B_4C). We found that Mg_3B_(50)C_8 has more metallic bonding charterer than B_4C, but the atomic structure still satisfies Wade's rules. The metallic bonding significantly affects the failure mechanisms of Mg_3B_(50)C_8 compared with B_4C. In Mg_3B_(50)C_8, the B_(12) icosahedral clusters are rotated in order to accommodate to the extensive shear strain without deconstruction. In addition, the critical failure strength of Mg_3B_(50)C_8 is slightly higher than that of B_4C under indentation stress conditions. Our results suggested that the ductility of Mg_3B_(50)C_8 is drastically enhanced compared with B_4C while the hardness is slightly higher than B_4C.