  We examine the effects produced by replacing B-B interlayer bonds by C-C
bonds in AlMgB14-related boron network on its mechanical properties. The
elastic constants, Vickers hardness and shear strength are evaluated by means
of first principle computer simulations on the basis of density functional
theory. The results of simulations suggest a possibility of existence of
several orthorhombic boron carbide phases with strongly enhanced mechanical
properties with the Young modulus and Vickers hardness being within the range
of 550-600 GPa and 43-50 GPa, respectively
