Elastic properties of boron carbides

Boron carbides made by hot pressing exist as a single phase with compositions between 10 and 20 atomic percent carbon. The longitudinal and shear ultrasonic velocities were measured on fully dense boron carbides with compositions throughout the single-phase region. Both longitudinal and shear velocities decreased rapidly when the carbon content was below 13 atomic percent. Young's modulus, shear modulus, bulk modulus, compressibility, and Poisson's ratio, were calculated from the ultrasonic velocities. The results show that boron carbides become substantially more compressible for carbon concentrations below 13%. This softening is consistent with B12 icosahedra replacing B11C icosahedra in this composition range