Behavior of disordered boron carbide under stress

Gibbs free-energy calculations based on density functional theory have been used to determine the possible source of failure of boron carbide just above the Hugoniot elastic limit (HEL). A range of B4C polytypes is found to be stable at room pressure. The energetic barrier for shock amorphization of boron carbide is by far the lowest for the B-12(CCC) polytype, requiring only 6 GPa approximate to P(HEL) for collapse under hydrostatic conditions. The results clearly demonstrate that the collapse of the B-12(CCC) phase leads to segregation of B-12 and amorphous carbon in the form of 2-3 nm bands along the (113) lattice direction, in excellent agreement with recent transmission electron microscopy results.