The formation mechanisms of amorphous bands of boron carbide nanopillars under uniaxial compressions and their effects on mechanical properties from molecular dynamics simulation

Boron carbide is considered as an ideal armor material, but its abnormal failure under high speed impact hinders its application. In order to explore the compression deformation behavior and mechanism of boron carbide, molecular dynamics simulation of reactive force-field (ReaxFF) interatomic potential is adopted to simulate the uniaxial compression deformation of boron carbide nanopillars in different directions at room temperature. The compression deformation behaviors of B11CpCBC nanopillars show significant anisotropy. No twin formation is observed during any compression. The B11CpCBC nanopillars have similar uniaxial compressive strength and corresponding strain along c-axis and b-axis, which are 59.21GPa and 0.185, 54.9GPa and 0.187, respectively, while along d-axis are higher, which are 82.06GPa and 0.234. During the compression along axes of b, c and d, the nanopillars undergo the same deformation processes in sequence as: symmetry reducing region is formed, extends aslant through the model and forms amorphous band. The formation mechanisms of amorphous bands of B11CpCBC nanopillars are as follows. During the compression along axis c, the three-atom chains constantly bend and the bonds among the three polar site B atoms of the icosahedron fracture at large strain leading to icosahedron collapse. During the compression along axes b and d, new bonds are formed between vertical adjacent icosahedrons at large strain leading to icosahedron collapse. In all three compression cases, the collapse extends aslant through the nanopillars under shear stress, during which polar site B atoms of icosahedrons bond with C and B atoms of triatomic chains, and finally forms amorphous bands. The formation of amorphous bands results in the decrease of compressive strength of B11CpCBC.