Local amorphization in boron carbide at finite temperature: Strategies toward improved ductility

Boron carbide (${\mathrm{B}}_{4}\mathrm{C}$) is superhard, but its engineering applications are limited by the abnormal brittle failure arising from amorphous shear band formation. Mitigating the local amorphization is essential to improve the ductility of ${\mathrm{B}}_{4}\mathrm{C}$. Here, we carried out ab initio molecular dynamics (AIMD) simulations to examine the response of ${\mathrm{B}}_{4}\mathrm{C}$ to shear along two plausible slip systems $(001)/[100]$ and $(111)/[11\overline{2}]$ at finite temperature. We found that the icosahedra of ${\mathrm{B}}_{4}\mathrm{C}$ gradually deconstruct at finite temperature, resulting in local amorphization, thereby giving rise to a lower stress barrier than that of density functional theory simulations at zero temperature. The deconstruction of the icosahedral clusters arises from the interaction with the neighboring chains. The failure mechanism at the finite temperature suggested that the local amorphization can be suppressed by altering the structure of the 12-atom icosahedron and the 3-atom chain of ${\mathrm{B}}_{4}\mathrm{C}$. To demonstrate this, we replaced the ${\mathrm{B}}_{11}{\mathrm{C}}^{\mathrm{P}}$ icosahedron in ${\mathrm{B}}_{4}\mathrm{C}$ with ${\mathrm{B}}_{12}$ icosahedron to form boron-rich boron carbide (${\mathrm{B}}_{13}{\mathrm{C}}_{2}$) and then performed the same shear deformations. We found that local amorphization significantly decreases, which results from the modified icosahedral interaction. We also altered the 3-atom C-B-C chain to the 2-atom P-P chain and found that the accumulated shear stress can be released through icosahedral slipping, which is achieved by breaking the chains. The icosahedral slipping then prevents the destruction of icosahedra under shear deformations, expected to improve the ductility. Our results demonstrate two design strategies toward improved ductility of ${\mathrm{B}}_{4}\mathrm{C}$: (1) boron enrichment can alleviate amorphous shear band formation in boron carbide, and (2) altering the 3-atom chain to a 2-atom chain and meanwhile decreasing the strength of the chain to make it less stable than the icosahedron cage may lead to an icosahedral-slipping-dominated mechanism, thereby avoiding icosahedral fracture.