Atomistic origin of shear induced quasi-plastic deformation in boron carbide

Superhard materials typically exhibit limited plastic deformation due to their strong covalent and ionic bonding. Identifying the deformation mechanisms is crucial to enhance their mechanical properties. By combining molecular dynamics simulations with transmission electron microscopy experiments, we determine the shear-induced crystal defects and their related deformation mechanisms, referred to quasi-plasticity, in superhard boron carbide (B ₄C). Surprisingly, we identify an anomalous quasi-plastic deformation in B ₄ C with a larger failure strain under ideal shear deformation along the (111)[ 1 ¯ 1 ¯ 2] slip system. This behavior arises from slip band formation and propagation, which is stabilized by the newly formed bonding between icosahedra clusters and atomic chains during deformation. The slip band and crystal matrix share a dislocation-like interface, which serves as the nucleation site for the amorphous band formation at a large shear strain. In contrast, the dislocation-like mechanism mediated amorphization is observed in B ₄C as it shears along (021)[ 5 ¯ 1 2 ¯ ], ( 1 ¯ 1 1 ¯ )[21 1 ¯ ] and (5 1 ¯ 2)[ 1 ¯ 5 ¯ 3 ¯ ] slip systems. We derive the activation volume and energy for the quasi-plastic deformation, which are much lower than those for other deformation mechanisms (e.g. shear banding in amorphous alloys). Therefore, the quasi-plastic deformation may play an essential role in the brittle failure of single crystal B ₄C.