Atomistic explanation of compression-induced deformation mechanisms in boron carbide

The compression-induced deformation mechanisms of boron carbide (B 4 C) have not been well understood due to its complex crystal structure. Here, we investigate the mechanical behaviors and deformation mechanisms of B 4 C under various compression conditions, using molecular dynamics simulations with a machine-learning force field. Two structural transitions in B 4 C under bulk compression are identified: the formation of new chains–icosahedra bonds, and icosahedral deconstruction. The former triggers the “pop-in” event observed in the nanoindentation experiments due to chain bending, while the latter facilitates icosahedral sliding and amorphization. Furthermore, the results reveal a mechanism involving an intermediate structure characterized by the development of new chains–icosahedra bonds across the entire model, followed by the amorphization process. This mechanism induced by increased stress in icosahedra due to the new chains–icosahedra bond formation plays an important role in significantly improving the strength and ductility of B 4 C. In contrast, B 4 C with free surfaces or nanopores exhibits a direct transformation from chain bending to icosahedral deconstruction without the intermediate structure formation, consequently reducing strength and ductility. However, this intermediate configuration is not stable and maintained only under stress, because structural recovery within non-amorphized regions occurs after amorphization under quasi-static compression with a relaxed stress state. This study provides a thorough understanding of the deformation behaviors and mechanisms of B 4 C.