Ductility in crystalline boron subphosphide (B12P2) for large strain indentation

Abstract

Our studies of brittle fracture in B4C showed that shear-induced cracking of the (B11C) icosahedra leading to amorphous B4C regions induced cavitation and failure. This suggested that to obtain hard boron-rich phases that are ductile, we need to replace the CBC chains of B4C with two-atom chains that can migrate between icosahedra during shear without cracking the icosahedra. We report here a quantum mechanism (QM) simulation showing that under indentation stress conditions, superhard boron subphosphide (B12P2) displays such a unique deformation mechanism. Thus, stress accumulated as shear increases is released by slip of the icosahedra planes through breaking and then reforming the P–P chain bonds without fracturing the (B12) icosahedra. This icosahedral slip may facilitate formation of mobile dislocation and deformation twinning in B12P2 under high-stress conditions, leading to high ductility. However, the presence of twin boundaries (TBs) in B12P2 will weaken the icosahedra along TBs, leading to the fracture of (B12) icosahedra under indentation stress conditions. These results suggest that crystalline B12P2 is an ideal superhard material to achieve high ductility.