Ductility in Crystalline Boron Subphosphide (B_(12)P_2) for Large Strain Indentation

Abstract

Our studies of brittle fracture in B_4C showed that shear-induced cracking of the (B_(11)C) icosahedra leading to amorphous B_4C 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 B_4C 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 (B_(12)P_2) 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 (B_(12)) icosahedra. This icosahedral slip may facilitate formation of mobile dislocation and deformation twinning in B_(12)P_2 under high-stress conditions, leading to high ductility. However, the presence of twin boundaries (TBs) in B_(12)P_2 will weaken the icosahedra along TBs, leading to the fracture of (B_(12)) icosahedra under indentation stress conditions. These results suggest that crystalline B_(12)P_2 is an ideal superhard material to achieve high ductility.

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