Influence of high pressure on the structure,hardness and brittle-to-ductile transition of NbSi2 ceramics

We apply first-principles calculations to study the influence of pressure on the structure, elastic modulus, hardness, brittle-or-ductile behavior and melting point of NbSi₂. Four NbSi₂ phases: C40, C11_b, C54 and C49 are considered based on the structural feature. The results show that the calculated formation enthalpy of the four NbSi₂ phases is negative within the pressure range of 0–60 GPa, indicating that they are thermodynamically stable in whole pressure. In particular, the calculated formation enthalpy of the C54 NbSi₂ is smaller than the other NbSi₂ phases, indicating that the C54 NbSi₂ is more thermodynamically stable than the other NbSi₂ phases. The calculated elastic modulus and Vickers hardness of NbSi₂ increase with increasing pressure. Note that the pressure results in brittle-to-ductile transition of the C40 NbSi₂, C11_b NbSi₂ and C54 NbSi₂ between 30 GPa and 60 GPa. Naturally, the increasing of mechanical properties is that the pressure enhances the electronic hybridization between Nb and Si, which is demonstrated by the chemical bonding such as Nb–Si bond and Si–Si bond.