Energetics and nucleation of point defects in aluminum under extreme tensile hydrostatic stresses

Density functional theory calculations are employed to investigate the energetics of point defects-monovacancy, self-interstitials (tetrahedral, octahedral, and dumbbell), and Frenkel pairs-in aluminum under tensile hydrostatic stresses. Our study suggests that the defect core energy of a vacancy, which is governed solely by the electronic structure at the core, significantly depends on the macroscopic hydrostatic stress, and that this constitutes an important contribution to the formation enthalpy, especially in the regime of extreme tensile hydrostatic stresses. This finding is in contrast to widely used elastic formulations of point defects based on formation volume that ignore the defect core-energy contribution. The formation enthalpies of all point defects considered in the present study monotonically decrease with increasing tensile hydrostatic stress. Furthermore, we find that the formation enthalpies of vacancies and Frenkel pairs are negative beyond critical tensile hydrostatic stresses (9 GPa for vacancies and 12 GPa for Frenkel pairs), which suggests a spontaneous nucleation of these point defects and this has important implications to nucleation mechanisms leading to spall failure. In particular, the present findings suggest two possible defect nucleation mechanisms leading to spall failure: (i) a heterogeneous nucleation of vacancies from defect sources and (ii) a homogeneous nucleation of Frenkel pairs at higher hydrostatic stresses.