Ab initio pseudopotential study of vacancies and self-interstitials in hcp titanium

Abstract

By means of an ab initio plane-wave pseudopotential method, monovacancy, divacancy and self-interstitials in hcp titanium are investigated. The calculated monovacancy formation energy is 1.97 eV, which is in excellent agreement with other theoretical calculations, and agrees qualitatively with published experimental results. The relaxation of the atoms around a single vacancy is observed to be small. Two divacancy configurations, the in-plane and the off-plane, have also been shown to be equally stable. With regards to the interstitials, of the eight configurations studied, two (octahedral and basal octahedral) have relatively lower formation energies and are, thus, the most likely stable configurations. We find small energy differences between them, suggesting their possible co-existence. It is also observed that the tetrahedral configuration decays to a split dumbbell configuration, whereas both the basal tetrahedral and the basal pseudocrowdion interstitials decay to the basal octahedral configuration. Using the nudged elastic band method (NEB), we determine a possible minimum energy path (MEP) for the diffusion of self-interstitial titanium atoms from an octahedral site to the nearest octahedral site. The energy barrier for this migration mechanism is shown to be about 0.20 eV.

Keywords:

• self-interstitial
• ab initio
• density-functional theory
• ion implantation
• point defects
• residual stress
• titanium

Acknowledgements

This work was supported by the South African National Research Foundation (NRF). One of the authors, ATR, wishes to acknowledge the Mori Foundation for a scholarship award through the Abdus Salam ICTP for his visit to the institute. We have also benefited immensely from our discussions with Roger Rousseau. Most of the computations were done using the facilities at CINECA, the Italian supercomputing centre.