Core structure of screw dislocations in hcp Ti: an ab initio DFT study

The core structure of screw dislocations in -Ti was studied in the cluster approach with ab initio DFT-GGA, and in both the cluster and quadrupole approaches with a recently highly optimized EAM central force potential. With the EAM potential we have shown that finite-size effects, in the cluster approach, are negligible down to the size studied in the ab initio DFT calculations that have shown unambiguously a preferential prismatic core spreading for the dislocation. Our results are in agreement with previously published approximated calculations using empirical or semi-empirical interaction models: only approximated interaction models, taking explicitly into account the covalent directional bonding of the d electrons, can properly account for the preferential prismatic core spreading against the basal one; and empirical interaction models without angular force components are inadequate. Interestingly, at first sight, the relaxed core structures (basal or prismatic) obtained with empirical or semi-empirical interaction models are almost identical to the ones obtained with the ab initio DFT calculations.