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

The core structure of sigma/3<11<(2)over bar>0> screw dislocations in alpha-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 it 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.