Enhanced solid solution hardening by off-center substitutional solute atoms in α-Ti

Abstract

Most recently, some substitutional solute atoms in α-Ti have been predicted to occupy unexpectedly the low-symmetry (LS) positions away from the high-symmetry (HS) lattice site, which was speculated to result in enhanced solid solution hardening (SSH). In the present work, the SSH induced by the LS off-center solute atom is evaluated within the framework of continuum elasticity theory, in comparison with that induced by its HS lattice-site counterpart. The interaction energy and force between the solute atom and the basal/prismatic edge/screw 〈a〉 dislocations in α-Ti solid solution are calculated with the elastic dipole model, with which the strength increments induced by the solute atoms are evaluated with the Labusch model. We show that, in general, the LS solute atom interacts much more strongly with the dislocations than its HS counterpart does. The calculated interaction energies suggest that the LS solute atom forms atmosphere above/below the slip plane of the basal 〈a〉 dislocations but on the slip plane of the prismatic 〈a〉 dislocations regardless of the dislocation types (edge or screw). The strength increments caused by most of the LS solute atoms are more than an order of magnitude higher than those by their HS counterparts. The SSH effect induced by the LS solute atom is mainly determined by the strength of the Jahn-Teller splitting of the $d$-orbitals of the solute atom, dissimilar to that induced by HS solute atom where the atomic size mismatch dominates.