Solid solution strengthening of hexagonal titanium alloys: Restoring forces and stacking faults calculated from first principles

The solid solution strengthening of alpha-Ti was investigated in respect of dislocation nucleation and dissociation in all four active glide modes. A series of Ti+X alloys (X = Al, Sn, V, Zr and O) was selected to analyze the impact of solute valence structure (Al, Sn - p type elements, V, Zr - d type elements) and lattice site (interstitial O) on the mechanisms responsible for variation of mechanical properties. The computational procedure relied on the generalized stacking fault energy (GSFE) concept combined with the nudged elastic band method that enables full atomic relaxation and determination of the true, minimum energy GSFE path. Additionally, various concentrations of solutes and their distance to the glide plane were considered as well. Our study revealed a strong, nonlinear influence of X position on GSFE and migration of O atoms during the crystal slip. These new phenomena allowed one to determine three solution strengthening mechanisms: (I) hindrance of < a > prismatic dislocation emission and reconfiguration of 1/3 < 11 (2) over bar0 > screw dislocation cores (p type solutes), (II) hindrance of < a > prismatic dislocation emission (V) and SFE reduction in other modes (both d type solutes) and (III) suppression of dislocation nucleation in all modes caused by O. We found that the stacking faults formed by the single partial dislocations have a thickness of few atomic layers and exhibit a highly non-uniform structure. Their ability to accommodate the lattice deformation introduced by solute elements greatly affects the stacking fault energies of the alpha-Ti alloys. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.