Generalized stacking fault energies and critical resolved shear stresses of random α-Ti-Al alloys from first-principles calculations.

The critical resolved shear stress (CRSS) and its related plastic deformation of titanium with hexagonal close packed structure are highly anisotropic, leading to low ductility of the material. Understanding the alloying effect on the CRSS is crucial for the improvement of the mechanical properties through rational composition design. Accurate prediction of the CRSS is not straightforward due to the atomic randomness of the alloy. The generalized stacking fault energies (GSFEs) for the basal and prismatic plane 〈 a 〉 slips of random α-Ti 1− x Al x alloys (0 ≤ x ≤ 0.1875) are calculated by using first-principles methods including exact muffin-tin orbitals (EMTO) and plane-wave psuedopotential (VASP) methods in this work. The random distribution of Al in the alloy is treated by using both coherent potential approximation (CPA) for EMTO and special quasirandom structure (SQS) techniques for VASP. The CRSSs are then evaluated within the frame work of semi-discrete variational Peierls-Nabarro model. The VASP-SQS calculations with atomic relaxation generate reasonably good GSFE and CRSS compared with the EMTO-CPA and VASP-SQS calculations without atomic relaxation. For pure Ti, the unstable stacking fault energy (γ u s f ) and CRSS (τ a) for the basal 〈 a 〉 slip are higher than those for the prismatic 〈 a 〉 slip. With increasing Al concentration x , γ u s f b for the basal 〈 a 〉 slip decreases whereas γ u s f p for the prismatic 〈 a 〉 increases. The CRSSs for the basal 〈 a 〉 slip (τ a b) and the prismatic 〈 a 〉 slip (τ a p) both increase with x while τ a p increases faster than τ a b such that τ a b approaches to τ a p. The calculated CRSSs explain successfully our recently measured mechanical properties (yield strength, fracture toughness, ultimate strength, and elongation) of the α-Ti 1− x Al x alloy against x. • The article calculated GSFEs and CRSSs for basal and prismatic plane (a) slips of random α-Ti 1-x Al x alloys (0 ≤ x ≤ 0.1875). • The local lattice distortion influences significantly on the GSFEs and CRSSs. • Strength and fracture toughness of α-Ti 1-x Al x should increase whereas the plasticity reduces with increasing x. • Basal and prismatic plane slips may be activated during plastic deformation with increasing x. • The unstable stacking fault energy alone does not always work for the description of the mobility of some slip system. [ABSTRACT FROM AUTHOR]