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Solute atoms such as Fe, Co, and Ni diffuse abnormally fast in α -Ti, which influences significantly the mechanical properties of the titanium alloys. Various mechanisms (e.g., the interstitial diffusion mechanism and solute-vacancy complex mechanism) have been proposed to account for the fast diffusion of these solutes in α -Ti. To elucidate such diffusion mechanism, a first-principles method is employed to calculate the formation energies, migration energy barriers, and solute-vacancy binding energies of the substitutional and interstitial solute atoms including Al, Si, Sn, V, Mn, Fe, Co, Ni, and Cu in α -Ti. Based on the calculated parameters, the diffusion mechanisms are discussed. Comparing the formation energies of the substitutional and interstitial solutes, we find that all the solute atoms prefer the substitutional configuration to the interstitial one. The interstitial migration energy barriers are quite low for all the solutes. Al and Sn diffuse mainly through normal vacancy mediated mechanism due to the high substitutional to interstitial preferential energy that leads to very low fraction of interstitial solutes (about 10 − 15 ∼ 10 − 16 at 1000 K) at thermal equilibrium state and high interstitial diffusion activation energy. The 3d metal solute atoms, especially Mn, Fe, and Co, are fast diffusers and the diffusion coefficients are dominated by the interstitial mechanism because of their sizable thermal equilibrium interstitial fractions (several percent at 1000 K). The solute-vacancy complex mechanism is not likely to account for the fast diffusions in α -Ti. We show that the direct chemical interaction between the solute and matrix atoms determines the site-occupancy of the solute atoms in α -Ti besides the atomic size effect that was commonly believed to be responsible for the fast diffusions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hydrogen atoms are more mobile near the surface than in bulk of titanium. • Hydrogen atoms are prone to accumulate beneath the surface of titanium, facilitating the formation of brittle hydride. • Surface energy reduces with increasing hydrogen coverage, enhancing the decohesion of titanium. • Desorption of hydrogen atoms to form gaseous H 2 cost energy, ruling out the hydrogen atmospheric pressure induced HE of titanium. Hydrogen embrittlement (HE) caused by the interaction between hydrogen and crack tip is the key factors for the stress corrosion crack (SCC) of titanium alloys utilized in marine environment. In the present work, the interactions between hydrogen and the fresh surface of crack tip in titanium, including the adsorption, desorption, diffusion of hydrogen atom near the surface, are investigated systematically by using a first-principles method. The surface energy of titanium with H adsorption is calculated. We show that H atoms are prone to adsorb on the surface of titanium and then accumulate beneath the surface whereas both desorption of H atoms to form H 2 and diffusion of H atom into bulk cost energy. The surface energy decreases with increasing coverage of H, which reduces the fracture work of titanium. The H-induced reduction of the fracture work at the crack tip is expected to accelerate the crack propagation. The accumulation of H beneath the surface facilitates the formation of hydride ahead the crack tip which is critical for the hydride induced embrittlement. The H 2 pressure mechanism is not supposed to be responsible for the HE of titanium because the desorption of H atoms from the surface to form H 2 is energetically unfavorable. [ABSTRACT FROM AUTHOR]

A plane wave pseudo-potential method based on density functional theory is employed to calculate the migration energy barrier for the atomic self-diffusion in HCP metals including Mg, Zn, Ti, Zr, and Hf. The influences of some key factors (plane-wave cutoff energy, k -mesh, supercell size, and geometric optimization scheme) on the calculated migration energy barrier and its anisotropy are systematically investigated. We show that the supercell size affects heavily the migration energy barrier and its anisotropy for the metals with valence d electrons (Ti, Zr, and Hf) but not for the ones with only valence s metals (Mg). In general, the anisotropy of the migration energy barrier reduces with increasing size of the supercell especially for Ti, Zr, and Hf. The optimization of the shape and volume of the supercell matters for the migration energy barrier calculated with the small size supercell but not for that calculated with the large supercell. With the calculated migration energy barrier, the self-diffusion coefficients are evaluated based on the transition state theory and compared with other first-principles calculations and the experimental measurements. [ABSTRACT FROM AUTHOR]

Precipitation strengthening of silicides plays an important role in improving the creep resistance of high‐temperature titanium alloys. The oversize and concentrated silicides remarkably reduce the ductility of the alloys, which shorten the serving life of the key components in aerospace engines and gas turbines. To investigate the alloying effect on the stability of Ti5Si3, the enthalpy of solution for IIIB–VIB and IIIA–IVA elements in Ti5Si3, α matrix, and β phase are calculated and compared. It is proposed that Zr and Hf stabilize Ti5Si3 and promote the nucleation and refinement of silicide particles. The β stabilizers and simple metals destabilize Ti5Si3 and would suppress the precipitation of Ti5Si3. [ABSTRACT FROM AUTHOR]

Phase decomposition influences significantly the mechanical properties of high entropy alloys (HEAs). Prediction of the phase decomposition of HEA is greatly hindered by the hyper-dimensional composition space of the alloys. In the present work, we propose to represent the HEAs as various pseudo-binary alloys of which the temperature dependent free energies as functions of compositions may be readily calculated by using first-principles methods in combination with thermodynamic models. With the calculated free energies, the phase diagrams of the pseudo-binary alloys may be constructed and the phase decomposition can be predicted. This procedure is applied to Hf-Nb-Ta-Ti-Zr alloy with body-centered cubic (BCC) structure. We predict that the equiatomic HfNbTaTiZr HEA suffers from phase decomposition below critical temperature of 1298 K. The HEA decomposes most favorably to BCC NbTa-rich and HfZr-rich phases. The BCC HfZr-rich phase transfers to a hexagonal close-packed structure (HCP) phase at low temperature. The predicted compositions of the decomposed phases are in good agreement with experiment and Thermal-Calc modeling. Furthermore, the effect of the phase decomposition on the strength of the HEA is evaluated by considering the solid-solution and precipitation strengthening mechanisms. The precipitation strengthening effect is stronger than the solid-solution strengthening at the low annealing temperature but becomes weaker at high annealing temperature. [Display omitted]. [ABSTRACT FROM AUTHOR]