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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]

Abstract: The origin of the superior high temperature strength of Ti-rich γ-TiAl with addition of Nb is a highly controversial issue. In this paper, we calculated the interaction energies between point defects in these alloys using a first-principles plane-wave pseudopotential method. It is found that, the nearest neighbouring Nb atoms on Al-sublattice (NbAl) repel whereas the next nearest neighbouring NbAl atoms attract each other, indicating that NbAl atoms may form short-range ordered structure. Based on this finding, we propose that the short-range order may be responsible for the high strength of the Ti-rich TiAl-Nb alloys. [Copyright &y& Elsevier]

Abstract: The interaction energies between substitutional rare earth (RE) atoms, between RE and interstitial C, N, O, H atoms, as well as between RE and vacancies in α-Ti are calculated via first-principles density-functional theory with projector augmented-wave (PAW) pseudopotentials. The results show that the RE–vacancy and RE–RE interactions are attractive due to the weaker RE–Ti bond than the host Ti–Ti bond. All of the RE atoms investigated in this paper are repulsive to C and N, but attractive to H. RE–O interactions are repulsive for the light RE atoms, though the interactions are very weak for the heavy RE atoms. The mechanism underlying the interactions and their possible influence on the properties of Ti alloys are discussed. [Copyright &y& Elsevier]

The alloying effects of Nb, Zr, and/or Sn on the phase stability and elastic properties of Ti are investigated by using a first-principles method. Our calculation results indicate that a carefully designed Ti–Nb–Zr–Sn system can be a good candidate for low modulus biomedical materials. We find that the well-known correlation between the e/a ratio and both elastic and phase stabilities for Ti alloyed with transition metal elements breaks down for the Ti–Sn alloy. [ABSTRACT FROM AUTHOR]

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]

First-principles method is used to characterize segregation and magnetic properties of alloyed Ti/TiO 2 interface. We calculate the segregation energy of the doped Ti/TiO 2 interface to investigate alloying atom's distribution. The oxidation resistance of Ti/TiO 2 interface is enhanced by elements Fe and Ni but reduced by element Co. Magnetism could be produced by alloying elements such as Co, Fe and Ni in the bulk of titanium and the surface of Ti at Ti/TiO 2 interface. The presence of these alloying elements could transform the non-magnetic titanium alloys into magnetic systems. We have also calculated the temperature dependence of magnetic permeability for the doped and pure Ti/TiO 2 interfaces. Alloying effects on the Curie temperature of the Ti/TiO 2 interface have been elaborated. [ABSTRACT FROM AUTHOR]

Hydride induced embrittlement is one of the main causes for the stress corrosion crack of titanium alloys. The formation process of the hydride at atomic level remains controversial as it is not accessible directly by experiments. In the present work, the formation of titanium hydride is investigated by using a first principles method. By calculating the formation energies of TiH m with various types of H occupation state in HCP and FCC Ti matrix, we show that there exists phase equilibrium between α-Ti-H solid solution and δ/ε hydrides with χ and γ hydrides appearing as metastable phases. The energy barrier for the HCP → FCC structure transformation for the formation of the hydrides decreases with increasing H:Ti ratio m. The structure transformation is accompanied with spontaneous shift of half amounts of the H atoms from the octahedral to the tetrahedral interstices. The energy barrier for the migration of the remaining half of the H atoms from the octahedral to the tetrahedral interstices reduces with increasing m and approaches to a constant value. The hydrostatic compressive stress acting on TiH m , resulted from the H-induced lattice dilation tendency and the constraint of the surrounding matrix, does not facilitate the HCP → FCC transformation and the migration of the H atoms. The present work sheds light on the hydride formation process at atomic level. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: The crystallographic structure and stability of the phase relative to the and phases in Ti–x M (M=Ta, Nb, V, Mo) alloys are investigated by using the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation. We show that, with increasing concentration of the alloying elements, the structure of the orthorhombic- phase evolutes from the hcp- to the bcc- phase, i.e., the lattice parameters b/a and c/a as well as the basal shuffle y decreases from those corresponding to the phase to those of the phase. The compositional and phase boundaries are determined by comparing the total energies of the phases. The predicted phase boundaries are about 10.2, 10.5, 11.5, 4.5at% for Ti–V, Ti–Nb, Ti–Ta, and Ti–Mo, respectively, in reasonable agreement with experiments. The phase boundaries are higher than the experimental values, possibly due to the absence of temperature effect in the first-principles calculations. Analyzing the electronic density of states, we propose that the stability of the phase is controlled by the compromise between the strength of the covalent and metallic bonds. [Copyright &y& Elsevier]

Graphical abstract Highlights • The interface energies and GSFE of β / ω interface in Ti alloy are calculated. • The strong anisotropy of the interface energy contributes to the ellipsoid-like morphology of β particles, with its major axis parallel to 〈1 1 1〉 β direction. • The β / ω interface is prone to crack initiation and propagation as compared to the bulk and contributes to the ω -embrittlement of β -Ti alloys. Abstract ω phase precipitating in a β -Ti alloy is generally detrimental to the ductility and plasticity of the alloy.To understand the influence of ω phase on the mechanical properties of β -Ti alloys, the property of the β / ω interface is demanded.In this regard, we calculate the interface energies ( γ int ), cleavage energies ( γ cl ), and generalized stacking fault energies (γ) of the β / ω interfaces with different orientations using a first-principles method.Our calculations predict interface energies of 0.32 J m−2 and 0.03 J m−2 for the (1 1 1 ¯) β / (0 0 0 1) ω and (1 1 2) β / (1 0 1 ¯ 0) ω interfaces, respectively.The strong anisotropy of the interface energy contributes to the ellipsoid-like morphology of ω phase with its major axis parallel to the 〈 1 1 1 〉 β direction.The influence of the interface on the ductility of the alloy is discussed with the calculated cleavage energies and the unstable stacking fault energies (γ us) extracted from the generalized stacking fault energy (GSFE). The γ cl / γ us ratio of the (1 1 2) β / (1 0 1 ¯ 0) ω interface is lower than those of the main slip planes of the bulk phases, indicating that this interface is prone to crack initiation and propagation as compared to the bulk and contributes to the ω -embrittlement of β -Ti alloys. [ABSTRACT FROM AUTHOR]

Graphical abstract Effects of the shear stress and composition on the ω -variant selection during the β → ω phase transformation of Ti- x Mo alloys from first-principles. Highlights • A model is developed to investigate the variant selection from first-principles. • Both bulk and interface contributions to the variant selection are considered. • The variant selection becomes stronger with increasing applied shear stress. • There may exists a critical concentration where there is no variant selection. Abstract Variant selection during solid–solid phase transformation in titanium alloys affects greatly the microstructure and mechanical properties of the alloys. Theoretical investigations of the variant selection were generally performed by using phase field simulation that considers solely the elastic coherency strain energy. In the present work, we develop a model to determine the variant selection directly from first-principles calculations. The source of the variant selection is considered to be the varying free energy gains (including both bulk and interface contributions) induced by the transformation from the parent phase to different variants of the product phase. This model is applied to investigate the effects of shear stress and alloy composition on ω variant selection during athermal β → ω transformation in binary Ti- x Mo (x ≤ 25 at.%) alloys. The random distribution of the atoms in the alloy is described by using virtual crystal approximation (VCA). We show that the tendency of variant selection becomes stronger with increasing shear stress. With increasing Mo concentration, the favorable ω variant transfers from one to another, except for the ω phase with very small particle size (e.g., radius R ≤ 1 nm) where one variant is always selected. The critical Mo concentration for the transfer of the favorable ω variant approaches to 5 at.% with increasing size of the ω phase particles and remains almost unchanged against the shear stress. At the critical Mo concentration, the considered variants have equal free energy gains and there is no variant selection. This finding opens the possibility of controlling the variant selection and the associated microstructure by changing the composition of the alloys. [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]

The phase stability and elastic property of binary Ti- x TM (TM = V, Cr, Nb, Mo) and ternary Ti-15TM- y Al alloys are investigated systematically by using a first-principles method. The coherent potential approximation is employed to describe the random distribution of the alloying atoms in the alloys. We show that the transition metal (TM) elements V, Cr, Nb, Mo increase the elastic stability of the β phase but decreases that of the α phase. The polycrystalline bulk modulus of the α phase increases with the concentration of the TM alloying elements whereas the Young's modulus and shear modulus are weakened. For the β phase, all the polycrystalline elastic moduli hardens with the addition of the TM alloying elements. The influence of the simple metal (SM) element Al on the elastic properties is much weaker than the TM ones. The TM alloying elements increase the stability of the β relative to the α and ω phases. Al stabilizes significantly the β phase over the ω phase. Our calculations suggest that the d electron density dominates the elastic modulus but not the relative phase stability. The interaction between the TM and SM alloying elements in titanium alloys matters to the relative stability. [ABSTRACT FROM AUTHOR]

[Display omitted] • X-Al interaction energy is proposed as an index for screening alloying element X in high temperature Ti alloys. • Effects of temperature and off-center occupation of alloying atom on interaction energy are considered. • Mo, Tc, Ru, W, Re, and Os are identifed to benifit both strength and thermal stability of high temperature Ti alloys. Increasing the solubility of Al and inhibiting the growth of Ti 3 Al precipitates in high temperature titanium alloy improve both the thermal strength and thermal stability of the alloy. In principle, this may be achieved by adding some other alloying atoms which attract Al so as to serve as traps for Al in the alloy. In the present work, the interaction energies between Al and alloying atoms X with X covering all the transition metal (TM) and noble metal (NM) elements in the Chemical Elemental Period Table (CEPT) are calculated by using a first principles method in order to screen the traps for Al. The effects of temperature (taking Mo-Al interaction as an example) as well as the newly found off-center site-occupation of alloying atoms (Acta Mater. 197, 2020, 91) on the interaction energies are considered. We show that the interactions between Al and the TM alloying atoms early in the CEPT are weak. The middle TM alloying atoms attract but the late TM and NM ones repel Al. At finite temperature, the lattice vibration enhances whereas the configurational entropy weakens the Mo-Al attraction. Both effects become stronger with increasing temperature. The physical origins underlying the X-Al interactions are revealed by analyzing the electronic density of states and bonding charge density. Alloying atoms such as Mo, Tc, Ru, W, Re, and Os are identified to benefit the strength and thermal stability of high temperature titanium alloys. This work is helpful to the rational design of high temperature titanium alloys. [ABSTRACT FROM AUTHOR]